Format Markdown with prettier in make fmt and make fmt-check (closes #19)

script/fmt and script/fmt-check run prettier over every Markdown file
again, next to gofmt. prettier is pinned by hash through package.json
and yarn.lock, copied from the prompts repo with .prettierrc and
.prettierignore, and is never installed on a host: a new prettier stage
of the Dockerfile installs it into a digest-pinned node image, and both
scripts build that stage and run it with the repository mounted. CI
checks the Markdown in a markdown stage that the build stage waits on.
Because make fmt-check now runs docker, the Dockerfile runs gofmt
directly in its lint stage instead. All Markdown is reformatted.

Model: opus-5-5

.dockerignore

@@ -1,9 +1,12 @@
-.git
+# .git is sent without its config. Without a VERSION build argument the
+# stage that compiles runs `git describe --tags --always` on .git, which
+# does not need .git/config; that file can hold a credential, such as a
+# password in a remote URL or the token the CI checkout step stores there.
+.git/config
+
 .claude
 .DS_Store
 sfdupes
-files.dat
-node_modules
 *.log
 *.out
 *.test

.gitea/workflows/check.yml

@@ -6,4 +6,6 @@ jobs:
         steps:
             # actions/checkout v4.2.2, 2026-02-22
             - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
+              with:
+                  fetch-depth: 0
             - run: script/cibuild

.gitignore

@@ -27,7 +27,6 @@ node_modules/
 *.log
 
 # Local scan data
-files.dat
 *.sqlite
 *.sqlite-shm
 *.sqlite-wal

Dockerfile

@@ -12,9 +12,9 @@ COPY . .
 # build would exit 0 having run nothing. script/cibuild and
 # script/docker pass a fresh CHECK_EPOCH on every invocation.
 #
-# Two properties this depends on. ARG is per-stage, so the build stage
-# below declares it again; one declaration here would leave that
-# stage's gate cacheable. And each gate RUN must reference the value,
+# Two properties this depends on. ARG is per-stage, so the markdown and
+# build stages below declare it again; one declaration here would leave
+# their gates cacheable. And each gate RUN must reference the value,
 # because BuildKit hashes the expanded command: a declared but
 # unreferenced ARG invalidates nothing.
 #
@@ -27,12 +27,16 @@ ARG CHECK_EPOCH
 # target now runs `docker build -f Dockerfile.lint`, and a docker build
 # cannot run a docker build: routing the gate through make would mean
 # nesting docker inside this image. Same reason `make check` is gone
-# from the build stage below.
-#
-# `make fmt-check` is not run in this stage: it now also runs prettier
-# over Markdown, and this golangci-lint image has no node. The gate runs
-# in the build stage below, where script/bootstrap installs node and
-# prettier.
+# from the build stage below, and `make fmt-check` from both stages: it
+# runs prettier through docker too. Its gofmt half is the step below,
+# its Markdown half the markdown stage further down. gofmt's output is
+# assigned to a variable first so that its own exit status, as when it
+# cannot parse a file, still fails the step.
+RUN echo "gate gofmt, epoch ${CHECK_EPOCH}" && \
+    files="$(gofmt -s -l .)" && \
+    if [ -n "$files" ]; then \
+        echo "gofmt: files not formatted:" >&2; echo "$files" >&2; exit 1; \
+    fi
 
 # The FROM above and the one in Dockerfile.lint pin the same linter
 # twice, and nothing else keeps them in sync; this fails the build when
@@ -49,71 +53,125 @@ RUN echo "gate config verify, epoch ${CHECK_EPOCH}" && \
 RUN echo "gate lint, epoch ${CHECK_EPOCH}" && \
     golangci-lint run --config .golangci.yml ./...
 
+# Prettier stage: the prettier that formats this repository's Markdown,
+# never installed on a host. script/fmt and script/fmt-check build this
+# stage alone and run it with the repository mounted on /src. prettier
+# is installed in /tools so that the repository, mounted or copied onto
+# /src, cannot hide it.
+# node:22-alpine, 2026-02-22
+FROM node@sha256:e4bf2a82ad0a4037d28035ae71529873c069b13eb0455466ae0bc13363826e34 AS prettier
+WORKDIR /tools
+# yarn.lock pins prettier by hash, and --frozen-lockfile fails rather
+# than install anything yarn.lock does not name.
+COPY package.json yarn.lock ./
+RUN yarn install --frozen-lockfile
+ENV PATH=/tools/node_modules/.bin:$PATH
+WORKDIR /src
+
+# Markdown stage: the Markdown half of `make fmt-check`, as a gate.
+FROM prettier AS markdown
+COPY . .
+# Second per-stage declaration of the gate cache-buster; see the lint
+# stage above.
+ARG CHECK_EPOCH
+RUN echo "gate prettier, epoch ${CHECK_EPOCH}" && \
+    prettier --check '**/*.md' --tab-width 4 --prose-wrap always
+
 # Build stage
 # golang:1.25-alpine, 2026-07-23
 FROM golang@sha256:56961d79ea8129efddcc0b8643fd8a5416b4e6228cfd477e3fd61deb2672c587 AS builder
 
 # We never build or run as root. Create an unprivileged user and point
-# HOME and the Go caches at its home so go build and go test can write
-# their caches when we drop to it below. $GOPATH/bin is deliberately not
-# on PATH: script/bootstrap no longer `go install`s anything (the linter
-# runs from a pinned image, never from a host install), so nothing lands
+# HOME and the build cache at its home so go build and go test can write
+# it when we drop to it below. $GOPATH/bin is deliberately not on PATH:
+# script/bootstrap no longer `go install`s anything (the linter runs
+# from a pinned image, never from a host install), so nothing lands
 # there and adding it would only widen what this image resolves.
+#
+# The module cache is kept outside that home, at the base image's
+# default /go/pkg/mod, and belongs to root: script/bootstrap fills it as
+# root. Do not move it into the home and hand it over with `chown -R`:
+# that walks every file in it, which took from about 80 s to over ten
+# minutes on a shared host, depending on load.
 RUN adduser -D -u 1000 builder
 ENV HOME=/home/builder
 ENV GOPATH=/home/builder/go
+ENV GOMODCACHE=/go/pkg/mod
 ENV GOCACHE=/home/builder/.cache/go-build
 
 WORKDIR /src
 
-# No-op file copy whose only purpose is the build-graph edge: it is what
-# makes this stage depend on the lint stage, and so what forces BuildKit
-# to finish fmt-check, the pin guard and lint before compilation and
-# tests start. Remove it and the fail-fast design dies silently — the
-# build stops gating on lint and still exits 0. It replaces a copy of
-# the linter binary itself, which is no longer wanted here: nothing in
-# this stage runs the linter, because `make lint` is now a docker build
-# and a docker build cannot run inside one.
+# No-op file copies whose only purpose is the build-graph edge: they are
+# what make this stage depend on the lint and markdown stages, and so
+# what forces BuildKit to finish gofmt, the pin guard, lint and prettier
+# before compilation and tests start. Remove one and the fail-fast
+# design dies silently — the build stops gating on that stage and still
+# exits 0. The first replaces a copy of the linter binary itself, which
+# is no longer wanted here: nothing in this stage runs the linter,
+# because `make lint` is now a docker build and a docker build cannot
+# run inside one.
 COPY --from=lint /src/go.sum /dev/null
+COPY --from=markdown /src/go.sum /dev/null
 
 # Install development prerequisites the same way a developer does,
 # rather than duplicating the installs inline. Only script/ and the
 # dependency manifests are copied first, nothing else, so this layer
 # stays cached until the scripts or the dependencies change — bootstrap
-# runs `go mod download` and `yarn install`, which is why there is no
-# separate invocation of either here. The JS manifests (package.json,
-# yarn.lock) are copied too so the yarn install layer caches alongside
-# the Go one.
+# ends in `go mod download`, which is why there is no separate
+# invocation of it here.
 COPY script/ script/
-COPY go.mod go.sum package.json yarn.lock ./
+COPY go.mod go.sum ./
 RUN script/bootstrap
 
-COPY . .
+# Hand builder only what it writes to, without walking the module cache.
+# This layer stays cached with bootstrap.
+# - /src itself: make build writes the binary into it, and git refuses
+#   a repository whose top directory belongs to another user.
+# - the module cache's cache/download directory itself, not what is in
+#   it: Go only reads the downloaded modules, but make build saves its
+#   lookup of this module's own version from git there, in a new
+#   directory named after the module path.
+# - builder's home: the go commands bootstrap ran as root left Go's
+#   telemetry files there, a few small files.
+RUN chown builder:builder /src /go/pkg/mod/cache/download && \
+    chown -R builder:builder /home/builder
 
-# Hand the sources and caches to the unprivileged user, then drop root
-# before running any checks or builds.
-RUN chown -R builder:builder /src /home/builder
+# The sources are handed to builder as they are copied, so no layer has
+# to walk them. Then drop root before running any checks or builds.
+COPY --chown=builder:builder . .
 USER builder
 
 # Fail the build unless the branch is green. Runs as non-root so the
 # permission-denied test paths are exercised legitimately (root would
 # bypass the chmod(0) the tests rely on).
 #
-# The gates are the individual targets, not `make check`: that aggregate
-# runs `script/lint`, which is now a docker build, and nothing inside an
-# image build may shell out to docker. Lint is not skipped by this — it
-# ran in the lint stage above, which this stage's COPY --from makes a
-# prerequisite. `make`, not the scripts directly, because the Makefile's
+# The gate is `make test`, not `make check`: that aggregate runs
+# `script/lint` and `script/fmt-check`, which both run docker, and
+# nothing inside an image build may shell out to docker. Lint and the
+# format checks are not skipped by this — they ran in the lint and
+# markdown stages above, which this stage's COPY --from lines make
+# prerequisites. `make`, not the script directly, because the Makefile's
 # `export CGO_ENABLED = 0` applies only to what it invokes.
 #
-# Second per-stage declaration of the gate cache-buster; see the lint
+# Third per-stage declaration of the gate cache-buster; see the lint
 # stage above for why one is not enough. It is placed after USER so the
 # drop to the unprivileged user still happens before the checks run.
 ARG CHECK_EPOCH
 RUN echo "gate test, epoch ${CHECK_EPOCH}" && make test
-RUN echo "gate fmt-check, epoch ${CHECK_EPOCH}" && make fmt-check
 
-RUN make build
+# The version stamped into the binary: the VERSION build argument when
+# one is given, otherwise `git describe --tags --always` of the .git in
+# the build context (git is installed by script/bootstrap above). A
+# context that carries .git and still yields no version fails the build;
+# with neither, as from a source tarball, it is "dev".
+ARG VERSION
+RUN version="${VERSION:-$(git describe --tags --always || echo dev)}"; \
+    if [ -e .git ] && { [ -z "$version" ] || [ "$version" = dev ] || \
+        [ "$version" = unknown ]; }; then \
+        echo "no version could be derived although the build context carries .git" >&2; \
+        exit 1; \
+    fi; \
+    make build VERSION="$version"
 
 # Runtime stage
 # alpine:3.22, 2026-07-23

Makefile

@@ -46,4 +46,4 @@ hooks:
 	@script/install-precommit
 
 clean:
-	rm -f $(BINARY) files.dat
+	rm -f $(BINARY)

README.md

@@ -4,16 +4,21 @@
 
 `sfdupes` is an MIT-licensed Go CLI tool by [@sneak](https://sneak.berlin) that
 quickly identifies _candidate_ duplicate files — and, ultimately, entire
-duplicate directory trees — across very large filesystems without reading full
-file contents. Files are considered duplicates when they have identical size,
-identical SHA-256 of their first 1024 bytes, and identical SHA-256 of their last
-1024 bytes. This is a strong candidate signal, not proof of identical content
-(the middle of the file is never read); the intended use is finding duplicate
-downloads and duplicated directory trees on multi-terabyte ZFS servers where
-reading every byte is prohibitively expensive. `scan` maintains a persistent
-SQLite database of file signatures that survives between runs, so it can be run
-from cron and the reports can be generated at any time from the most recent
-scan.
+duplicate directory trees — across very large filesystems without reading every
+byte of every file. Files are considered duplicates when their sizes are equal
+and they agree on a short ladder of hashes. A file under 10 MiB is hashed in
+full and compared directly. A larger file is gated first on the SHA-256 of its
+first 64 KiB and of its last 64 KiB, and only when its size and both of those
+match another file's is it read for a content hash to compare — the SHA-256 of
+the whole file when it is under 50 MiB, or of gigabyte-spaced 1 MiB samples when
+it is 50 MiB or larger. Below 50 MiB the content hash is proof of identical
+content; at or above 50 MiB it is a strong candidate signal rather than proof,
+because the gaps between samples are never read. The intended use is finding
+duplicate downloads and duplicated directory trees on multi-terabyte ZFS servers
+where reading every byte of every file is prohibitively expensive. `scan`
+maintains a persistent SQLite database of file signatures that survives between
+runs, so it can be run from cron and the reports can be generated at any time
+from the most recent scan.
 
 This README is the complete and authoritative specification.
 
@@ -28,8 +33,9 @@ export SFDUPES_DATABASE="$HOME/.local/share/sfdupes/db.sqlite"
 ```
 
 `scan` walks one or more filesystem trees and maintains one database record per
-regular file (path, size, mtime, head hash, tail hash). The database persists
-between runs; a rescan only hashes files that are new or changed, and removes
+regular file (path, size, mtime, head hash, tail hash, content hash). The
+database persists between runs; a rescan only hashes files that are new or
+changed, or that may have gained a duplicate since the last scan, and removes
 records for files that no longer exist. `report` reads the database and prints
 the file-level duplicates report. `trees` reads the same database and prints the
 duplicate-tree report. A missing/invalid subcommand — or a `scan` invocation
@@ -40,18 +46,134 @@ by setting `SFDUPES_DATABASE`. The intended deployment is a daily `sfdupes scan`
 cron job, with the reporting commands run interactively whenever needed; their
 results are as fresh as the last completed scan.
 
+### Install
+
+With Go installed, this builds and installs the current `main` branch:
+
+```sh
+go install sneak.berlin/go/sfdupes@main
+```
+
+The binary goes to `$(go env GOPATH)/bin`, or to `$GOBIN` when that is set. A
+binary installed this way reports its version as `dev`; one built from a clone
+or into the Docker image carries the git tag or commit it was built from.
+
+From a clone, `make build` writes the binary to `./sfdupes`:
+
+```sh
+git clone https://git.eeqj.de/sneak/sfdupes.git
+cd sfdupes
+make build
+```
+
+Copy the binary to `/usr/local/bin` for the cron job below.
+
+`make docker` builds the Docker image, tagged `sfdupes`, after running the tests
+and the linter (see "Build"). The image runs `sfdupes` as root with the database
+at its default path, so a bind mount of `/var/lib/sfdupes` keeps the database
+between runs. Mount the scanned tree at the same path inside the container as on
+the host; read-only is enough. The database records paths as the container sees
+them, so the reports then name the host's paths.
+
+```sh
+make docker
+docker run --rm -v /srv:/srv:ro -v /var/lib/sfdupes:/var/lib/sfdupes \
+    sfdupes scan /srv
+docker run --rm -v /var/lib/sfdupes:/var/lib/sfdupes sfdupes report > dupes.tsv
+```
+
+### Daily scan from cron
+
+Run `scan` as root, so that it can read every file: a path it cannot read is
+skipped with a warning and loses its database record (see "Rules for the walk").
+As a file `/etc/cron.d/sfdupes`:
+
+```
+30 3 * * * root /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log
+```
+
+- The database is `/var/lib/sfdupes/db.sqlite`, created with its directory by
+  the first scan. To keep it elsewhere, set
+  `SFDUPES_DATABASE=/path/to/db.sqlite` before the command on the same line.
+- `scan` writes nothing to stdout. Its stderr, appended here to
+  `/var/log/sfdupes.log`, holds a plain progress line as each phase starts and
+  then at most every 5 seconds, a warning for each path it skips, and the
+  summary line (see "Progress" and "`scan` mode"). The log grows with every
+  scan; rotate it like any other.
+- Skipped paths do not fail a scan: it still exits 0, and cron sends nothing. A
+  scan that fails, or is stopped by `SIGINT` or `SIGTERM`, exits 1 with the
+  reason among the last lines of the log; `tail` prints them, and cron mails
+  them to root if the host can send mail.
+- A scan still running when the next one starts carries on. The new one fails at
+  once, and the lines cron mails include
+  `sfdupes: another scan is running (lock held on /var/lib/sfdupes/db.sqlite.lock)`.
+- `report` and `trees` need only read access to the database (see "Database").
+  Under the usual umask of `022` the first scan creates it readable by every
+  user, so an unprivileged user can run them against root's database.
+
+### Reading the reports
+
+Each row of `report` names two copies of one file, and each row of `trees` two
+copies of one directory tree (see "Report output format" and "Trees output
+format"). In a group of copies, the path that sorts first byte by byte is
+`first` and every other path is a `dupe` of it. `first` says nothing about which
+copy is the original or the oldest; which copy to keep is your choice.
+
+A row is a candidate, not proof:
+
+- The reports read only the database, so they show the files as of the last
+  scan; a file may have changed or gone since.
+- A file of 50 MiB or more is compared only on samples of its content (see
+  "Duplicate detection").
+- Paths that are hard links to one file are listed as duplicates, but they share
+  their data, so removing one frees nothing.
+
+Compare a pair byte for byte before removing either copy. For the row
+`/srv/a/big.iso`, `/srv/b/big-copy.iso`, `4294967296`:
+
+```sh
+cmp /srv/a/big.iso /srv/b/big-copy.iso && echo identical
+[ /srv/a/big.iso -ef /srv/b/big-copy.iso ] && echo "hard links"
+```
+
+`cmp` prints nothing and exits 0 only when every byte matches, and otherwise
+reports where the files differ. The second line prints `hard links` when the two
+paths are the same file, so removing either frees nothing.
+
+A path holding a backslash, tab, newline or carriage return is escaped in the
+reports (see "Report output format"). Undo the escapes before using it.
+`printf '%b'` does exactly that, because every backslash in an escaped path
+starts one of the four escapes. Command substitution drops trailing newlines, so
+print an `x` after the path and remove it afterwards, or a path that ends in a
+newline names a different file:
+
+```sh
+p="$(printf '%bx' '/srv/a/tab\tname.txt')"; p="${p%x}"
+cmp "$p" /srv/b/tab-copy.txt
+```
+
+Check a `trees` row with `diff -r`, which compares the two trees file by file
+and also names anything present in only one of them, such as an empty directory
+or a symlink, which `trees` does not see.
+
 ## Rationale
 
 Duplicate finders that hash entire files do not scale to the target environment:
 ~10 million files and ~150 TB on possibly slow or busy disks (a ZFS pool under
-resilver). Reading at most 2 KiB per file — and only from files whose size at
-least one other file shares, since a size-unique file cannot be a duplicate —
-makes a full-filesystem sweep tractable, and the signatures are kept in a
-persistent database, so the expensive filesystem pass is incremental: a rescan
-re-hashes only files whose recorded mtime or size changed, and all analysis
-happens offline from the database alone. The end goal is not individual files
-but whole duplicated trees — duplicate extractions, duplicate downloads, copied
-project trees — which an operator can consider removing as a unit.
+resilver). sfdupes spends disk I/O only on files whose size at least one other
+file shares, since a size-unique file cannot be a duplicate. Of those, a file
+under 10 MiB is read in full; a larger one has its cheap end windows read first,
+and is read for a content hash only when its size and both end windows match
+another file's — the whole file below 50 MiB, but only gigabyte-spaced samples
+at or above 50 MiB, so the largest files are never read in full. This keeps a
+full-filesystem sweep tractable, and the signatures are kept in a persistent
+database, so the expensive filesystem pass is incremental: a rescan re-hashes
+only files whose recorded mtime or size changed, plus — for its content hash — a
+file of 10 MiB or more whose size and end windows have come to match another
+file's. All analysis happens offline from the database alone. The end goal is
+not individual files but whole duplicated trees — duplicate extractions,
+duplicate downloads, copied project trees — which an operator can consider
+removing as a unit.
 
 ## Design
 
@@ -63,27 +185,43 @@ Goals, in order:
     trees), so the operator can consider removing an entire subtree at once.
     File-level duplicate detection is the foundation; tree-level detection is
     built on top of it.
-2.  **Never read full file contents.** At most 2 KiB is read per file (first and
-    last 1024 bytes), and only files whose size at least one other file shares
-    are read at all — a size-unique file cannot be a duplicate. Scale target:
-    tens of millions of files, ~150 TB filesystem, possibly slow or busy disks
-    (ZFS pool under resilver). Holding one small record (path, size, mtime) per
-    file in memory during a scan is acceptable; holding every file's hashes is
-    not (they stay in the database).
+2.  **Spend I/O in proportion to duplicate likelihood.** Only files whose size
+    at least one other file shares are read at all — a size-unique file cannot
+    be a duplicate. Those are compared by the ladder in "Duplicate detection"
+    below: a file under 10 MiB is hashed in full, while a larger file is gated
+    on cheap 64 KiB end windows first, and gets a content hash only when its
+    size and both end windows match another file's. That hash reads the whole
+    file below 50 MiB but only gigabyte-spaced 1 MiB samples at or above it, so
+    the very largest files are still never read in full. Scale target: tens of
+    millions of files, ~150 TB filesystem, possibly slow or busy disks (ZFS pool
+    under resilver). Holding one small record (path, size, mtime) per file in
+    memory during a scan is acceptable; holding every file's hashes is not (they
+    stay in the database). The reporting commands do not hold every file's
+    hashes either: `report` lets SQLite group and order the records and writes
+    each row as it reads it, so its memory does not grow with the database, and
+    `trees` reads the records in path order and keeps each directory's path,
+    digest and totals, plus the hashes of only the files in the directories
+    holding the record being read, so its memory grows with the number of
+    directories and with the size of the largest directory.
 3.  **Scan incrementally, analyze offline.** The expensive filesystem scan
     maintains a persistent database; an unchanged file is never read again on a
-    rescan. All analysis (`report`, `trees`) works from the database alone and
-    must never touch the scanned filesystem again. `scan` is designed to be
-    cronned; the reports run at any time against the last completed scan.
+    rescan, except to compute its content hash once a file of 10 MiB or more
+    comes to match another on size and both end windows. All analysis (`report`,
+    `trees`) works from the database alone and must never touch the scanned
+    filesystem again. `scan` is designed to be cronned; the reports run at any
+    time against the last completed scan.
 4.  **Clean stream separation.** Everything on stdout is machine-readable data.
-    All progress, warnings, and summaries go to stderr. Never mix them.
+    All progress, warnings, summaries, and help and usage text go to stderr.
+    Never mix them.
 
 ### Constraints
 
 - Language: Go (module `sneak.berlin/go/sfdupes`). Binary name: `sfdupes`.
 - Dependencies: standard library, `github.com/spf13/cobra` for the CLI, **one
-  progress-bar library** (`github.com/schollz/progressbar/v3`), and **one SQLite
-  driver** (`modernc.org/sqlite`, pure Go, so builds keep cgo disabled).
+  progress-bar library** (`github.com/schollz/progressbar/v3`),
+  `golang.org/x/term` to tell whether stderr is a terminal, **one SQLite
+  driver** (`modernc.org/sqlite`, pure Go, so builds keep cgo disabled), and
+  `golang.org/x/sys` for `flock(2)` (the scan lock, see "Database").
   `github.com/spf13/viper` is permitted if configuration-file support is ever
   needed, but is not currently used. No other third-party deps.
 - Cross-compilation is not a concern. Builds run with cgo disabled (the
@@ -107,8 +245,18 @@ Three subcommands, all implemented:
 sfdupes scan [--workers N] [-x] PATH...
 sfdupes report > dupes.tsv
 sfdupes trees > dupetrees.tsv
+sfdupes --version
+sfdupes [command] --help
 ```
 
+`--workers N` sets the size of each `scan` worker pool (default: the number of
+CPUs), and `-x` (`--one-file-system`) keeps the walk of each operand on that
+operand's filesystem; both are described under "`scan` mode".
+`sfdupes --version` (or `-v`) prints one line, `sfdupes VERSION`, to stdout and
+exits 0, writing nothing to stderr. `-h` or `--help`, alone or after a
+subcommand, prints the help text to stderr and exits 0, writing nothing to
+stdout.
+
 ### Database
 
 All three subcommands operate on a single SQLite database file:
@@ -119,33 +267,112 @@ All three subcommands operate on a single SQLite database file:
 - `scan` creates the database (and its parent directory) on first use. `report`
   and `trees` require an existing database; a missing database file is a fatal
   error (exit 1) telling the user to run `scan` first.
-- The database uses WAL journal mode and a busy timeout, so running a report
-  while a cron `scan` is in progress is safe. The filesystem is authoritative;
-  the database is an eventually-consistent reflection of it. Hashed records are
-  committed in batched transactions while the scan is still running (keeping the
-  WAL small and letting concurrent reports observe progress), so a report may
-  see a scan's changes partially applied, and a scan that dies partway leaves a
-  valid database holding everything hashed so far; the next scan skips those
-  records and converges toward the filesystem.
+- Only one `scan` runs against a database at a time. For its whole run, `scan`
+  holds an exclusive `flock(2)` lock on a lock file beside the database, named
+  by appending `.lock` to the database path (`/var/lib/sfdupes/db.sqlite.lock`
+  by default), taken before it walks the filesystem or opens the database. A
+  second `scan` against the same database does not wait: it fails at once with a
+  one-line error naming the lock file and exits 1, without walking anything or
+  opening the database, and the running scan carries on. The lock file is
+  created on first use, open to its owner only, and left in place: a leftover
+  file blocks nothing, because the lock ends with the process holding it however
+  it ends, a fatal error or an interrupt included, and deleting the file while a
+  scan runs would let a second scan start. `report` and `trees` never take the
+  lock, so they run during a scan.
+- While `scan` runs, the database is in WAL journal mode with a busy timeout, so
+  running a report while a cron `scan` is in progress is safe. The filesystem is
+  authoritative; the database is an eventually-consistent reflection of it.
+  Hashed records are committed in batched transactions while the scan is still
+  running (keeping the WAL small and letting concurrent reports observe
+  progress), so a report may see a scan's changes partially applied, and a scan
+  that dies partway leaves a valid database holding every batch committed so far
+  (an interrupted scan also commits the batch in progress, see "Error handling
+  and exit codes"); the next scan skips those records and converges toward the
+  filesystem.
+- `scan` switches the database back to rollback-journal mode when it closes it,
+  so between scans the database file alone holds the whole database. Each switch
+  needs the database to itself: a `scan` that starts while a report is still
+  reading waits for it up to the 10-second busy timeout, then fails; a `scan`
+  that ends while a report has the database open warns and leaves the database
+  in WAL mode until the next scan. `report` writes each row as it reads it, so
+  it is still reading while its output is paused (a pager, a stalled pipe), and
+  a `scan` started then fails after the busy timeout.
+- `report` and `trees` open the database read-only and need only read access to
+  the database file, and no write access to its directory. While the database is
+  in WAL mode they also read the `-wal` and `-shm` files beside it, which SQLite
+  creates with the database file's permissions.
 - Schema (`PRAGMA user_version` is the schema version, currently 1; a database
-  with any other version is a fatal error):
+  with any other version is a fatal error. `scan` creates the schema and sets
+  the version in one transaction, so a first scan stopped while doing so leaves
+  an empty database the next scan sets up. A database at version 0 that already
+  has a `files` table was therefore not made by sfdupes; every subcommand
+  refuses it with an error telling the user to remove the file and rescan):
 
     ```sql
     CREATE TABLE files (
         path    BLOB PRIMARY KEY,  -- absolute path, raw bytes
         size    INTEGER NOT NULL,  -- bytes, from lstat
         mtime   INTEGER NOT NULL,  -- Unix seconds, from lstat
-        head  TEXT NOT NULL,     -- lowercase-hex SHA-256, first 1 KiB
-        tail  TEXT NOT NULL      -- lowercase-hex SHA-256, last 1 KiB
+        head    TEXT NOT NULL,     -- lowercase-hex SHA-256; first 64 KiB, or whole file under 10 MiB
+        tail    TEXT NOT NULL,     -- lowercase-hex SHA-256; last 64 KiB, or whole file under 10 MiB
+        content TEXT NOT NULL      -- lowercase-hex SHA-256, whole file or samples
     ) WITHOUT ROWID;
+    CREATE INDEX files_signature ON files (size, head, tail, content);
     ```
 
     Paths are stored as BLOBs because Unix paths are raw bytes, not guaranteed
     UTF-8. `mtime` is used only for change detection; it is not part of the
-    duplicate key. `head` and `tail` are empty strings when the file has never
-    been hashed because its size was unique as of the last scan that covered it;
-    such records still define the file for tree reconstruction but never
-    participate in duplicate groups.
+    duplicate key. For a file under 10 MiB `head`, `tail`, and `content` all
+    hold the whole-file hash (that range is hashed in full, with no end
+    windows); for a larger file `head` and `tail` hold the first- and last-64
+    KiB hashes and `content` the whole-file or sampled hash. All three are empty
+    strings when the file has never been hashed because its size was unique as
+    of the last scan that covered it. For a file of 10 MiB or more, `content`
+    stays empty until the content phase of a scan (see "`scan` mode" below) has
+    read the file. A record with an empty `content` is never part of a duplicate
+    group, though it still defines the file for tree reconstruction. The
+    `files_signature` index lets SQLite group the records by signature for
+    `report` without sorting the whole table.
+
+### Duplicate detection
+
+Two files are duplicates only when they agree on every rung of this ladder; a
+mismatch at any rung means they are not duplicates. `scan` stores each file's
+hashes, and `report` and `trees` group files by the whole signature — size,
+`head`, `tail`, and `content` — so the grouping is exactly this ladder applied
+across everything scanned into the database, even across separate scans.
+
+1.  **Size.** Files of different sizes are never compared. Only files whose size
+    at least one other file shares are hashed at all.
+2.  **Under 10 MiB: whole file.** A file smaller than 10 MiB is hashed in full
+    and compared directly, with no separate end-window step — small files are
+    cheap to read to the last byte, and doing so makes the comparison exact.
+    `head`, `tail`, and `content` all hold this whole-file SHA-256, so such a
+    file's signature is decided entirely by its size and its content.
+3.  **10 MiB and above: head and tail.** For a larger file, the SHA-256 of the
+    first 64 KiB (`head`) and of the last 64 KiB (`tail`) are a cheap gate that
+    eliminates most same-size pairs before any bulk reading: the content hash of
+    the next two rungs is computed only for a file whose size, `head`, and
+    `tail` match another file's, whether that file is scanned in the same run or
+    stored by an earlier scan. A stored file that first gains such a match in a
+    later scan gets its content hash then; until it has one, its `content` is
+    empty and it is not a duplicate. At 10 MiB and above the two windows never
+    overlap.
+4.  **10 MiB and above, content below 50 MiB.** The SHA-256 of the entire file.
+    Agreement here is proof of identical content (barring a SHA-256 collision).
+5.  **10 MiB and above, content 50 MiB and above.** A sampled SHA-256: the 1 MiB
+    window at each gigabyte-aligned offset (0, 1 GiB, 2 GiB, … while inside the
+    file, the final window truncated at end of file) is fed, in order, into one
+    hash. This is **deliberately probabilistic** — the gaps between samples are
+    never read, so two large files that agree on every sample are reported as
+    duplicates without being read in full. It is the price of never reading a
+    150 GB file end to end. Because size is already part of the signature, only
+    equal-size files reach this rung, so their sample boundaries always align.
+
+`head`, `tail`, and `content` are one column each. A file below 10 MiB and one
+at or above it never share a size, and neither do a file below 50 MiB and one at
+or above it, so a stored value is never ambiguous between the whole-file,
+end-window, and sampled forms.
 
 ### `scan` mode
 
@@ -161,14 +388,25 @@ pool. Overlapping operands are harmless — an operand that duplicates another o
 lies under another is dropped before walking, so every file is reached exactly
 once and produces one database record.
 
+An operand that is a symlink (never followed, not even as an operand), socket,
+FIFO, or device node, or a directory named `.zfs`, is not scanned. `scan` prints
+a one-line warning naming the path and what it is, counts it as skipped, and
+drops it from the scanned operands before reading the database. Another operand
+beneath it is still scanned. The records stored beneath it are not deleted: they
+are treated like any other record outside the scanned operands, including the
+content-phase exception below. If it lies under another operand, they are under
+that operand instead, and are deleted like any other record there that this scan
+did not verify. This is not an error: a scan whose every operand is dropped
+walks nothing and exits 0.
+
 `scan` synchronizes the database with the filesystem state under the scanned
 operands:
 
 - Only a file whose size at least one other file shares is ever read: a
   size-unique file cannot be a duplicate, so it is recorded without hashes
-  (`head` and `tail` empty). The size census covers every file walked this scan
-  plus every database record outside the scanned operands, so a possible
-  duplicate of a separately scanned tree is still recognized.
+  (`head`, `tail`, and `content` empty). The size census covers every file
+  walked this scan plus every database record outside the scanned operands, so a
+  possible duplicate of a separately scanned tree is still recognized.
 - A file not yet in the database is inserted: hashed when its size is shared,
   without hashes otherwise.
 - A file already in the database is **skipped without reading its contents**
@@ -176,7 +414,9 @@ operands:
   than the recorded mtime. This is what makes a daily rescan cheap. Exception:
   an unchanged file whose record lacks hashes is hashed — and its record updated
   — once its size becomes shared, so hashing deferred by size-uniqueness happens
-  as soon as it could matter.
+  as soon as it could matter. Likewise, an unchanged file of 10 MiB or more
+  whose record has no `content` hash is read for one by the content phase below
+  once its size, `head`, and `tail` match another record's.
 - A file whose mtime is newer than recorded, or whose size differs, is processed
   as if new: re-hashed, or recorded without hashes, per the shared-size rule.
 - A database record whose path lies under one of the scanned operands but was
@@ -184,11 +424,16 @@ operands:
   deleted files. It also removes records for paths that failed to stat or hash
   this run: the database only ever contains signatures verified by the most
   recent scan that covered them (a subsequent successful scan re-adds such
-  files).
+  files). A failure in the content phase below removes nothing: the record is
+  left as it is.
 - Database records outside the scanned operands are untouched, so disjoint trees
-  can be scanned on different schedules into the same database.
+  can be scanned on different schedules into the same database. The one
+  exception is the content phase below: a stored file of 10 MiB or more without
+  a `content` hash is read for one, wherever it lies, once its size, `head`, and
+  `tail` match another record's. If that file is gone or has changed since its
+  record was written, the record is left as it is.
 
-`scan` runs **three sequential phases over the whole scan**. Parallelism lives
+`scan` runs **four sequential phases over the whole scan**. Parallelism lives
 inside each phase; batched database writes begin during the hash phase:
 
 1.  **walk + stat** — enumerate the trees under all `PATH` operands concurrently
@@ -204,10 +449,11 @@ inside each phase; batched database writes begin during the hash phase:
 2.  **hash** — with the census complete, each carried file's size decides its
     fate. Size-unique files are never read: new or changed ones are recorded
     without hashes in the update phase, unchanged unhashed ones simply keep
-    their records. Every file with a shared size is hashed by the worker pool:
-    read the first `min(1024, size)` bytes and the last `min(1024, size)` bytes
-    (one read when `size <= 1024`, since the two windows coincide) and compute
-    the SHA-256 of each. Zero-length files have constant hashes and are never
+    their records. Every file with a shared size is hashed by the worker pool as
+    described in "Duplicate detection" above: a file under 10 MiB in full, which
+    gives its `head`, `tail`, and `content` alike, and a larger file only in its
+    end windows, which give its `head` and `tail`; its content hash is left to
+    the content phase. Zero-length files have constant hashes and are never
     opened. Files are hashed in **inode order** (minimizing seeks on spinning
     disks), and paths that are hard links to the same inode are **read once**,
     all sharing the one result — a hard-link backup farm costs one read per
@@ -219,13 +465,36 @@ inside each phase; batched database writes begin during the hash phase:
 3.  **update** — commit the final partial batch, the hash-less records for
     size-unique new and changed files, and the deletions for records the scan
     did not verify (vanished files, plus paths that failed to stat or hash).
+4.  **content** — find every record of 10 MiB or more without a `content` hash
+    whose size, `head`, and `tail` equal another record's, anywhere in the
+    database: records from this scan and records stored by earlier scans, inside
+    or outside the scanned operands. SQLite finds them, so only the records to
+    be read are kept in memory, never every file's hashes. Every record sharing
+    their size, `head`, and `tail`, including one that already has a `content`
+    hash, has its file checked with `lstat` first. A file that is gone, is no
+    longer a regular file, or has changed (a different size, or an mtime newer
+    than recorded) keeps its record as it is and does not count as a match for
+    the others. Any other `lstat` error is warned about and counted as skipped,
+    with the same result. If such a record has no `content` hash, it stays out
+    of duplicate groups; if it has one, it is still reported until a scan
+    covering its own tree updates or removes it. The files that pass and have no
+    `content` hash are read only if at least two of those records pass, so a
+    file whose only matches are stale costs no read; a file that already has a
+    `content` hash is never read again. They are read by a worker pool as in the
+    hash phase, in inode order and once per inode, and their content hashes are
+    committed in batches. A failed read is warned about and counted as skipped;
+    its record keeps an empty `content`, so it is not a duplicate, and a later
+    scan tries again.
 
 Rules for the walk:
 
 - Only regular files. Skip directories, symlinks (do not follow, including
-  symlink operands), sockets, FIFOs, and device nodes.
+  symlink operands), sockets, FIFOs, and device nodes. An operand that is a
+  symlink, socket, FIFO, or device node is dropped as described in "`scan` mode"
+  above.
 - Never descend into a directory named `.zfs` (ZFS snapshot pseudo-dirs; walking
-  them would list every file once per snapshot).
+  them would list every file once per snapshot), not even when it is an operand;
+  such an operand is dropped the same way.
 - Filesystem boundaries are crossed by default. With `-x` (long form
   `--one-file-system`, following the GNU `du`/`rsync` convention), never descend
   into a directory on a different filesystem than its `PATH` operand; each
@@ -234,17 +503,20 @@ Rules for the walk:
   unreadable): print a one-line warning to stderr, skip the path, and continue.
   Per-file errors never abort the run; the final summary reports how many were
   skipped. As specified above, a skipped path that has a database record from an
-  earlier scan loses that record; an unreadable directory subtree likewise loses
-  its records (accepted: the database mirrors what the latest scan could
-  actually verify).
+  earlier scan loses that record, unless it failed only in the content phase, or
+  is an operand dropped before the database was read that lies under no other
+  operand; an unreadable directory subtree likewise loses its records (accepted:
+  the database mirrors what the latest scan could actually verify).
 
-Concurrency: the walk phase (which also stats files) and the hash phase each use
-a worker pool of `--workers` workers (default `runtime.NumCPU()`); the walk
-parallelizes across directories, hashing across files. Both phases are
-seek-bound on spinning disks, so raising `--workers` well past the core count
-can help on pools with many spindles. The main goroutine owns partitioning,
-database writes, and progress rendering; progress display must never block the
-workers.
+Concurrency: the walk phase (which also stats files), the hash phase, and the
+content phase each use a worker pool of `--workers` workers (default
+`runtime.NumCPU()`); the walk parallelizes across directories, hashing across
+files. `--workers` must be at least 1: a smaller value is a usage error,
+reported in one line on stderr with exit 2 before anything is scanned. All three
+phases are seek-bound on spinning disks, so raising `--workers` well past the
+core count can help on pools with many spindles. The main goroutine owns
+partitioning, database writes, and progress rendering; progress display must
+never block the workers.
 
 `scan` writes nothing to stdout. The summary line on stderr reports the files
 seen this run broken down by disposition, plus skips:
@@ -262,14 +534,17 @@ total.)
 
 **`report` must never touch the filesystem being analyzed.** It does not stat,
 open, or otherwise access any path that appears in the records; its only I/O is
-reading the database and writing stdout/stderr. It must produce identical output
+reading the database, writing stdout/stderr, and the temporary file SQLite sorts
+in when the duplicate rows do not fit in memory. SQLite puts that file in
+`$SQLITE_TMPDIR` or `$TMPDIR` when set, otherwise in `/var/tmp` (or `/tmp`), and
+deletes it as soon as it has opened it. `report` must produce identical output
 whether or not the scanned filesystem is still mounted.
 
 Processing:
 
-- Records without hashes (size-unique when last scanned) are excluded: their
+- Records without a `content` hash (see "Database" above) are excluded: their
   content is unknown, so they are never reported as duplicates.
-- Group the remaining records by the key `(size, head_hash, tail_hash)`.
+- Group the remaining records by the key `(size, head, tail, content)`.
 - Every group with two or more paths is a duplicate group.
 - Within each group, sort paths lexicographically (byte order). The first path
   is the group's `first`; every other path is a `dupe`.
@@ -288,6 +563,14 @@ first	dupe	size
 /srv/a/big.iso	/srv/c/big-copy2.iso	4294967296
 ```
 
+Paths are raw bytes and may hold any byte except NUL, so the path columns
+(`first` and `dupe`) are escaped to keep every row one line of tab-separated
+fields: a backslash is written as `\\`, a tab as `\t`, a newline as `\n`, and a
+carriage return as `\r`. Every other byte is written unchanged, including bytes
+that are not valid UTF-8. Undoing those four escapes gives back the stored path.
+Grouping and ordering use the stored path, not the escaped one. The warnings
+`scan` prints on stderr are escaped the same way, so each warning is one line.
+
 Summary to stderr: records read, number of duplicate groups, number of dupe
 files, and total reclaimable bytes (sum of `size` over all dupe rows) in human
 units.
@@ -304,10 +587,10 @@ records, split on `/`.
 
 Definitions:
 
-- A file's **signature** is `(size, head_hash, tail_hash)` — mtime is
-  informational and excluded. An unhashed record (empty hashes) has unknown
+- A file's **signature** is `(size, head, tail, content)` — mtime is
+  informational and excluded. A record without a `content` hash has unknown
   content: its signature is treated as unique to that file, so a tree containing
-  an unhashed file never compares equal to any other tree.
+  such a file never compares equal to any other tree.
 - A directory's **digest** is a SHA-256 Merkle digest computed bottom-up:
   serialize the directory's child entries — for a file child, its name and
   signature; for a subdirectory child, its name and that subdirectory's digest —
@@ -353,6 +636,9 @@ first	dupe	files	size
 /srv/a/project	/srv/backup/project	3417	104857600
 ```
 
+The `first` and `dupe` paths are escaped as described under "Report output
+format". The root directory's path is `/`.
+
 Summary to stderr: records read, number of duplicate-tree groups, number of dupe
 trees, and total reclaimable bytes (sum of `size` over all dupe rows) in human
 units.
@@ -365,9 +651,12 @@ Use the progress-bar library for all scan progress; rendering in the style of
 Each phase gets its own display, rendered the moment the phase starts — a scan
 must never look hung. Loading the existing-record index (`load`) and the walk
 have no known totals while running: show a live count, rate, and elapsed time
-(spinner-style, no percentage or ETA). The hash and update phases have exact
-totals — only files that actually need hashing appear in the hash total, so its
-ETA is meaningful. Required elements for the bars with known totals:
+(spinner-style, no percentage or ETA). The content phase's display (`content`)
+starts the same way, counting the records checked while SQLite finds the files
+to read and `lstat` checks them, then shows a bar once reading starts. The hash
+and update phases, and the content phase's reads, have exact totals — only files
+that actually need hashing appear in the hash and content totals, so their ETAs
+are meaningful. Required elements for the bars with known totals:
 
 - elapsed time
 - estimated time remaining
@@ -382,21 +671,57 @@ hash: [12345/98765] 12% |████      | 92 files/s elapsed 2:32 eta 17:54
 
 Additional requirements:
 
-- When stderr is not a TTY, do not emit ANSI redraws: print a plain one-line
-  progress update no more often than every 5 seconds instead.
+- When stderr is not a terminal (a pipe, a file, `/dev/null`), do not emit ANSI
+  redraws: print a plain one-line progress update the moment each phase starts,
+  then no more often than every 5 seconds.
 - Progress updates are driven from the main goroutine and must be non-blocking
-  with respect to the worker pool.
+  with respect to the worker pool. On a terminal the spinner-style displays also
+  redraw on their own several times a second, so their count and elapsed time
+  stay current while a phase waits for its next item.
+- A warning printed during a phase always lands on a line of its own, never
+  inside the progress display.
+- A bar whose phase stops short of its total, as an interrupted one does, is
+  left as last drawn rather than filled up.
 - `report` and `trees` modes need no progress display, only their stderr
   summaries.
 
 ### Error handling and exit codes
 
 - `0`: success, even if individual files were skipped with warnings.
-- `1`: fatal error (e.g., a `PATH` operand does not exist, the database cannot
-  be created/opened/read/written, a missing database for `report`/`trees`,
-  stdout write failure).
-- `2`: usage error (including `scan` with no `PATH` operand and `report`/`trees`
-  with any positional argument).
+- `1`: fatal error (e.g., a `PATH` operand does not exist, another `scan` is
+  already running against the same database, the database cannot be
+  created/opened/read/written, a missing database for `report`/`trees`, stdout
+  write failure), or a `scan` stopped by `SIGINT` or `SIGTERM` (see below).
+- `2`: usage error (including `scan` with no `PATH` operand, `scan` with
+  `--workers` below 1, and `report`/`trees` with any positional argument).
+
+A stdout write failure, such as a full disk, is reported in one line on stderr
+and exits 1. Two cases never reach sfdupes as a failed write:
+
+- When the reader of a stdout pipe exits early, as in `sfdupes report | head`,
+  the next write ends sfdupes with `SIGPIPE`, quietly and without a summary, the
+  way `cat` or `sort` end. The shell reports the signal (status 141 in most
+  shells), not exit 1.
+- When stdout is closed outright (`sfdupes report >&-`), the Go runtime opens
+  `/dev/null` in its place before sfdupes starts, so the output is discarded and
+  the run succeeds, as with `> /dev/null`.
+
+`scan` stops cleanly on `SIGINT` (Ctrl-C) or `SIGTERM`. Its workers stop taking
+work, each finishing at most the directory listing or file it is reading; the
+progress display is finished; and the records it has hashed but not yet
+committed are committed, so the next scan does not hash them again. Apart from
+that commit it starts no further writes or deletions: records are deleted only
+after a complete walk, so those under paths an interrupted walk never reached
+are kept. The database is closed and the lock released as on any other exit, the
+line `scan: interrupted after N files` goes to stderr, N being the number of
+files the walk reached, and the exit code is 1. The next scan skips the records
+already written and converges as usual.
+
+After the first signal `scan` stops catching them, so a second one ends it at
+once, as an uncaught signal does: the records not yet committed are lost, and
+the database is left valid, as when any scan dies (see "Database"). A `SIGINT`
+that `scan` inherits as ignored, as a script's background job does, stays
+ignored.
 
 ## Entrypoints
 
@@ -409,18 +734,14 @@ from any working directory, and may be invoked directly. The provided
 entrypoints are:
 
 - `script/bootstrap` — install everything needed to build and develop this
-  repository, idempotently, assuming nothing is present. `git`, `make`, `go`,
-  and `node` come from the first of nix, apt, brew, or apk found on the host,
-  and are presence-checked only; `node` is an unpinned host runtime like the
-  rest, because nvm's prebuilt node is glibc-linked and does not run on this
-  repo's musl/Alpine build image. The Markdown formatter itself — `prettier` —
-  is pinned by `yarn.lock`'s integrity hash and installed with
-  `yarn install --frozen-lockfile`. `golangci-lint` is deliberately **not**
-  installed: it runs from a digest-pinned image via `script/lint` and never from
-  a host install, so there is no host copy to drift from the pin. A missing
+  repository, idempotently, assuming nothing is present. `git`, `make`, and `go`
+  come from the first of nix, apt, brew, or apk found on the host, and are
+  presence-checked only. `golangci-lint` and prettier are deliberately **not**
+  installed: they run in Docker (see `script/lint` and `script/fmt`) and never
+  from a host install, so there is no host copy to drift from the pin. A missing
   `docker` is warned about rather than installed or treated as fatal —
-  everything except linting works without it. Ends with `go mod download` and
-  the `yarn` install.
+  everything except linting and formatting works without it. Ends with
+  `go mod download`.
 - `script/setup` — make a fresh clone ready for development: runs
   `script/bootstrap`, then `script/install-precommit`.
 - `script/projectname` — print this project's name (`sfdupes`). Scripts that
@@ -445,15 +766,22 @@ entrypoints are:
   entirely offline, until `go.mod` or `go.sum` changes and the download layer
   goes cold again. Because the daemon only ever sees a build context, this works
   when the docker daemon is remote and bind mounts are impossible.
-- `script/fmt` — format in place: `gofmt -s -w` for Go sources and `prettier`
-  for Markdown (`--tab-width 4 --prose-wrap always`, the house settings, also
-  carried in `.prettierrc`). prettier is the pinned devDependency in
-  `package.json`/`yarn.lock`, installed by `script/bootstrap`.
-- `script/fmt-check` — the read-only counterpart of `script/fmt`: runs both
-  checks, reports each independently so it is clear which failed, and exits
-  non-zero if either found unformatted files instead of writing.
+- `script/fmt` — format in place: the Go sources with `gofmt -s -w`, and every
+  Markdown file with prettier, at the settings in `.prettierrc` (4-space
+  indents, prose wrapped at 80 columns). prettier is pinned by hash through
+  `package.json` and `yarn.lock` and never installed on the host: this builds
+  the `Dockerfile`'s `prettier` stage, a digest-pinned node image into which
+  `yarn install --frozen-lockfile` installs it, tagged `sfdupes-prettier`, and
+  runs that with the repository mounted, as the calling user. Needs `docker`,
+  and because of the mount, unlike `script/lint`, a local docker daemon.
+- `script/fmt-check` — the read-only counterpart of `script/fmt`, with the
+  repository mounted read-only: prints any unformatted file and exits non-zero
+  instead of writing. gofmt and prettier both run every time, and each names
+  itself when it fails. The `Dockerfile` runs the same two checks as gates: the
+  gofmt check in its lint stage, prettier in its `markdown` stage.
 - `script/check` — run `script/test`, `script/lint`, and `script/fmt-check`, in
-  that order. Modifies nothing. Needs `docker`, because `script/lint` does.
+  that order. Modifies nothing. Needs `docker`, because `script/lint` and
+  `script/fmt-check` do.
 - `script/docker` — build the Docker image, tagged with the name from
   `script/projectname`. The `Dockerfile` runs the gates as build steps, so this
   is also the check a developer or reviewer runs by hand.
@@ -507,10 +835,11 @@ compile recipe:
   failure).
 - `make lint` — run `golangci-lint` with the repo config, in Docker (see
   `script/lint`); requires `docker`.
-- `make fmt` / `make fmt-check` — format Go and Markdown sources / verify both
-  without writing.
+- `make fmt` / `make fmt-check` — format the Go sources and the Markdown /
+  verify formatting without writing; requires `docker`, for prettier (see
+  `script/fmt`).
 - `make check` — `test`, `lint`, and `fmt-check`; modifies nothing. Requires
-  `docker`, via `lint`.
+  `docker`, via `lint` and `fmt-check`.
 - `make docker` — build the Docker image, which runs the gates as build stages.
 - `make hooks` — install the pre-commit hook.
 - `make clean` — remove the binary.
@@ -519,14 +848,14 @@ compile recipe:
 
 All of the following, run in this directory, must pass:
 
-1.  `make check` passes (tests, lint, `gofmt`).
+1.  `make check` passes (tests, lint, `gofmt`, prettier).
 2.  `make docker` succeeds.
 3.  Smoke test — create a throwaway tree in a temp dir (never test against real
     data):
 
     ```sh
     d=$(mktemp -d)
-    export SFDUPES_DATABASE="$d/db.sqlite"
+    export SFDUPES_DATABASE="$(mktemp -d)/db.sqlite"
     mkdir -p "$d/a" "$d/b"
     head -c 2000 /dev/urandom > "$d/a/one.bin"
     cp "$d/a/one.bin" "$d/b/copy.bin"
@@ -554,8 +883,9 @@ All of the following, run in this directory, must pass:
     ./sfdupes report
     ```
 
-    (The scan database lives inside `$d` here purely for test hygiene; scanning
-    `$d` therefore also records the SQLite file itself, which is harmless.)
+    (The database lives in a temp directory of its own: inside `$d`, the scan
+    would record it, and its empty lock file would join the `empty1`/`empty2`
+    group.)
 
     Expected from the first `report`: `one.bin`/`copy.bin`/`copy2.bin` form one
     group (two dupe rows, `first` is the lexicographically smallest path);
@@ -584,8 +914,10 @@ Tracked in [TODO.md](TODO.md).
 
 ## Non-goals
 
-- No full-content verification, no byte-for-byte compare, no deletion or linking
-  of duplicates. The reports are advisory; acting on them is the user's job.
+- No byte-for-byte compare, and no deletion or linking of duplicates. Files that
+  match are compared by a SHA-256 of the whole file below 50 MiB, and only by
+  samples at 50 MiB and over. The reports are advisory; acting on them is the
+  user's job.
 - No persistence beyond the SQLite database described above; no export/import
   formats.
 - No daemon or filesystem watcher; scheduling rescans is cron's job.

TODO.md

@@ -28,9 +28,102 @@
 
 # Completed Steps
 
-- restore Markdown formatting in `script/fmt`/`fmt-check` and reformat all
-  Markdown to the house prettier settings (2026-09-21, closes
+- `make fmt` and `make fmt-check` run prettier over all Markdown, in Docker, and
+  CI checks it; all Markdown reformatted (2026-10-04,
   https://git.eeqj.de/sneak/sfdupes/issues/19)
+
+- `scan` rejects `--workers` below 1 as a usage error instead of running
+  single-threaded (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/10)
+
+- a test fails when either walk cancellation check in `scan.go` is removed
+  (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/81)
+
+- test that `scan` refuses a database with another schema version (2026-10-04,
+  https://git.eeqj.de/sneak/sfdupes/issues/64)
+
+- correct four inaccurate comments in `cancel_test.go` and rename
+  `walkCancelInFlightDirs` to `walkCancelInFlightFiles` (2026-10-04,
+  https://git.eeqj.de/sneak/sfdupes/issues/33)
+
+- test the `-x` filesystem-boundary rules in `subdirJob` (2026-10-04,
+  https://git.eeqj.de/sneak/sfdupes/issues/17)
+
+- `scan` creates the schema in one transaction; a version-0 database with a
+  `files` table is refused with a clear schema-version error (2026-10-04,
+  https://git.eeqj.de/sneak/sfdupes/issues/11)
+
+- README documents install, Docker, a daily cron scan and how to read and check
+  the reports (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/54)
+
+- the `Dockerfile` build stage keeps the Go module cache out of `builder`'s home
+  and copies the sources with `--chown`, so no `chown -R` walks them
+  (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/43)
+
+- `--version` prints `sfdupes VERSION` to stdout; README documents it and
+  `--help` (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/15)
+
+- `scan` stops cleanly on `SIGINT` or `SIGTERM`: commits what it has hashed,
+  deletes nothing more, exits 1 (2026-10-04,
+  https://git.eeqj.de/sneak/sfdupes/issues/5)
+
+- `report` and `trees` stream the records instead of holding them all in memory;
+  the schema gains the `files_signature` index (2026-10-04,
+  https://git.eeqj.de/sneak/sfdupes/issues/14)
+
+- progress prints at once on a non-terminal, uses a real terminal test, and
+  prints warnings through a spinner instead of racing its redraw (2026-10-03,
+  https://git.eeqj.de/sneak/sfdupes/issues/13)
+
+- warn about and skip symlink, socket, FIFO, device and `.zfs` operands, keeping
+  the records beneath them (2026-10-03,
+  https://git.eeqj.de/sneak/sfdupes/issues/9)
+
+- `scan` holds a lock on a lock file beside the database for its whole run, so a
+  second `scan` fails at once with exit 1 (2026-10-03,
+  https://git.eeqj.de/sneak/sfdupes/issues/53)
+
+- test stdout write failures in `report` and `trees`; README states that
+  `| head` ends sfdupes by `SIGPIPE` and `>&-` writes to `/dev/null`
+  (2026-10-03, https://git.eeqj.de/sneak/sfdupes/issues/30)
+
+- `report` and `trees` open the database read-only, and `scan` leaves it out of
+  WAL mode, so reading needs only read access (2026-10-03, closes
+  https://git.eeqj.de/sneak/sfdupes/issues/8)
+
+- escape tabs, newlines, carriage returns and backslashes in report, trees and
+  warning paths; the root directory's path is `/` (2026-10-03,
+  https://git.eeqj.de/sneak/sfdupes/issues/7)
+
+- stamp the git tag or short commit in a plain `docker build .` instead of `dev`
+  (2026-10-02, branch `next`, closes
+  https://git.eeqj.de/sneak/sfdupes/issues/67): `.dockerignore` now sends
+  `.git`, without `.git/config`, and the `Dockerfile` build stage takes the
+  `VERSION` build argument when one is given, otherwise
+  `git describe --tags --always` of that `.git`. The build fails if the context
+  carries `.git` and the version still comes out empty, `dev` or `unknown`. The
+  CI checkout step fetches the full history (`fetch-depth: 0`) so CI sees the
+  tag and stamps the same value as `make build`.
+
+- replace the 1 KiB end-window sampling with the head/tail plus content-hash
+  ladder (2026-09-22, branch `next`, closes
+  https://git.eeqj.de/sneak/sfdupes/issues/61): a file under 10 MiB is hashed in
+  full and compared directly, with no end-window step — its `head`, `tail`, and
+  `content` all hold the whole-file hash. A file at 10 MiB or above gets only
+  the 64 KiB `head` and `tail` in the hash phase; a new content phase, after the
+  update phase, reads it for its `content` hash — the whole file below 50 MiB,
+  gigabyte-spaced 1 MiB samples at or above — only when its size, `head`, and
+  `tail` match another record's, from the same scan or stored by an earlier one,
+  so a stored file gains its content hash when it gains a match. A file that is
+  gone or has changed since its record was written is not read. The `content`
+  column is part of the version 1 schema. `report` and `trees` group by the
+  extended signature and leave out any record without a `content` hash, so the
+  ladder is applied across the whole database. README "Duplicate detection"
+  documents every rung including the probabilistic large-file path.
+
+- remove the dead `files.dat` references from `Makefile`, `.gitignore` and
+  `.dockerignore` (2026-09-21, branch `next`, closes
+  https://git.eeqj.de/sneak/sfdupes/issues/22)
+
 - fix the lint-image pin comments and `FROM` form in `Dockerfile` and
   `Dockerfile.lint` (2026-08-10, branch `next`, closes
   https://git.eeqj.de/sneak/sfdupes/issues/25): dropped the false

cancel_test.go

@@ -4,20 +4,35 @@ import (
 	"context"
 	"database/sql"
 	"errors"
+	"fmt"
 	"os"
+	"os/signal"
 	"path/filepath"
+	"slices"
 	"strconv"
+	"strings"
 	"sync"
 	"sync/atomic"
+	"syscall"
 	"testing"
 	"time"
 )
 
-// poolUnwind bounds how long a goroutine is given to leave a pool
-// after its context is cancelled. Only a failing run ever waits this
-// long: a pool that ignored its cancellation parks forever, and this
-// is what turns that into a failed assertion instead of a suite that
-// hangs until the test binary's own timeout.
+// This file gathers the tests for scan cancellation and worker-pool
+// unwinding. Everything it exercises lives in scan.go, so by the repo's
+// convention of one test file per source file it would belong in
+// scan_test.go. It is kept separate on purpose: cancellation behaviour
+// cuts across both the walk pool and the hash pool as a single concern,
+// and scan_test.go is already over 1,600 lines. That is the deliberate
+// exception the convention otherwise expects to be stated.
+
+// poolUnwind bounds how long a test waits for a cancellation to take
+// effect: for a goroutine to return or a channel to close once its
+// context is cancelled, or for a signal to cancel the scan's context.
+// Only a failing run waits this long, and the bound is what makes that
+// failure an assertion instead of a hang. A call made without it, as
+// most of this file's scans are, has no bound: a regression that parks
+// it is caught only as the test binary's own timeout.
 const poolUnwind = 2 * time.Second
 
 // walkClock is a context whose cancellation is driven by the scan's
@@ -28,9 +43,14 @@ const poolUnwind = 2 * time.Second
 //
 // The accounting behind the n chosen by each test: every blocking
 // channel operation in the walk selects on Done, so the walk spends
-// one consultation per file event plus a couple per directory, while
-// the index load that runs ahead of it spends a small fixed number
-// (three) whatever the record count.
+// one consultation per file event plus a couple per directory. The
+// index load that runs ahead of it also consults Done, but a bounded
+// number of times that does not grow with the record count. The tests
+// depend on that property, not on the bound's exact value: each test
+// sets n from the consultations of the walk, plus those of the hash
+// phase when it cancels mid-hash, far from both ends of the phase it
+// interrupts, so the cancellation lands inside that phase whatever the
+// record count.
 type walkClock struct {
 	n    int64
 	seen atomic.Int64
@@ -90,7 +110,10 @@ const (
 	walkCancelFilesPerDir = 20
 	walkCancelFiles       = walkCancelDirs * walkCancelFilesPerDir
 	walkCancelWorkers     = 4
-	walkCancelInFlightDirs = walkCancelWorkers * walkCancelFilesPerDir
+	// The most files the walkCancelWorkers directories already in
+	// flight when the scan is cancelled can still emit, at
+	// walkCancelFilesPerDir each. A file count, not a directory count.
+	walkCancelInFlightFiles = walkCancelWorkers * walkCancelFilesPerDir
 )
 
 // walkCancelAtDone is the consultation on which the fixture's context
@@ -150,9 +173,13 @@ func assertRecordsIntact(t *testing.T, db *sql.DB, before []string) {
 // Every one of those records would look vanished to the update phase.
 // The guard is what stops the scan there, and this test is what
 // notices if it stops doing so: deleting the guard, or making it
-// unreachable, makes the scan carry its truncated view into a later
-// phase and fail there instead, with a wrapped error rather than the
-// bare cancellation.
+// unreachable, makes the scan carry its truncated view into the update
+// phase, which counts every record the walk never reached for removal.
+//
+// The syncScan call here is not bounded by poolUnwind: a regression
+// that left a worker pool parked would hang it, and that regression is
+// caught only by the test binary's own timeout, not by a quick
+// assertion.
 //
 //nolint:paralleltest // counts goroutines: must not run beside others
 func TestSyncScanCancelledMidWalkKeepsRecords(t *testing.T) {
@@ -182,10 +209,10 @@ func TestSyncScanCancelledMidWalkKeepsRecords(t *testing.T) {
 
 // assertWalkGuardAborted checks that the scan stopped at the post-walk
 // guard: with a census that is neither empty (the walk really ran)
-// nor complete (it really was cut short), and with the guard's own
-// bare cancellation as the error. A wrapped error means the partial
-// census was carried past the guard into the hash or update phase,
-// which is the failure this test exists to catch.
+// nor complete (it really was cut short), and with no record counted
+// for removal. A removal count means the partial census was carried
+// past the guard into the update phase, which is the failure this test
+// exists to catch.
 func assertWalkGuardAborted(t *testing.T, st scanStats, err error) {
 	t.Helper()
 
@@ -194,12 +221,6 @@ func assertWalkGuardAborted(t *testing.T, st scanStats, err error) {
 			err, context.Canceled)
 	}
 
-	if errors.Unwrap(err) != nil {
-		t.Errorf("syncScan reported %q, want the guard's bare "+
-			"cancellation: a wrapped error means the truncated census "+
-			"reached a later phase", err)
-	}
-
 	if st.unchanged == 0 {
 		t.Fatalf("stats = %+v: the census is empty, so the walk never "+
 			"ran and the guard was reached for the wrong reason", st)
@@ -211,10 +232,11 @@ func assertWalkGuardAborted(t *testing.T, st scanStats, err error) {
 	}
 
 	// The workers drop every directory still queued once the scan is
-	// cancelled, so only the directories already in flight can add to
-	// the census after the fact. A census beyond that bound would mean
-	// the cancellation was not observed where it should have been.
-	limit := walkCancelAtDone + walkCancelInFlightDirs
+	// cancelled, so only the files in the directories already in flight
+	// can add to the census after the fact. A census beyond that bound
+	// would mean the cancellation was not observed where it should have
+	// been.
+	limit := walkCancelAtDone + walkCancelInFlightFiles
 	if st.unchanged > limit {
 		t.Errorf("census covers %d files, want at most %d: the walk kept "+
 			"taking directories off the queue after cancellation",
@@ -260,6 +282,242 @@ func TestSyncScanCancelledBeforeLoadIndex(t *testing.T) {
 	assertRecordsIntact(t, db, before)
 }
 
+// hashCancelAtDone is the consultation on which the mid-hash test's
+// context cancels itself. The walk of buildWalkCancelTree spends about
+// one per file and three per directory, and the hash phase then one per
+// file hashed, so this lands about half way through the hash phase.
+const hashCancelAtDone = walkCancelFiles + 3*walkCancelDirs +
+	walkCancelFiles/2
+
+// TestSyncScanCancelledMidHashKeepsHashedRecords cancels a first scan
+// part-way through its hash phase. The fixture holds fewer files than a
+// batch, so every file hashed is still waiting to be committed: the scan
+// must commit them all before it returns, and the next scan must hash
+// only the rest.
+func TestSyncScanCancelledMidHashKeepsHashedRecords(t *testing.T) {
+	t.Parallel()
+
+	dir := buildWalkCancelTree(t)
+	db := openTestDB(t)
+
+	st, err := syncScan(newWalkClock(hashCancelAtDone), db,
+		[]string{dir}, walkCancelWorkers, false)
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("syncScan cancelled mid-hash = %v, want %v",
+			err, context.Canceled)
+	}
+
+	if st.walked != walkCancelFiles || st.added == 0 ||
+		st.added >= walkCancelFiles {
+		t.Fatalf("stats = %+v: want the walk complete and the hash phase "+
+			"cut short", st)
+	}
+
+	if got := len(dbRecords(t, db)); got != st.added {
+		t.Errorf("%d records after the cancelled scan, want the %d it hashed",
+			got, st.added)
+	}
+
+	hashed := st.added
+
+	st = syncTree(t, db, dir)
+	if st.added != walkCancelFiles-hashed || st.unchanged != hashed {
+		t.Errorf("next scan stats = %+v, want %d added %d unchanged",
+			st, walkCancelFiles-hashed, hashed)
+	}
+}
+
+// storedPaths opens the database at path as report does, which fails
+// unless it is a valid database, and returns its records' paths.
+func storedPaths(t *testing.T, path string) []string {
+	t.Helper()
+
+	db, err := openReportDatabase(t.Context(), path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer func() { _ = db.Close() }()
+
+	return recordPaths(dbRecords(t, db))
+}
+
+// TestRunScanInterrupted calls the scan entrypoint with a context that
+// is already cancelled, as when a signal arrives at once. It must return
+// errInterrupted promptly with its one line on stderr, leave the
+// database valid and as it was, and leave nothing in the way of the
+// next scan, which must bring the database up to date.
+func TestRunScanInterrupted(t *testing.T) {
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	stderr := captureStderr(t)
+	dir := buildSmokeTree(t)
+
+	err := runScan(t.Context(), []string{dir}, walkCancelWorkers, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	before := storedPaths(t, path)
+
+	// A vanished file and a new one: the interrupted scan records
+	// neither.
+	gone := filepath.Join(dir, "a", "unique.bin")
+
+	err = os.Remove(gone)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	added := writeFile(t, dir, "a/new.bin", pattern(50, 10))
+	shown := len(stderr())
+	done := make(chan struct{})
+
+	go func() {
+		defer close(done)
+
+		err = runScan(cancelledContext(t), []string{dir}, walkCancelWorkers,
+			false)
+	}()
+
+	awaitReturn(t, done, "runScan")
+
+	if !errors.Is(err, errInterrupted) {
+		t.Fatalf("runScan on a cancelled context = %v, want %v",
+			err, errInterrupted)
+	}
+
+	want := "scan: interrupted after 0 files\n"
+	if got := stderr()[shown:]; got != want {
+		t.Errorf("stderr = %q, want %q", got, want)
+	}
+
+	assertNoSidecars(t, path)
+
+	if got := storedPaths(t, path); !slices.Equal(got, before) {
+		t.Errorf("records = %q after the interrupted scan, want %q",
+			got, before)
+	}
+
+	err = runScan(t.Context(), []string{dir}, walkCancelWorkers, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	got := storedPaths(t, path)
+	if slices.Contains(got, gone) || !slices.Contains(got, added) {
+		t.Errorf("records = %q after the next scan, want %q gone and %q "+
+			"added", got, gone, added)
+	}
+}
+
+// TestRunScanInterruptedMidHash interrupts the scan entrypoint part-way
+// through its hash phase, after the database is open. It must return
+// errInterrupted, release the lock, end stderr with its line counting
+// every file the walk reached, close the database out of WAL mode, and
+// keep the records it hashed.
+func TestRunScanInterruptedMidHash(t *testing.T) {
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	stderr := captureStderr(t)
+	dir := buildWalkCancelTree(t)
+
+	err := runScan(newWalkClock(hashCancelAtDone), []string{dir},
+		walkCancelWorkers, false)
+	if !errors.Is(err, errInterrupted) {
+		t.Fatalf("runScan interrupted mid-hash = %v, want %v",
+			err, errInterrupted)
+	}
+
+	holdScanLock(t, path)
+
+	want := fmt.Sprintf("scan: interrupted after %d files\n", walkCancelFiles)
+	if got := stderr(); !strings.HasSuffix(got, want) {
+		t.Errorf("stderr = %q, want it to end with %q", got, want)
+	}
+
+	assertNoSidecars(t, path)
+
+	db, err := openReportDatabase(t.Context(), path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer func() { _ = db.Close() }()
+
+	// A plain close also removes the sidecars, but leaves WAL mode on.
+	var mode string
+
+	err = db.QueryRowContext(t.Context(), "PRAGMA journal_mode").Scan(&mode)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if mode != "delete" {
+		t.Errorf("journal mode = %q after the interrupted scan, want %q",
+			mode, "delete")
+	}
+
+	kept := len(dbRecords(t, db))
+	if kept == 0 || kept >= walkCancelFiles {
+		t.Errorf("%d records after the interrupted scan, want those it "+
+			"hashed: some but not all of the %d files", kept, walkCancelFiles)
+	}
+}
+
+// TestInterruptContextCatchesSIGTERM sends SIGTERM to the test process
+// while the scan's handler is installed, and checks that it cancels the
+// scan's context.
+//
+//nolint:paralleltest // signals the whole process: must not run beside a scan
+func TestInterruptContextCatchesSIGTERM(t *testing.T) {
+	// Caught here as well, so that a handler that misses SIGTERM fails
+	// this test instead of ending the test process.
+	caught := make(chan os.Signal, 1)
+	signal.Notify(caught, syscall.SIGTERM)
+
+	defer signal.Stop(caught)
+
+	ctx, stop := interruptContext(t.Context())
+	defer stop()
+
+	err := syscall.Kill(os.Getpid(), syscall.SIGTERM)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	select {
+	case <-ctx.Done():
+	case <-time.After(poolUnwind):
+		t.Fatal("SIGTERM did not cancel the scan's context")
+	}
+}
+
+// TestCommitFullBatchKeepsFailedBatch checks that a full batch whose
+// commit fails, as it does once the scan is interrupted, stays in the
+// batch, so that syncScan's final commit saves it.
+func TestCommitFullBatchKeepsFailedBatch(t *testing.T) {
+	t.Parallel()
+
+	s := &scanState{db: openTestDB(t)}
+	for i := range updateBatchSize {
+		s.batch = append(s.batch, scanRec{path: "/f" + strconv.Itoa(i)})
+	}
+
+	err := s.commitFullBatch(cancelledContext(t))
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("commitFullBatch on a cancelled context = %v, want %v",
+			err, context.Canceled)
+	}
+
+	if len(s.batch) != updateBatchSize {
+		t.Errorf("batch holds %d records after the failed commit, want %d",
+			len(s.batch), updateBatchSize)
+	}
+}
+
 // drainClosed counts the values received from ch until it closes,
 // failing the test if it does not close within poolUnwind. A pool that
 // ignored its cancellation leaves its channel open with its goroutines
@@ -328,20 +586,49 @@ func TestSendEventAbandonsBlockedSend(t *testing.T) {
 	awaitReturn(t, done, "sendEvent")
 }
 
-// TestWalkWorkersDropQueuedDirs checks that cancelled walk workers keep
-// reading jobs and drop the directories rather than stopping their
-// read: the range over jobs has to run out for the pool to tear down
-// and close its event stream.
-func TestWalkWorkersDropQueuedDirs(t *testing.T) {
+// TestWalkOneDirStopsWhenCancelled checks that a cancelled scan stops
+// reading a directory instead of going through the rest of its
+// entries. A walk that kept going would return the subdirectory below
+// to descend into. Unlike a file event, that return is not a send the
+// cancellation can abandon, so the test catches the regression every
+// time.
+func TestWalkOneDirStopsWhenCancelled(t *testing.T) {
 	t.Parallel()
 
 	dir := t.TempDir()
-	writeEmptyFiles(t, dir, walkCancelFilesPerDir)
+
+	err := os.Mkdir(filepath.Join(dir, "sub"), 0o750)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Unbuffered and unread: on a cancelled scan every send gives up.
+	events := make(chan walkEvent)
+
+	subs := walkOneDir(cancelledContext(t), dirJob{path: dir}, false, events)
+	if len(subs) != 0 {
+		t.Errorf("cancelled walkOneDir returned %+v to descend into, "+
+			"want none", subs)
+	}
+}
+
+// TestWalkWorkersDropQueuedDirs checks that cancelled walk workers keep
+// reading jobs and drop the directories rather than stopping their
+// read: the range over jobs has to run out for the pool to tear down
+// and close its event stream. The queued directory does not exist, so
+// a worker that walked it anyway would send a warning before
+// walkOneDir's own cancellation check could stop it. On a cancelled
+// scan that send delivers or gives up at random, so with 64 jobs
+// queued the regression has a one in 2^64 chance of passing.
+func TestWalkWorkersDropQueuedDirs(t *testing.T) {
+	t.Parallel()
+
+	missing := filepath.Join(t.TempDir(), "missing")
 
 	jobs, _, events := startWalkWorkers(cancelledContext(t), 2, false)
 
-	for range 4 {
-		jobs <- dirJob{path: dir}
+	for range 64 {
+		jobs <- dirJob{path: missing}
 	}
 
 	close(jobs)
@@ -412,7 +699,11 @@ func TestDispatchDirsClosesJobsWhenCancelled(t *testing.T) {
 
 // TestFeedHashJobsClosesJobsWhenCancelled checks that the hash feeder
 // abandons the runs it has not queued yet and still closes the job
-// channel, which is what lets the workers' range terminate.
+// channel, which is what lets the workers' range terminate. The
+// receive on jobs below is not bounded: a feeder that returned without
+// closing jobs would leave that receive with no sender and no close, so
+// this regression is caught by the test binary's timeout rather than by
+// a bounded assertion.
 func TestFeedHashJobsClosesJobsWhenCancelled(t *testing.T) {
 	t.Parallel()
 
@@ -440,6 +731,16 @@ func TestFeedHashJobsClosesJobsWhenCancelled(t *testing.T) {
 // nobody wants the hashes of — while still letting the range run out
 // so the pool tears down. The queued run names a file that does not
 // exist, so a worker that hashed it anyway would produce a result.
+//
+// hashWorker's other cancellation exit, abandoning the send of a
+// result, is reachable from the scan: stop cancels the pool before it
+// drains results, so a worker waiting on that send can leave through
+// it. The tests that stop a scan mid-hash, among them
+// TestScanHashWriteFailureUnwindsPool, reach it in some runs only,
+// depending on timing, and no test fails without it, since stop's
+// drain frees a waiting worker anyway. This test, for its part, catches
+// a removed drop check in some runs only: a worker that hashes the run
+// anyway then picks at random between sending the result and leaving.
 func TestHashWorkerDropsQueuedRuns(t *testing.T) {
 	t.Parallel()
 
@@ -456,7 +757,7 @@ func TestHashWorkerDropsQueuedRuns(t *testing.T) {
 	go func() {
 		defer close(done)
 
-		hashWorker(cancelledContext(t), jobs, results)
+		hashWorker(cancelledContext(t), jobs, results, hashSignature)
 	}()
 
 	awaitReturn(t, done, "hashWorker")
@@ -472,7 +773,10 @@ func TestHashWorkerDropsQueuedRuns(t *testing.T) {
 // TestHashPhaseCancelledReturnsContextError checks the result loop's
 // own exit: with the pool cancelled, no result will ever arrive, and
 // the loop must leave through the cancellation rather than wait for a
-// receive that cannot happen.
+// receive that cannot happen. This call is not bounded by poolUnwind: a
+// loop that dropped its cancellation case would block on that receive,
+// so the regression surfaces as the test binary's timeout rather than
+// as a bounded assertion.
 func TestHashPhaseCancelledReturnsContextError(t *testing.T) {
 	t.Parallel()
 

db.go

@@ -11,6 +11,7 @@ import (
 	"slices"
 	"strconv"
 
+	"golang.org/x/sys/unix"
 	// The pure-Go SQLite driver, registered as "sqlite"; keeps cgo
 	// disabled.
 	_ "modernc.org/sqlite"
@@ -32,6 +33,11 @@ const schemaVersion = 1
 // scan.
 const dbDirPerm = 0o755
 
+// lockFilePerm is the mode for the scan lock file. Anyone who can open
+// the file can hold the lock and keep every scan from running, so it
+// is open to its owner only.
+const lockFilePerm = 0o600
+
 // createTableSQL is the schema applied to a fresh database. Paths are
 // BLOBs because Unix paths are raw bytes, not guaranteed UTF-8.
 const createTableSQL = `
@@ -40,18 +46,26 @@ CREATE TABLE files (
     size    INTEGER NOT NULL,
     mtime   INTEGER NOT NULL,
     head    TEXT NOT NULL,
-    tail  TEXT NOT NULL
+    tail    TEXT NOT NULL,
+    content TEXT NOT NULL
 ) WITHOUT ROWID
 `
 
+// createIndexSQL indexes the records by signature, so report can have
+// SQLite group them without sorting the whole table.
+const createIndexSQL = `
+CREATE INDEX files_signature ON files (size, head, tail, content)
+`
+
 // upsertSQL inserts one file record, replacing any existing record for
 // the same path.
 const upsertSQL = `
-INSERT INTO files (path, size, mtime, head, tail)
-VALUES (?, ?, ?, ?, ?)
+INSERT INTO files (path, size, mtime, head, tail, content)
+VALUES (?, ?, ?, ?, ?, ?)
 ON CONFLICT (path) DO UPDATE SET
     size = excluded.size, mtime = excluded.mtime,
-    head = excluded.head, tail = excluded.tail
+    head = excluded.head, tail = excluded.tail,
+    content = excluded.content
 `
 
 // errNoDatabase reports a missing database file for report/trees.
@@ -62,6 +76,10 @@ var errNoDatabase = errors.New(
 // does not understand.
 var errSchemaVersion = errors.New("unsupported database schema version")
 
+// errScanRunning reports that another scan holds the lock on the
+// database.
+var errScanRunning = errors.New("another scan is running")
+
 // databasePath resolves the database location: SFDUPES_DATABASE when
 // set and non-empty, the compiled-in default otherwise.
 func databasePath() string {
@@ -72,16 +90,24 @@ func databasePath() string {
 	return defaultDatabasePath
 }
 
-// openDB opens the SQLite database at path with WAL journaling and a
-// busy timeout, so a report can run while a cron scan is in progress.
-// It does not create or verify the schema.
-func openDB(path string) (*sql.DB, error) {
-	dsn := "file:" + path +
-		"?_pragma=busy_timeout(10000)" +
+// scanParams are the connection parameters for scan: read-write, with
+// WAL journaling and a busy timeout, so a report can run while a cron
+// scan is in progress. closeScanDatabase leaves WAL mode again.
+const scanParams = "_pragma=busy_timeout(10000)" +
 	"&_pragma=journal_mode(WAL)" +
 	"&_pragma=synchronous(NORMAL)"
 
-	db, err := sql.Open("sqlite", dsn)
+// reportParams are the connection parameters for report and trees:
+// read-only, with the same busy timeout. They set no journal mode,
+// because setting one is a write.
+const reportParams = "mode=ro" +
+	"&_pragma=busy_timeout(10000)" +
+	"&_pragma=query_only(1)"
+
+// openDB opens the SQLite database at path with the connection
+// parameters params. It does not create or verify the schema.
+func openDB(path, params string) (*sql.DB, error) {
+	db, err := sql.Open("sqlite", "file:"+path+"?"+params)
 	if err != nil {
 		return nil, fmt.Errorf("open database %s: %w", path, err)
 	}
@@ -94,6 +120,43 @@ func openDB(path string) (*sql.DB, error) {
 	return db, nil
 }
 
+// lockScanDatabase takes the lock that keeps a second scan off the
+// database at path: an exclusive flock(2) on the file beside it named
+// path with ".lock" appended, created along with the database's parent
+// directory if missing. A lock held by another scan fails at once
+// instead of waiting. The lock lasts until the returned file is closed
+// or the process ends. The file is never deleted: a scan that deleted
+// it would let the next scan lock a new file while another still holds
+// the old one.
+func lockScanDatabase(path string) (*os.File, error) {
+	err := os.MkdirAll(filepath.Dir(path), dbDirPerm)
+	if err != nil {
+		return nil, fmt.Errorf("create database directory: %w", err)
+	}
+
+	lockPath := path + ".lock"
+
+	//nolint:gosec // the operator chooses the database path
+	f, err := os.OpenFile(lockPath, os.O_RDWR|os.O_CREATE, lockFilePerm)
+	if err != nil {
+		return nil, err
+	}
+
+	err = unix.Flock(int(f.Fd()), unix.LOCK_EX|unix.LOCK_NB)
+	if err != nil {
+		_ = f.Close()
+
+		if errors.Is(err, unix.EWOULDBLOCK) {
+			return nil, fmt.Errorf("%w (lock held on %s)",
+				errScanRunning, lockPath)
+		}
+
+		return nil, fmt.Errorf("lock %s: %w", lockPath, err)
+	}
+
+	return f, nil
+}
+
 // openScanDatabase opens the database for the scan subcommand, creating
 // the file, its parent directory, and the schema as needed.
 func openScanDatabase(ctx context.Context, path string) (*sql.DB, error) {
@@ -102,7 +165,7 @@ func openScanDatabase(ctx context.Context, path string) (*sql.DB, error) {
 		return nil, fmt.Errorf("create database directory: %w", err)
 	}
 
-	db, err := openDB(path)
+	db, err := openDB(path, scanParams)
 	if err != nil {
 		return nil, err
 	}
@@ -117,6 +180,24 @@ func openScanDatabase(ctx context.Context, path string) (*sql.DB, error) {
 	return db, nil
 }
 
+// closeScanDatabase switches the database at path from WAL back to
+// rollback-journal mode and closes it. Out of WAL mode the database
+// file alone holds the whole database, so a reader needs no -wal or
+// -shm file beside it, nor write access to create them. The switch
+// fails while a report has the database open; the database then stays
+// in WAL mode, still readable, until a later scan closes it.
+func closeScanDatabase(ctx context.Context, db *sql.DB, path string) {
+	// Runs on the way out of a cancelled scan too.
+	_, err := db.ExecContext(context.WithoutCancel(ctx),
+		"PRAGMA journal_mode = DELETE")
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "scan: database %s left in WAL mode: %v\n",
+			path, err)
+	}
+
+	_ = db.Close()
+}
+
 // openReportDatabase opens an existing database for the report and
 // trees subcommands. A missing database file is an error directing the
 // user to run scan first; the schema version must match exactly.
@@ -132,12 +213,18 @@ func openReportDatabase(ctx context.Context,
 		return nil, fmt.Errorf("database: %w", err)
 	}
 
-	db, err := openDB(path)
+	db, err := openDB(path, reportParams)
 	if err != nil {
 		return nil, err
 	}
 
 	v, err := userVersion(ctx, db)
+	if err == nil && v == 0 {
+		// An empty database passes this check and fails the version
+		// check below.
+		err = checkUnversioned(ctx, db)
+	}
+
 	if err != nil {
 		_ = db.Close()
 
@@ -164,6 +251,11 @@ func initSchema(ctx context.Context, db *sql.DB) error {
 
 	switch v {
 	case 0:
+		err = checkUnversioned(ctx, db)
+		if err != nil {
+			return err
+		}
+
 		return createSchema(ctx, db)
 	case schemaVersion:
 		return nil
@@ -173,20 +265,65 @@ func initSchema(ctx context.Context, db *sql.DB) error {
 	}
 }
 
+// checkUnversioned checks a database at user_version 0 before it is
+// taken for an empty one. createSchema creates the files table and
+// sets the version together, so a files table at version 0 was made by
+// something else. Adopting it could corrupt unrelated data, so that is
+// a schema-version error telling the operator to remove the file and
+// rescan.
+func checkUnversioned(ctx context.Context, db *sql.DB) error {
+	var name string
+
+	err := db.QueryRowContext(ctx,
+		"SELECT name FROM sqlite_master "+
+			"WHERE type = 'table' AND name = 'files'").Scan(&name)
+
+	switch {
+	case err == nil:
+		return fmt.Errorf(
+			"has a files table but no schema version; "+
+				"remove the file and rescan: %w", errSchemaVersion)
+	case errors.Is(err, sql.ErrNoRows):
+		return nil
+	default:
+		return fmt.Errorf("check for files table: %w", err)
+	}
+}
+
 // createSchema applies the schema to a fresh database and stamps the
-// schema version.
+// schema version in one transaction, so a creation stopped partway, by
+// an interrupt or an error, leaves an empty database the next scan
+// sets up, never a files table at version 0, which checkUnversioned
+// refuses.
 func createSchema(ctx context.Context, db *sql.DB) error {
-	_, err := db.ExecContext(ctx, createTableSQL)
+	tx, err := db.BeginTx(ctx, nil)
 	if err != nil {
 		return fmt.Errorf("create schema: %w", err)
 	}
 
-	_, err = db.ExecContext(ctx,
+	defer func() { _ = tx.Rollback() }()
+
+	_, err = tx.ExecContext(ctx, createTableSQL)
+	if err != nil {
+		return fmt.Errorf("create schema: %w", err)
+	}
+
+	_, err = tx.ExecContext(ctx, createIndexSQL)
+	if err != nil {
+		return fmt.Errorf("create schema: %w", err)
+	}
+
+	_, err = tx.ExecContext(ctx,
 		"PRAGMA user_version = "+strconv.Itoa(schemaVersion))
 	if err != nil {
 		return fmt.Errorf("set schema version: %w", err)
 	}
 
+	err = tx.Commit()
+	if err != nil {
+		return fmt.Errorf("create schema: %w", err)
+	}
+
 	return nil
 }
 
@@ -202,39 +339,113 @@ func userVersion(ctx context.Context, db *sql.DB) (int, error) {
 	return v, nil
 }
 
-// loadFileRows reads every record from the files table.
-func loadFileRows(ctx context.Context, db *sql.DB) ([]scanRec, error) {
+// loadFileRows streams every record to fn in path order: byte order,
+// which is the order of the primary key, so SQLite does not sort.
+func loadFileRows(ctx context.Context, db *sql.DB, fn func(r scanRec)) error {
 	rows, err := db.QueryContext(ctx,
-		"SELECT path, size, mtime, head, tail FROM files")
+		"SELECT path, size, mtime, head, tail, content FROM files "+
+			"ORDER BY path")
 	if err != nil {
-		return nil, fmt.Errorf("read records: %w", err)
+		return fmt.Errorf("read records: %w", err)
 	}
 
 	defer func() { _ = rows.Close() }()
 
-	var recs []scanRec
-
 	for rows.Next() {
 		var (
 			path []byte
 			r    scanRec
 		)
 
-		err = rows.Scan(&path, &r.size, &r.mtime, &r.head, &r.tail)
+		err = rows.Scan(&path, &r.size, &r.mtime, &r.head, &r.tail,
+			&r.content)
 		if err != nil {
-			return nil, fmt.Errorf("read record: %w", err)
+			return fmt.Errorf("read record: %w", err)
 		}
 
 		r.path = string(path)
-		recs = append(recs, r)
+		fn(r)
 	}
 
 	err = rows.Err()
 	if err != nil {
-		return nil, fmt.Errorf("read records: %w", err)
+		return fmt.Errorf("read records: %w", err)
 	}
 
-	return recs, nil
+	return nil
+}
+
+// dupeRowsSQL selects every record in a duplicate group, with the
+// group's first path. A group is the records with a content hash that
+// share a size, head, tail, and content, when there are two or more of
+// them. The rows come in report order: groups by size descending, then
+// by first path, and each group's paths ascending.
+const dupeRowsSQL = `
+SELECT g.first, f.path, f.size
+FROM files AS f
+JOIN (
+    SELECT size, head, tail, content, MIN(path) AS first
+    FROM files
+    WHERE content <> ''
+    GROUP BY size, head, tail, content
+    HAVING COUNT(*) > 1
+) AS g USING (size, head, tail, content)
+ORDER BY f.size DESC, g.first, f.path
+`
+
+// loadDupeRows streams the rows of dupeRowsSQL to fn and returns the
+// number of records in the database. The count and the rows are read
+// in one transaction, so they agree while a scan is committing. An
+// error from fn stops the reading and is returned as it is.
+func loadDupeRows(ctx context.Context, db *sql.DB,
+	fn func(first, path string, size int64) error,
+) (int, error) {
+	// Everything goes through tx: the report connection is the only
+	// one, so a query on db would wait for tx forever.
+	tx, err := db.BeginTx(ctx, &sql.TxOptions{ReadOnly: true})
+	if err != nil {
+		return 0, fmt.Errorf("read records: %w", err)
+	}
+
+	defer func() { _ = tx.Rollback() }()
+
+	var records int
+
+	err = tx.QueryRowContext(ctx, "SELECT COUNT(*) FROM files").Scan(&records)
+	if err != nil {
+		return 0, fmt.Errorf("read records: %w", err)
+	}
+
+	rows, err := tx.QueryContext(ctx, dupeRowsSQL)
+	if err != nil {
+		return 0, fmt.Errorf("read records: %w", err)
+	}
+
+	defer func() { _ = rows.Close() }()
+
+	for rows.Next() {
+		var (
+			first, path []byte
+			size        int64
+		)
+
+		err = rows.Scan(&first, &path, &size)
+		if err != nil {
+			return 0, fmt.Errorf("read record: %w", err)
+		}
+
+		err = fn(string(first), string(path), size)
+		if err != nil {
+			return 0, err
+		}
+	}
+
+	err = rows.Err()
+	if err != nil {
+		return 0, fmt.Errorf("read records: %w", err)
+	}
+
+	return records, nil
 }
 
 // loadFileMeta streams every record's path, size, mtime, and whether
@@ -275,6 +486,62 @@ func loadFileMeta(ctx context.Context, db *sql.DB,
 	return nil
 }
 
+// contentCandidatesSQL selects every record of at least headTailMin
+// bytes whose size, head, and tail equal another record's, in each
+// group (the records sharing a size, head, and tail) where at least one
+// record has no content hash, with whether each record has one. SQLite
+// does the grouping, so no other record's hashes are loaded into
+// memory; the rows come ordered by size, head, and tail, so each
+// group's rows arrive together.
+const contentCandidatesSQL = `
+SELECT f.path, f.size, f.mtime, f.head, f.tail, f.content <> ''
+FROM files AS f
+JOIN (
+    SELECT size, head, tail
+    FROM files
+    WHERE size >= ? AND head <> ''
+    GROUP BY size, head, tail
+    HAVING COUNT(*) > 1 AND SUM(content = '') > 0
+) AS g USING (size, head, tail)
+ORDER BY size, head, tail
+`
+
+// loadContentCandidates streams the rows of contentCandidatesSQL to fn:
+// each record, without its content hash, and whether it has one.
+func loadContentCandidates(ctx context.Context, db *sql.DB,
+	fn func(r scanRec, hashed bool),
+) error {
+	rows, err := db.QueryContext(ctx, contentCandidatesSQL, headTailMin)
+	if err != nil {
+		return fmt.Errorf("read records: %w", err)
+	}
+
+	defer func() { _ = rows.Close() }()
+
+	for rows.Next() {
+		var (
+			path   []byte
+			r      scanRec
+			hashed int64
+		)
+
+		err = rows.Scan(&path, &r.size, &r.mtime, &r.head, &r.tail, &hashed)
+		if err != nil {
+			return fmt.Errorf("read record: %w", err)
+		}
+
+		r.path = string(path)
+		fn(r, hashed != 0)
+	}
+
+	err = rows.Err()
+	if err != nil {
+		return fmt.Errorf("read records: %w", err)
+	}
+
+	return nil
+}
+
 // updateBatchSize is the number of record changes committed per
 // transaction during the update pass. The filesystem is authoritative
 // and the database an eventually-consistent reflection of it, so
@@ -348,7 +615,7 @@ func execUpserts(ctx context.Context, tx *sql.Tx, upserts []scanRec,
 
 	for _, r := range upserts {
 		_, err = st.ExecContext(ctx,
-			[]byte(r.path), r.size, r.mtime, r.head, r.tail)
+			[]byte(r.path), r.size, r.mtime, r.head, r.tail, r.content)
 		if err != nil {
 			return fmt.Errorf("upsert %s: %w", r.path, err)
 		}

db_test.go

@@ -5,6 +5,7 @@ import (
 	"database/sql"
 	"errors"
 	"fmt"
+	"os"
 	"path/filepath"
 	"slices"
 	"strings"
@@ -72,9 +73,78 @@ func TestOpenScanDatabaseCreates(t *testing.T) {
 
 	defer func() { _ = db.Close() }()
 
-	recs, err := loadFileRows(t.Context(), db)
-	if err != nil || len(recs) != 0 {
-		t.Fatalf("loadFileRows = %v, %v; want empty, nil", recs, err)
+	if recs := dbRecords(t, db); len(recs) != 0 {
+		t.Fatalf("records = %v, want none", recs)
+	}
+}
+
+func TestOpenDatabaseUnversionedForeign(t *testing.T) {
+	t.Parallel()
+
+	// A database that has a files table but user_version 0, written by
+	// some other tool. report, trees and scan must refuse it with the
+	// schema-version error, not adopt it and not emit a raw SQLite
+	// "table files already exists".
+	path := testDBPath(t)
+
+	db, err := sql.Open("sqlite", path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = db.ExecContext(t.Context(), "CREATE TABLE files (x INTEGER)")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_ = db.Close()
+
+	_, err = openReportDatabase(t.Context(), path)
+	if !errors.Is(err, errSchemaVersion) ||
+		!strings.Contains(err.Error(), "remove the file and rescan") {
+		t.Fatalf("report: err = %v, want errSchemaVersion telling the "+
+			"operator to remove the file and rescan", err)
+	}
+
+	_, err = openScanDatabase(t.Context(), path)
+	if !errors.Is(err, errSchemaVersion) ||
+		!strings.Contains(err.Error(), "remove the file and rescan") {
+		t.Fatalf("scan: err = %v, want errSchemaVersion telling the "+
+			"operator to remove the file and rescan", err)
+	}
+}
+
+func TestSchemaCreationStoppedPartway(t *testing.T) {
+	t.Parallel()
+
+	// A first scan stopped while creating the schema must leave a
+	// database the next scan accepts. max_page_count(2) leaves room for
+	// the files table but not its index, so schema creation fails right
+	// after CREATE TABLE, a point an interrupt could also stop it at.
+	path := testDBPath(t)
+
+	db, err := openDB(path, scanParams+"&_pragma=max_page_count(2)")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = initSchema(t.Context(), db)
+	_ = db.Close()
+
+	if err == nil {
+		t.Fatal("initSchema with no room for the index succeeded")
+	}
+
+	db, err = openScanDatabase(t.Context(), path)
+	if err != nil {
+		t.Fatalf("next scan: %v", err)
+	}
+
+	defer func() { _ = db.Close() }()
+
+	v, err := userVersion(t.Context(), db)
+	if err != nil || v != schemaVersion {
+		t.Fatalf("userVersion = %d, %v; want %d, nil", v, err, schemaVersion)
 	}
 }
 
@@ -87,9 +157,11 @@ func TestOpenReportDatabaseMissing(t *testing.T) {
 	}
 }
 
-func TestOpenReportDatabaseVersionMismatch(t *testing.T) {
+func TestOpenDatabaseVersionMismatch(t *testing.T) {
 	t.Parallel()
 
+	// A database stamped with a schema version other than 0 and
+	// schemaVersion. report, trees and scan must all refuse it.
 	path := testDBPath(t)
 
 	db, err := openScanDatabase(t.Context(), path)
@@ -106,7 +178,12 @@ func TestOpenReportDatabaseVersionMismatch(t *testing.T) {
 
 	_, err = openReportDatabase(t.Context(), path)
 	if !errors.Is(err, errSchemaVersion) {
-		t.Fatalf("err = %v, want errSchemaVersion", err)
+		t.Fatalf("report: err = %v, want errSchemaVersion", err)
+	}
+
+	_, err = openScanDatabase(t.Context(), path)
+	if !errors.Is(err, errSchemaVersion) {
+		t.Fatalf("scan: err = %v, want errSchemaVersion", err)
 	}
 }
 
@@ -130,16 +207,63 @@ func TestOpenReportDatabaseOK(t *testing.T) {
 	_ = db.Close()
 }
 
+func TestCloseScanDatabaseWhileReportOpen(t *testing.T) {
+	t.Parallel()
+
+	// A report holding the database open stops scan from taking it out
+	// of WAL mode. The -wal and -shm files must then stay beside it, so
+	// that a later report still needs only read access.
+	path := testDBPath(t)
+
+	scanDB, err := openScanDatabase(t.Context(), path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	reportDB, err := openReportDatabase(t.Context(), path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	closeScanDatabase(t.Context(), scanDB, path)
+
+	_ = reportDB.Close()
+
+	_, err = os.Stat(path + "-wal")
+	if err != nil {
+		t.Fatalf("no -wal left: the switch out of WAL mode was not "+
+			"stopped: %v", err)
+	}
+
+	makeReadOnly(t, path)
+
+	reportDB, err = openReportDatabase(t.Context(), path)
+	if err != nil {
+		t.Fatalf("openReportDatabase: %v", err)
+	}
+
+	defer func() { _ = reportDB.Close() }()
+
+	err = loadFileRows(t.Context(), reportDB, func(scanRec) {})
+	if err != nil {
+		t.Fatalf("loadFileRows: %v", err)
+	}
+}
+
 func TestApplyChangesRoundTrip(t *testing.T) {
 	t.Parallel()
 
 	db := openTestDB(t)
 
 	// Paths may contain tabs and newlines; the database must store
-	// them byte-exactly.
+	// them byte-exactly. Every hash, content included, comes back as
+	// written.
 	recs := []scanRec{
-		{size: 2, mtime: 20, head: "h2", tail: "t2", path: "/a/tab\tnew\nline"},
-		{size: 1, mtime: 10, head: "h1", tail: "t1", path: "/a/x"},
+		{
+			size: 2, mtime: 20, head: "h2", tail: "t2", content: "c2",
+			path: "/a/tab\tnew\nline",
+		},
+		{size: 1, mtime: 10, head: "h1", tail: "t1", content: "c1", path: "/a/x"},
 	}
 
 	err := applyChanges(t.Context(), db, recs, nil,
@@ -148,22 +272,17 @@ func TestApplyChangesRoundTrip(t *testing.T) {
 		t.Fatalf("applyChanges: %v", err)
 	}
 
-	got, err := loadFileRows(t.Context(), db)
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	slices.SortFunc(got, func(a, b scanRec) int {
-		return strings.Compare(a.path, b.path)
-	})
-
+	// The records come back in path order, which is the order of recs.
+	got := dbRecords(t, db)
 	if !slices.Equal(got, recs) {
 		t.Fatalf("rows = %+v, want %+v", got, recs)
 	}
 
 	// An upsert for an existing path updates in place; a delete
 	// removes exactly its path.
-	upd := scanRec{size: 3, mtime: 30, head: "h3", tail: "t3", path: "/a/x"}
+	upd := scanRec{
+		size: 3, mtime: 30, head: "h3", tail: "t3", content: "c3", path: "/a/x",
+	}
 
 	err = applyChanges(t.Context(), db, []scanRec{upd},
 		[]string{"/a/tab\tnew\nline"}, newProgress("update", 2))
@@ -171,11 +290,7 @@ func TestApplyChangesRoundTrip(t *testing.T) {
 		t.Fatalf("applyChanges: %v", err)
 	}
 
-	got, err = loadFileRows(t.Context(), db)
-	if err != nil {
-		t.Fatal(err)
-	}
-
+	got = dbRecords(t, db)
 	if len(got) != 1 || got[0] != upd {
 		t.Fatalf("rows = %+v, want just %+v", got, upd)
 	}
@@ -204,9 +319,8 @@ func TestApplyChangesBatching(t *testing.T) {
 		t.Fatalf("applyChanges: %v", err)
 	}
 
-	got, err := loadFileRows(t.Context(), db)
-	if err != nil || len(got) != n {
-		t.Fatalf("loadFileRows = %d rows, %v; want %d", len(got), err, n)
+	if got := dbRecords(t, db); len(got) != n {
+		t.Fatalf("records = %d, want %d", len(got), n)
 	}
 
 	deletes := make([]string, 0, n)
@@ -220,8 +334,7 @@ func TestApplyChangesBatching(t *testing.T) {
 		t.Fatalf("applyChanges deletes: %v", err)
 	}
 
-	got, err = loadFileRows(t.Context(), db)
-	if err != nil || len(got) != 0 {
-		t.Fatalf("loadFileRows = %d rows, %v; want 0", len(got), err)
+	if got := dbRecords(t, db); len(got) != 0 {
+		t.Fatalf("records = %d, want 0", len(got))
 	}
 }

go.mod

@@ -5,6 +5,8 @@ go 1.25.7
 require (
 	github.com/schollz/progressbar/v3 v3.19.1
 	github.com/spf13/cobra v1.10.2
+	golang.org/x/sys v0.46.0
+	golang.org/x/term v0.44.0
 	modernc.org/sqlite v1.54.0
 )
 
@@ -18,8 +20,6 @@ require (
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
 	github.com/rivo/uniseg v0.4.7 // indirect
 	github.com/spf13/pflag v1.0.9 // indirect
-	golang.org/x/sys v0.46.0 // indirect
-	golang.org/x/term v0.44.0 // indirect
 	modernc.org/libc v1.74.1 // indirect
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect

main.go

@@ -1,16 +1,21 @@
 // Command sfdupes quickly identifies candidate duplicate files across
-// very large filesystems without reading full file contents. Files are
-// considered duplicates when they have identical size, identical SHA-256
-// of their first 1024 bytes, and identical SHA-256 of their last 1024
-// bytes. scan maintains a persistent SQLite database of file signatures
-// (SFDUPES_DATABASE, default /var/lib/sfdupes/db.sqlite) that the
-// reporting subcommands read.
+// very large filesystems without reading every byte of every file.
+// Files are considered duplicates when their sizes are equal and they
+// agree on a short ladder of SHA-256 hashes. A file under 10 MiB is
+// hashed in full. A larger file is compared on the hashes of its first
+// and last 64 KiB, and only when those match another file's is its
+// content hash computed and compared: of the whole file when it is
+// under 50 MiB, or of gigabyte-spaced 1 MiB samples when it is 50 MiB
+// or larger. scan maintains a persistent SQLite database of file
+// signatures (SFDUPES_DATABASE, default /var/lib/sfdupes/db.sqlite)
+// that the reporting subcommands read.
 //
 // Usage:
 //
 //	sfdupes scan [--workers N] [-x] PATH...
 //	sfdupes report > dupes.tsv
 //	sfdupes trees > dupetrees.tsv
+//	sfdupes --version
 //
 // See README.md for the complete specification.
 package main
@@ -46,6 +51,10 @@ const (
 // cobra prints for it is the whole message.
 var errNoSubcommand = errors.New("no subcommand")
 
+// errWorkersBelowOne is the usage error for a scan --workers value
+// below 1.
+var errWorkersBelowOne = errors.New("--workers must be at least 1")
+
 // Version is the build version, injected at link time via -ldflags
 // (see the Makefile); "dev" for a plain go build.
 //
@@ -53,22 +62,27 @@ var errNoSubcommand = errors.New("no subcommand")
 var Version = "dev"
 
 func main() {
-	os.Exit(run(os.Args[1:], os.Stderr))
+	// Once the reader of a stdout pipe has gone, as in "sfdupes report |
+	// head", the Go runtime ends the process with SIGPIPE on the next
+	// write instead of returning an error (README "Error handling").
+	// Registering for SIGPIPE with os/signal would change that.
+	os.Exit(run(os.Args[1:], os.Stdout, os.Stderr))
 }
 
 // run executes args against the command tree and returns the process
 // exit code. It is the program's single exit point: the subcommands
 // return their errors instead of exiting, so every deferred cleanup —
 // above all closing the database, which checkpoints the SQLite WAL —
-// runs before the process ends.
-func run(args []string, stderr io.Writer) int {
+// runs before the process ends. The report and trees subcommands write
+// their data to stdout.
+func run(args []string, stdout, stderr io.Writer) int {
 	// A nil slice makes cobra fall back to os.Args, which would let a
 	// test binary's own flags reach the command tree.
 	if args == nil {
 		args = []string{}
 	}
 
-	root := newRootCommand(stderr)
+	root := newRootCommand(stdout, stderr)
 	root.SetArgs(args)
 
 	err := root.Execute()
@@ -78,6 +92,9 @@ func run(args []string, stderr io.Writer) int {
 	switch {
 	case err == nil:
 		return exitOK
+	case errors.Is(err, errInterrupted):
+		// The interrupted scan has printed its own line.
+		return exitFatal
 	case errors.As(err, &fatal):
 		// The command ran and failed: a runtime error, reported
 		// without the usage text that a usage error gets.
@@ -85,22 +102,35 @@ func run(args []string, stderr io.Writer) int {
 
 		return exitFatal
 	default:
-		// A usage error: cobra has already printed the message and
-		// the usage text.
+		// A usage error, which cobra has already reported on stderr.
 		return exitUsage
 	}
 }
 
 // newRootCommand builds the command tree. Everything on stdout is
-// machine-readable data; all human-facing output (help, usage, errors)
-// goes to stderr.
-func newRootCommand(stderr io.Writer) *cobra.Command {
+// machine-readable data, the version line included; all human-facing
+// output (help, usage, errors) goes to stderr.
+func newRootCommand(stdout, stderr io.Writer) *cobra.Command {
+	var showVersion bool
+
+	printVersion := runE(func(context.Context, []string) error {
+		_, err := fmt.Fprintf(stdout, "sfdupes %s\n", Version)
+		if err != nil {
+			return fmt.Errorf("write stdout: %w", err)
+		}
+
+		return nil
+	})
+
 	root := &cobra.Command{
 		Use:   "sfdupes",
-		Short:   "Find candidate duplicate files by size and head/tail SHA-256",
-		Version: Version,
+		Short: "Find candidate duplicate files by size and head/tail/content SHA-256",
 		Args:  cobra.NoArgs,
-		RunE: func(cmd *cobra.Command, _ []string) error {
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if showVersion {
+				return printVersion(cmd, args)
+			}
+
 			// A missing subcommand prints usage and exits 2: cobra
 			// prints the usage text for the returned error, and run
 			// maps everything that is not a fatal error to exit 2.
@@ -113,6 +143,11 @@ func newRootCommand(stderr io.Writer) *cobra.Command {
 	root.SetErr(stderr)
 	root.CompletionOptions.DisableDefaultCmd = true
 
+	// Cobra's built-in version flag prints through the help writer,
+	// stderr; this one prints to stdout.
+	root.Flags().BoolVarP(&showVersion, "version", "v", false,
+		"print the version to stdout")
+
 	var (
 		scanWorkers int
 		scanOneFS   bool
@@ -122,12 +157,18 @@ func newRootCommand(stderr io.Writer) *cobra.Command {
 		Use:   cmdScan + " [--workers N] [-x] PATH...",
 		Short: "Walk trees and synchronize the scan database",
 		Args:  cobra.MinimumNArgs(1),
+		PreRunE: func(cmd *cobra.Command, _ []string) error {
+			return checkScanWorkers(cmd, scanWorkers)
+		},
 		RunE: runE(func(ctx context.Context, args []string) error {
+			ctx, stop := interruptContext(ctx)
+			defer stop()
+
 			return runScan(ctx, args, scanWorkers, scanOneFS)
 		}),
 	}
 	scanCmd.Flags().IntVar(&scanWorkers, "workers", runtime.NumCPU(),
-		"concurrent workers for the walk and hash phases")
+		"concurrent workers for the walk, hash, and content phases")
 	scanCmd.Flags().BoolVarP(&scanOneFS, "one-file-system", "x", false,
 		"do not cross filesystem boundaries")
 
@@ -136,7 +177,7 @@ func newRootCommand(stderr io.Writer) *cobra.Command {
 		Short: "Read the scan database and print the file-level duplicates report",
 		Args:  cobra.NoArgs,
 		RunE: runE(func(ctx context.Context, _ []string) error {
-			return runReport(ctx)
+			return runReport(ctx, stdout)
 		}),
 	}
 
@@ -145,7 +186,7 @@ func newRootCommand(stderr io.Writer) *cobra.Command {
 		Short: "Read the scan database and print the duplicate-tree report",
 		Args:  cobra.NoArgs,
 		RunE: runE(func(ctx context.Context, _ []string) error {
-			return runTrees(ctx)
+			return runTrees(ctx, stdout)
 		}),
 	}
 
@@ -154,13 +195,26 @@ func newRootCommand(stderr io.Writer) *cobra.Command {
 	return root
 }
 
-// runE adapts a subcommand implementation to cobra's RunE. Cobra
-// prints the error and the command's usage text for every error RunE
-// returns, but a subcommand that ran and failed has no usage problem
-// to report: both are silenced here, and the error is marked fatal so
-// that run reports it on stderr and exits 1 rather than 2. The command's
-// context is handed to the implementation: cancelling it unwinds the
-// scan's worker pools.
+// checkScanWorkers rejects a scan --workers value below 1. That is a
+// usage error reported in one line: cobra prints the returned message
+// without the usage text, and run exits 2.
+func checkScanWorkers(cmd *cobra.Command, workers int) error {
+	if workers >= 1 {
+		return nil
+	}
+
+	cmd.SilenceUsage = true
+
+	return fmt.Errorf("%w, got %d", errWorkersBelowOne, workers)
+}
+
+// runE adapts a subcommand implementation, or the version print, to
+// cobra's RunE. Cobra prints the error and the command's usage text for
+// every error RunE returns, but a subcommand that ran and failed has no
+// usage problem to report: both are silenced here, and the error is
+// marked fatal so that run reports it on stderr and exits 1 rather than
+// 2. The command's context is handed to the implementation: cancelling
+// it unwinds the scan's worker pools.
 func runE(
 	fn func(ctx context.Context, args []string) error,
 ) func(*cobra.Command, []string) error {

main_test.go

@@ -44,23 +44,61 @@ func assertNoSidecars(t *testing.T, path string) {
 	}
 }
 
-// captureStdout redirects os.Stdout to a file for the rest of the test
-// and returns a function reading back everything written to it. Only
-// machine-readable data belongs on stdout (README design goal 4), so
-// the tests assert on it directly.
-func captureStdout(t *testing.T) func() string {
+// makeReadOnly takes write permission away from the database at path,
+// from any WAL sidecar beside it, and from their directory, as for a
+// user reading a database that a root cron scan keeps. Root ignores
+// file permissions, so it skips the test when run as root.
+func makeReadOnly(t *testing.T, path string) {
 	t.Helper()
 
-	f, err := os.Create(filepath.Join(t.TempDir(), "stdout"))
+	if os.Geteuid() == 0 {
+		t.Skip("root ignores file permissions")
+	}
+
+	err := os.Chmod(path, 0o400)
 	if err != nil {
 		t.Fatal(err)
 	}
 
-	saved := os.Stdout
-	os.Stdout = f
+	for _, suffix := range walSuffixes {
+		err = os.Chmod(path+suffix, 0o400)
+		if err != nil && !errors.Is(err, fs.ErrNotExist) {
+			t.Fatal(err)
+		}
+	}
+
+	dir := filepath.Dir(path)
+
+	//nolint:gosec // reaching the database needs the search bit
+	err = os.Chmod(dir, 0o500)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Runs before t.TempDir's own cleanup, which must delete the files.
+	t.Cleanup(func() {
+		//nolint:gosec // removing the directory needs its search bit back
+		_ = os.Chmod(dir, 0o700)
+	})
+}
+
+// captureStderr redirects os.Stderr to a file for the rest of the test
+// and returns a function reading back everything written to it. scan
+// writes its warnings and summary straight to os.Stderr, not to the
+// stderr writer run is given.
+func captureStderr(t *testing.T) func() string {
+	t.Helper()
+
+	f, err := os.Create(filepath.Join(t.TempDir(), "stderr"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	saved := os.Stderr
+	os.Stderr = f
 
 	t.Cleanup(func() {
-		os.Stdout = saved
+		os.Stderr = saved
 
 		_ = f.Close()
 	})
@@ -91,13 +129,13 @@ func captureStdout(t *testing.T) func() string {
 // brokenDatabase writes a database that opens cleanly and passes the
 // schema-version check but has no files table, so the first query
 // fails with the database already open: a fatal error on a path that
-// owns an open database.
+// owns an open database. It closes the database the way scan does.
 func brokenDatabase(t *testing.T) string {
 	t.Helper()
 
 	path := testDBPath(t)
 
-	db, err := openDB(path)
+	db, err := openDB(path, scanParams)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -108,10 +146,7 @@ func brokenDatabase(t *testing.T) string {
 		t.Fatal(err)
 	}
 
-	err = db.Close()
-	if err != nil {
-		t.Fatal(err)
-	}
+	closeScanDatabase(t.Context(), db, path)
 
 	return path
 }
@@ -144,7 +179,10 @@ func TestOpenDatabaseKeepsWALWhileOpen(t *testing.T) {
 
 func TestRunFatalAfterOpenClosesDatabase(t *testing.T) {
 	// Every subcommand that owns an open database must close it when
-	// it fails: no os.Exit between the open and the return.
+	// it fails: no os.Exit between the open and the return. The
+	// sidecar check is evidence of the close only for scan: report and
+	// trees only read a database that is out of WAL mode, which leaves
+	// nothing on disk whether they close it or not.
 	cases := map[string][]string{
 		cmdScan:   {cmdScan},
 		cmdReport: {cmdReport},
@@ -160,17 +198,15 @@ func TestRunFatalAfterOpenClosesDatabase(t *testing.T) {
 				args = append(args, t.TempDir())
 			}
 
-			var stderr bytes.Buffer
+			var stdout, stderr bytes.Buffer
 
-			stdout := captureStdout(t)
-
-			code := run(args, &stderr)
+			code := run(args, &stdout, &stderr)
 			if code != exitFatal {
 				t.Errorf("run(%v) = %d, want %d", args, code, exitFatal)
 			}
 
 			assertNoSidecars(t, path)
-			assertFatalOutput(t, stderr.String(), stdout())
+			assertFatalOutput(t, stderr.String(), stdout.String())
 
 			// Proof that the failure happened after the open: only a
 			// query against the opened database can report this.
@@ -188,18 +224,16 @@ func TestRunMissingOperandIsFatalNotUsage(t *testing.T) {
 	// must not dump the usage text.
 	t.Setenv(databaseEnv, testDBPath(t))
 
-	var stderr bytes.Buffer
-
-	stdout := captureStdout(t)
+	var stdout, stderr bytes.Buffer
 
 	missing := filepath.Join(t.TempDir(), "nope")
 
-	code := run([]string{cmdScan, missing}, &stderr)
+	code := run([]string{cmdScan, missing}, &stdout, &stderr)
 	if code != exitFatal {
 		t.Errorf("run(scan %s) = %d, want %d", missing, code, exitFatal)
 	}
 
-	assertFatalOutput(t, stderr.String(), stdout())
+	assertFatalOutput(t, stderr.String(), stdout.String())
 }
 
 // assertFatalOutput checks that a fatal error was reported the way
@@ -243,11 +277,9 @@ func TestRunUsageErrors(t *testing.T) {
 			// path that does not exist.
 			t.Setenv(databaseEnv, testDBPath(t))
 
-			var stderr bytes.Buffer
+			var stdout, stderr bytes.Buffer
 
-			stdout := captureStdout(t)
-
-			code := run(tc.args, &stderr)
+			code := run(tc.args, &stdout, &stderr)
 			if code != exitUsage {
 				t.Errorf("run(%v) = %d, want %d", tc.args, code, exitUsage)
 			}
@@ -256,43 +288,115 @@ func TestRunUsageErrors(t *testing.T) {
 				t.Errorf("stderr = %q, want %q", stderr.String(), tc.want)
 			}
 
-			if got := stdout(); got != "" {
+			if got := stdout.String(); got != "" {
 				t.Errorf("stdout = %q, want nothing (data only)", got)
 			}
 		})
 	}
 }
 
-// TestRunHelpAndVersionSucceed checks that the two informational flags
-// exit 0 and keep their human-facing output on stderr.
-//
-//nolint:paralleltest // captureStdout replaces the process-wide os.Stdout
-func TestRunHelpAndVersionSucceed(t *testing.T) {
-	assertHumanOutput(t, "--help")
-	assertHumanOutput(t, "--version")
+func TestRunScanRejectsWorkersBelowOne(t *testing.T) {
+	// README §scan mode: --workers below 1 is a usage error reported in
+	// one line on stderr, before the scan opens the database.
+	for _, workers := range []string{"0", "-1"} {
+		t.Run(workers, func(t *testing.T) {
+			dbPath := testDBPath(t)
+			t.Setenv(databaseEnv, dbPath)
+
+			var stdout, stderr bytes.Buffer
+
+			args := []string{cmdScan, "--workers", workers, t.TempDir()}
+
+			code := run(args, &stdout, &stderr)
+			if code != exitUsage {
+				t.Errorf("run(%v) = %d, want %d", args, code, exitUsage)
 			}
 
-// assertHumanOutput runs sfdupes with one informational flag and checks
-// that it succeeds with its output on stderr and stdout untouched
-// (README design goal 4).
-func assertHumanOutput(t *testing.T, arg string) {
-	t.Helper()
+			want := "Error: --workers must be at least 1, got " + workers +
+				"\n"
+			if got := stderr.String(); got != want {
+				t.Errorf("stderr = %q, want %q", got, want)
+			}
 
-	var stderr bytes.Buffer
+			if got := stdout.String(); got != "" {
+				t.Errorf("stdout = %q, want nothing (data only)", got)
+			}
 
-	stdout := captureStdout(t)
+			_, err := os.Stat(dbPath)
+			if !errors.Is(err, fs.ErrNotExist) {
+				t.Errorf("stat %s: %v, want the database never created",
+					dbPath, err)
+			}
+		})
+	}
+}
 
-	code := run([]string{arg}, &stderr)
+func TestRunHelp(t *testing.T) {
+	t.Parallel()
+
+	// README §Subcommands: help goes to stderr, exits 0, and leaves
+	// stdout empty.
+	cases := [][]string{{"--help"}, {"-h"}, {cmdScan, "--help"}}
+
+	for _, args := range cases {
+		var stdout, stderr bytes.Buffer
+
+		code := run(args, &stdout, &stderr)
+		if code != exitOK {
+			t.Errorf("run(%v) = %d, want %d", args, code, exitOK)
+		}
+
+		if !strings.Contains(stderr.String(), usageMarker) {
+			t.Errorf("run(%v) stderr = %q, want the help text",
+				args, stderr.String())
+		}
+
+		if got := stdout.String(); got != "" {
+			t.Errorf("run(%v) stdout = %q, want nothing (data only)",
+				args, got)
+		}
+	}
+}
+
+func TestRunVersion(t *testing.T) {
+	t.Parallel()
+
+	// README §Subcommands: the version is one line on stdout, with
+	// nothing on stderr, and exits 0.
+	for _, arg := range []string{"--version", "-v"} {
+		var stdout, stderr bytes.Buffer
+
+		code := run([]string{arg}, &stdout, &stderr)
 		if code != exitOK {
 			t.Errorf("run(%s) = %d, want %d", arg, code, exitOK)
 		}
 
-	if stderr.Len() == 0 {
-		t.Errorf("run(%s) wrote nothing to stderr", arg)
+		want := "sfdupes " + Version + "\n"
+		if got := stdout.String(); got != want {
+			t.Errorf("run(%s) stdout = %q, want %q", arg, got, want)
 		}
 
-	if got := stdout(); got != "" {
-		t.Errorf("stdout = %q, want nothing (data only)", got)
+		if got := stderr.String(); got != "" {
+			t.Errorf("run(%s) stderr = %q, want nothing", arg, got)
+		}
+	}
+}
+
+func TestRunVersionWriteFailureIsFatal(t *testing.T) {
+	t.Parallel()
+
+	// README §Error handling: a stdout write failure exits 1, reported
+	// in one line on stderr.
+	var stderr bytes.Buffer
+
+	code := run([]string{"--version"}, failingWriter{}, &stderr)
+	if code != exitFatal {
+		t.Errorf("run(--version) = %d, want %d", code, exitFatal)
+	}
+
+	want := "sfdupes: write stdout: " + errWriteFailed.Error() + "\n"
+	if got := stderr.String(); got != want {
+		t.Errorf("stderr = %q, want %q", got, want)
 	}
 }
 
@@ -320,21 +424,31 @@ func scanFixture(t *testing.T) []string {
 		t.Fatal(err)
 	}
 
-	var stderr bytes.Buffer
+	scanOK(t, dir)
 
-	stdout := captureStdout(t)
-
-	code := run([]string{cmdScan, dir}, &stderr)
-	if code != exitOK {
-		t.Fatalf("run(scan) = %d, want %d; stderr: %s",
-			code, exitOK, stderr.String())
+	return dupes
 }
 
-	if got := stdout(); got != "" {
+// scanOK runs scan over operands, fails the test unless it exits 0 with
+// nothing on stdout, and returns everything it printed to stderr.
+func scanOK(t *testing.T, operands ...string) string {
+	t.Helper()
+
+	var stdout bytes.Buffer
+
+	stderr := captureStderr(t)
+
+	code := run(append([]string{cmdScan}, operands...), &stdout, os.Stderr)
+	if code != exitOK {
+		t.Fatalf("run(scan %q) = %d, want %d; stderr: %s",
+			operands, code, exitOK, stderr())
+	}
+
+	if got := stdout.String(); got != "" {
 		t.Errorf("scan stdout = %q, want nothing (data only)", got)
 	}
 
-	return dupes
+	return stderr()
 }
 
 func TestRunScanSucceedsDespiteWarnings(t *testing.T) {
@@ -345,24 +459,119 @@ func TestRunScanSucceedsDespiteWarnings(t *testing.T) {
 	assertNoSidecars(t, path)
 }
 
+func TestRunScanSkipsSymlinkOperand(t *testing.T) {
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	dir := t.TempDir()
+	writeFile(t, dir, "target/sub/f", pattern(1, 10))
+
+	link := filepath.Join(dir, "link")
+
+	err := os.Symlink(filepath.Join(dir, "target"), link)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Scanning a directory through the symlink stores a record beneath
+	// the symlink's own path for a file beneath its target.
+	scanOK(t, filepath.Join(link, "sub"))
+
+	assertOperandSkipped(t, path, link, "symlink",
+		filepath.Join(link, "sub", "f"))
+}
+
+func TestRunScanWalksOperandUnderSymlinkOperand(t *testing.T) {
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	dir := t.TempDir()
+	writeFile(t, dir, "target/sub/f", pattern(1, 10))
+
+	link := filepath.Join(dir, "link")
+
+	err := os.Symlink(filepath.Join(dir, "target"), link)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// link is dropped as a symlink, but link/sub must still be scanned,
+	// not dropped as lying under link.
+	scanOK(t, link, filepath.Join(link, "sub"))
+
+	db, err := openDB(path, reportParams)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Cleanup(func() { _ = db.Close() })
+
+	recordByPath(t, dbRecords(t, db), filepath.Join(link, "sub", "f"))
+}
+
+func TestRunScanSkipsZFSOperand(t *testing.T) {
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	zfs := filepath.Join(t.TempDir(), ".zfs")
+	snapshot := filepath.Join(zfs, "snapshot", "hourly")
+	f := writeFile(t, snapshot, "f", pattern(1, 10))
+
+	// An operand beneath a .zfs directory is walked, because it is not
+	// itself named .zfs.
+	scanOK(t, snapshot)
+
+	assertOperandSkipped(t, path, zfs, ".zfs directory", f)
+}
+
+// assertOperandSkipped scans operand alone and checks that it is skipped
+// as kind: a warning naming it, one skip in the summary, exit 0, and the
+// record for kept, which an earlier scan stored beneath operand, still
+// in the database at dbPath.
+func assertOperandSkipped(t *testing.T, dbPath, operand, kind,
+	kept string,
+) {
+	t.Helper()
+
+	stderr := scanOK(t, operand)
+
+	warning := "walk " + operand + ": skipping " + kind + " operand\n"
+	if !strings.Contains(stderr, warning) {
+		t.Errorf("stderr = %q, want %q", stderr, warning)
+	}
+
+	summary := "scan: 0 files seen (0 added, 0 updated, 0 removed, " +
+		"0 unchanged), 1 skipped\n"
+	if !strings.Contains(stderr, summary) {
+		t.Errorf("stderr = %q, want %q", stderr, summary)
+	}
+
+	db, err := openDB(dbPath, reportParams)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Cleanup(func() { _ = db.Close() })
+
+	recordByPath(t, dbRecords(t, db), kept)
+}
+
 func TestRunReportSucceeds(t *testing.T) {
 	path := testDBPath(t)
 	t.Setenv(databaseEnv, path)
 
 	dupes := scanFixture(t)
 
-	var stderr bytes.Buffer
+	var stdout, stderr bytes.Buffer
 
-	stdout := captureStdout(t)
-
-	code := run([]string{cmdReport}, &stderr)
+	code := run([]string{cmdReport}, &stdout, &stderr)
 	if code != exitOK {
 		t.Fatalf("run(report) = %d, want %d; stderr: %s",
 			code, exitOK, stderr.String())
 	}
 
 	want := "first\tdupe\tsize\n" + dupes[0] + "\t" + dupes[1] + "\t300\n"
-	if got := stdout(); got != want {
+	if got := stdout.String(); got != want {
 		t.Errorf("stdout = %q, want %q", got, want)
 	}
 
@@ -375,11 +584,9 @@ func TestRunTreesSucceeds(t *testing.T) {
 
 	dupes := scanFixture(t)
 
-	var stderr bytes.Buffer
+	var stdout, stderr bytes.Buffer
 
-	stdout := captureStdout(t)
-
-	code := run([]string{cmdTrees}, &stderr)
+	code := run([]string{cmdTrees}, &stdout, &stderr)
 	if code != exitOK {
 		t.Fatalf("run(trees) = %d, want %d; stderr: %s",
 			code, exitOK, stderr.String())
@@ -389,9 +596,197 @@ func TestRunTreesSucceeds(t *testing.T) {
 	// trees of each other.
 	want := "first\tdupe\tfiles\tsize\n" +
 		filepath.Dir(dupes[0]) + "\t" + filepath.Dir(dupes[1]) + "\t1\t300\n"
-	if got := stdout(); got != want {
+	if got := stdout.String(); got != want {
 		t.Errorf("stdout = %q, want %q", got, want)
 	}
 
 	assertNoSidecars(t, path)
 }
+
+func TestRunReportsNeedOnlyReadAccess(t *testing.T) {
+	// README §Database: report and trees need only read access to the
+	// database file. With its directory read-only as well, SQLite
+	// cannot create any file beside it.
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	dupes := scanFixture(t)
+	assertNoSidecars(t, path)
+	makeReadOnly(t, path)
+
+	cases := map[string]string{
+		cmdReport: "first\tdupe\tsize\n" +
+			dupes[0] + "\t" + dupes[1] + "\t300\n",
+		cmdTrees: "first\tdupe\tfiles\tsize\n" +
+			filepath.Dir(dupes[0]) + "\t" + filepath.Dir(dupes[1]) +
+			"\t1\t300\n",
+	}
+
+	for name, want := range cases {
+		var stdout, stderr bytes.Buffer
+
+		code := run([]string{name}, &stdout, &stderr)
+		if code != exitOK {
+			t.Errorf("run(%s) = %d, want %d; stderr: %s",
+				name, code, exitOK, stderr.String())
+
+			continue
+		}
+
+		if got := stdout.String(); got != want {
+			t.Errorf("%s stdout = %q, want %q", name, got, want)
+		}
+	}
+}
+
+// holdScanLock takes the lock on the database at path, as a running
+// scan does, and holds it until the test ends. It fails the test when
+// the lock is already held.
+func holdScanLock(t *testing.T, path string) {
+	t.Helper()
+
+	lock, err := lockScanDatabase(path)
+	if err != nil {
+		t.Fatalf("lock %s: %v", path, err)
+	}
+
+	t.Cleanup(func() { _ = lock.Close() })
+}
+
+func TestRunSecondScanFails(t *testing.T) {
+	// README §Database: while one scan holds the lock, a second scan
+	// fails at once, naming the lock file, without creating the
+	// database.
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	holdScanLock(t, path)
+
+	var stdout, stderr bytes.Buffer
+
+	code := run([]string{cmdScan, t.TempDir()}, &stdout, &stderr)
+	if code != exitFatal {
+		t.Errorf("run(scan) = %d, want %d", code, exitFatal)
+	}
+
+	want := "sfdupes: another scan is running (lock held on " +
+		path + ".lock)\n"
+	if got := stderr.String(); got != want {
+		t.Errorf("stderr = %q, want %q", got, want)
+	}
+
+	if got := stdout.String(); got != "" {
+		t.Errorf("stdout = %q, want nothing (data only)", got)
+	}
+
+	_, err := os.Stat(path)
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Errorf("stat %s = %v, want the database not created", path, err)
+	}
+}
+
+func TestRunScanReleasesLock(t *testing.T) {
+	// README §Database: a scan releases the lock however it ends.
+	t.Run("success", func(t *testing.T) {
+		path := testDBPath(t)
+		t.Setenv(databaseEnv, path)
+
+		scanFixture(t)
+		holdScanLock(t, path)
+	})
+
+	t.Run("fatal error", func(t *testing.T) {
+		path := brokenDatabase(t)
+		t.Setenv(databaseEnv, path)
+
+		code := run([]string{cmdScan, t.TempDir()}, io.Discard, io.Discard)
+		if code != exitFatal {
+			t.Fatalf("run(scan) = %d, want %d", code, exitFatal)
+		}
+
+		holdScanLock(t, path)
+	})
+}
+
+func TestRunReportsDuringScan(t *testing.T) {
+	// README §Database: report and trees never take the lock, so they
+	// run while a scan holds it.
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	scanFixture(t)
+	holdScanLock(t, path)
+
+	for _, name := range []string{cmdReport, cmdTrees} {
+		var stderr bytes.Buffer
+
+		code := run([]string{name}, io.Discard, &stderr)
+		if code != exitOK {
+			t.Errorf("run(%s) = %d, want %d; stderr: %s",
+				name, code, exitOK, stderr.String())
+		}
+	}
+}
+
+func TestRunStdoutClosedIsFatal(t *testing.T) {
+	// README §Error handling: a stdout write failure exits 1, reported
+	// in one line on stderr.
+	for _, name := range []string{cmdReport, cmdTrees} {
+		t.Run(name, func(t *testing.T) {
+			t.Setenv(databaseEnv, testDBPath(t))
+
+			scanFixture(t)
+
+			stdout, err := os.Create(filepath.Join(t.TempDir(), "stdout"))
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			err = stdout.Close()
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			var stderr bytes.Buffer
+
+			code := run([]string{name}, stdout, &stderr)
+			if code != exitFatal {
+				t.Errorf("run(%s) = %d, want %d", name, code, exitFatal)
+			}
+
+			got := stderr.String()
+			if !strings.HasPrefix(got, "sfdupes: write stdout: ") ||
+				!strings.Contains(got, os.ErrClosed.Error()) ||
+				strings.Count(got, "\n") != 1 {
+				t.Errorf("stderr = %q, want one line reporting the "+
+					"failed stdout write", got)
+			}
+		})
+	}
+}
+
+// errWriteFailed is the error failingWriter returns.
+var errWriteFailed = errors.New("write failed")
+
+// failingWriter is a stdout that fails every write.
+type failingWriter struct{}
+
+func (failingWriter) Write([]byte) (int, error) { return 0, errWriteFailed }
+
+func TestStdoutWriteErrorPropagates(t *testing.T) {
+	t.Setenv(databaseEnv, testDBPath(t))
+
+	scanFixture(t)
+
+	cases := map[string]func(context.Context, io.Writer) error{
+		cmdReport: runReport,
+		cmdTrees:  runTrees,
+	}
+
+	for name, fn := range cases {
+		err := fn(t.Context(), failingWriter{})
+		if !errors.Is(err, errWriteFailed) {
+			t.Errorf("%s: error = %v, want %v", name, err, errWriteFailed)
+		}
+	}
+}

progress.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/schollz/progressbar/v3"
+	"golang.org/x/term"
 )
 
 // plainInterval is the minimum time between progress lines when stderr
@@ -24,24 +25,23 @@ const percentScale = 100
 
 // stderrIsTTY reports whether stderr is attached to a terminal.
 func stderrIsTTY() bool {
-	fi, err := os.Stderr.Stat()
-	if err != nil {
-		return false
-	}
-
-	return fi.Mode()&os.ModeCharDevice != 0
+	return term.IsTerminal(int(os.Stderr.Fd()))
 }
 
 // progress renders one scan pass's progress on stderr. On a TTY it
 // delegates to the progressbar library (spinner style when the total is
 // unknown, full bar with count/percent/rate/elapsed/ETA otherwise). When
 // stderr is not a TTY it emits no ANSI redraws: it prints a plain
-// one-line update no more often than every plainInterval.
+// one-line update as the pass starts, then no more often than every
+// plainInterval.
 //
-// All methods must be called from the main goroutine only. A nil
-// *progress is a valid no-display receiver: every method is a no-op,
-// so batched database flushes during the streaming pass can reuse the
-// update-pass helpers without rendering anything.
+// All methods must be called from the main goroutine only. On a TTY
+// the library also redraws a spinner from its own goroutine, several
+// times a second, so its count and elapsed time stay current while a
+// pass waits for its next item. A nil *progress is a valid
+// no-display receiver: every method is a no-op, so batched database
+// flushes during the streaming pass can reuse the update-pass helpers
+// without rendering anything.
 type progress struct {
 	label string
 	total int64 // -1 when unknown (walk pass)
@@ -53,10 +53,22 @@ type progress struct {
 
 func newProgress(label string, total int64) *progress {
 	p := &progress{label: label, total: total, start: time.Now()}
-	if !stderrIsTTY() {
+	if stderrIsTTY() {
+		p.bar = newBar(label, total)
+
 		return p
 	}
 
+	// Print the zero state at once: the first item may take minutes,
+	// and a pass must never look hung.
+	p.last = p.start
+	fmt.Fprintln(os.Stderr, p.plainLine())
+
+	return p
+}
+
+// newBar builds the TTY display for newProgress.
+func newBar(label string, total int64) *progressbar.ProgressBar {
 	opts := []progressbar.Option{
 		progressbar.OptionSetWriter(os.Stderr),
 		progressbar.OptionSetDescription(label),
@@ -81,9 +93,7 @@ func newProgress(label string, total int64) *progress {
 		)
 	}
 
-	p.bar = progressbar.NewOptions64(total, opts...)
-
-	return p
+	return progressbar.NewOptions64(total, opts...)
 }
 
 // increment records one completed item and refreshes the display.
@@ -106,26 +116,45 @@ func (p *progress) increment() {
 }
 
 // warnf prints a one-line warning to stderr without corrupting the bar.
+// The whole message is escaped like a report's path columns, so a path
+// holding a newline cannot split the warning.
 func (p *progress) warnf(format string, args ...any) {
 	if p == nil {
 		return
 	}
 
+	msg := escapePath(fmt.Sprintf(format, args...))
+
+	if p.bar != nil && p.total < 0 {
+		// The library also redraws a spinner from its own goroutine, so
+		// a direct write could land inside a redraw. The bar prints the
+		// warning itself, just before its next redraw.
+		_, _ = progressbar.Bprintln(p.bar, msg)
+
+		return
+	}
+
 	if p.bar != nil {
 		_ = p.bar.Clear()
 	}
 
-	fmt.Fprintf(os.Stderr, format+"\n", args...)
+	fmt.Fprintln(os.Stderr, msg)
 }
 
-// finish terminates the pass's display.
+// finish terminates the pass's display. A bar whose pass stopped short
+// of its total, as an interrupted one does, is left as last drawn; the
+// library's Finish would fill it up.
 func (p *progress) finish() {
 	if p == nil {
 		return
 	}
 
 	if p.bar != nil {
+		if p.total >= 0 && p.count < p.total {
+			_ = p.bar.Exit()
+		} else {
 			_ = p.bar.Finish()
+		}
 
 		fmt.Fprintln(os.Stderr)
 

progress_test.go

@@ -0,0 +1,189 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+)
+
+// spinnerIdle comfortably outlasts the 100ms interval at which the
+// progressbar library redraws a spinner from its own goroutine.
+const spinnerIdle = 500 * time.Millisecond
+
+//nolint:paralleltest // replaces the process-wide os.Stderr
+func TestStderrIsTTYFalseForNonTerminals(t *testing.T) {
+	r, pipe, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	regular, err := os.Create(filepath.Join(t.TempDir(), "stderr"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	devNull, err := os.OpenFile(os.DevNull, os.O_WRONLY, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	saved := os.Stderr
+
+	t.Cleanup(func() {
+		os.Stderr = saved
+
+		for _, f := range []*os.File{r, pipe, regular, devNull} {
+			_ = f.Close()
+		}
+	})
+
+	cases := map[string]*os.File{
+		"a pipe":         pipe,
+		"a regular file": regular,
+		os.DevNull:       devNull,
+	}
+
+	for name, f := range cases {
+		os.Stderr = f
+
+		if stderrIsTTY() {
+			t.Errorf("stderrIsTTY() = true with stderr on %s", name)
+		}
+	}
+}
+
+// TestNewProgressPrintsBeforeFirstItem checks that each pass shows its
+// zero state the moment it starts when stderr is not a terminal, and
+// that the next line still waits for plainInterval.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
+func TestNewProgressPrintsBeforeFirstItem(t *testing.T) {
+	stderr := captureStderr(t)
+
+	newProgress("walk", -1).increment()
+	newProgress("hash", 10).increment()
+
+	want := "walk: 0 files, elapsed 0s\n" +
+		"hash: [0/10] 0% 0 files/s elapsed 0s eta ?\n"
+	if got := stderr(); got != want {
+		t.Errorf("stderr = %q, want %q", got, want)
+	}
+}
+
+// newWalkSpinner returns the walk pass's terminal display, writing to
+// os.Stderr whether or not it is a terminal, and stops the library's
+// redraws when the test ends.
+func newWalkSpinner(t *testing.T) *progress {
+	t.Helper()
+
+	p := &progress{
+		label: "walk", total: -1, start: time.Now(),
+		bar: newBar("walk", -1),
+	}
+	t.Cleanup(p.finish)
+
+	return p
+}
+
+// TestProgressWarningsOnOwnLines drives the terminal display of the walk
+// pass through a run of warnings with no items between them, as when the
+// walk meets many unreadable paths, for several of the spinner's
+// redraws: every warning must land on a line of its own, never inside a
+// redraw.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
+func TestProgressWarningsOnOwnLines(t *testing.T) {
+	stderr := captureStderr(t)
+	p := newWalkSpinner(t)
+
+	// No pause between warnings: one written straight to stderr is
+	// garbled only if a redraw lands while it is being written.
+	issued := 0
+	for start := time.Now(); time.Since(start) < spinnerIdle; issued++ {
+		p.warnf("warning")
+	}
+
+	// The spinner prints the warnings at its next redraw.
+	time.Sleep(spinnerIdle)
+
+	// A terminal shows each line as the text after its last carriage
+	// return.
+	shown := 0
+
+	for line := range strings.SplitSeq(stderr(), "\n") {
+		if !strings.Contains(line, "warning") {
+			continue
+		}
+
+		shown++
+
+		if text := line[strings.LastIndex(line, "\r")+1:]; text != "warning" {
+			t.Errorf("terminal shows %q, want %q", text, "warning")
+		}
+	}
+
+	if shown != issued {
+		t.Errorf("%d warning lines, want %d", shown, issued)
+	}
+}
+
+// TestSpinnerShowsCountAfterBurst checks that once a burst of items
+// faster than the redraw limit is over, the walk display shows every
+// item completed while it waits for the next one.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
+func TestSpinnerShowsCountAfterBurst(t *testing.T) {
+	stderr := captureStderr(t)
+	p := newWalkSpinner(t)
+
+	for range 50 {
+		p.increment()
+	}
+
+	time.Sleep(spinnerIdle)
+
+	if shown := lastFrame(stderr()); !strings.Contains(shown, "(50/-,") {
+		t.Errorf("terminal shows %q, want a count of 50", shown)
+	}
+}
+
+// lastFrame returns what a terminal shows of the frames a bar drew: the
+// last one. The library starts each frame with a carriage return and
+// erases the previous one with spaces first.
+func lastFrame(out string) string {
+	var shown string
+
+	for frame := range strings.SplitSeq(out, "\r") {
+		if strings.TrimSpace(frame) != "" {
+			shown = frame
+		}
+	}
+
+	return shown
+}
+
+// TestBarStoppedShortKeepsCount checks that the terminal display of a
+// pass that stops before its total, as an interrupted one does, is left
+// as last drawn instead of being filled up.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
+func TestBarStoppedShortKeepsCount(t *testing.T) {
+	stderr := captureStderr(t)
+	p := &progress{
+		label: "hash", total: 10, start: time.Now(),
+		bar: newBar("hash", 10),
+	}
+
+	p.increment()
+
+	// Past the redraw limit, so the bar draws the next count.
+	time.Sleep(2 * barThrottle)
+	p.increment()
+	p.finish()
+
+	if shown := lastFrame(stderr()); !strings.Contains(shown, "(2/10,") {
+		t.Errorf("terminal shows %q, want a count of 2 of 10", shown)
+	}
+}

report.go

@@ -4,8 +4,8 @@ import (
 	"bufio"
 	"context"
 	"fmt"
+	"io"
 	"os"
-	"slices"
 	"strings"
 )
 
@@ -17,82 +17,70 @@ const ioBufSize = 1 << 20
 const minGroupSize = 2
 
 // scanRec is one file record from the database. The signature (size,
-// head, tail) is the duplicate key; mtime is informational only and
-// used by scan for change detection.
+// head, tail, content) is the duplicate key; mtime is informational
+// only and used by scan for change detection.
 type scanRec struct {
 	size    int64
 	mtime   int64
 	head    string
 	tail    string
+	content string
 	path    string
 }
 
-// loadRecords opens the database and reads every file record for the
-// report and trees subcommands. Any database problem — including a
-// missing database — is fatal. The error is returned rather than
-// exiting, so that the deferred close — which checkpoints the SQLite
-// WAL — always runs; the database is closed before the caller formats
-// its output, so it stays closed even if that output fails.
-func loadRecords(ctx context.Context) ([]scanRec, error) {
+// runReport implements the report subcommand: it prints the file-level
+// duplicates report as TSV on stdout. SQLite groups and orders the
+// records, and each row is written as it is read, so no group is held
+// in memory. It never touches the scanned filesystem; its only I/O is
+// the database (with SQLite's temporary sort file), stdout, and stderr.
+// Any database problem, including a missing database, is fatal.
+func runReport(ctx context.Context, stdout io.Writer) error {
 	dbPath := databasePath()
 
 	db, err := openReportDatabase(ctx, dbPath)
 	if err != nil {
-		return nil, err
+		return err
 	}
 
 	defer func() { _ = db.Close() }()
 
-	recs, err := loadFileRows(ctx, db)
-	if err != nil {
-		return nil, fmt.Errorf("database %s: %w", dbPath, err)
-	}
-
-	return recs, nil
-}
-
-// dupeGroup is one set of candidate-duplicate files: identical size,
-// head hash, and tail hash. paths is sorted lexicographically; the
-// first entry is the group's "first", the rest are dupes.
-type dupeGroup struct {
-	size  int64
-	paths []string
-}
-
-// runReport implements the report subcommand: it reads every record
-// from the database and prints the file-level duplicates report as TSV
-// on stdout. It never touches the scanned filesystem; its only I/O is
-// the database, stdout, and stderr.
-func runReport(ctx context.Context) error {
-	recs, err := loadRecords(ctx)
-	if err != nil {
-		return err
-	}
-
-	dupes := collectDupeGroups(recs)
-
-	out := bufio.NewWriterSize(os.Stdout, ioBufSize)
+	out := bufio.NewWriterSize(stdout, ioBufSize)
 
 	_, err = fmt.Fprintln(out, "first\tdupe\tsize")
 	if err != nil {
 		return fmt.Errorf("write stdout: %w", err)
 	}
 
-	dupeFiles := 0
+	var (
+		groups, dupeFiles int
+		reclaimable       int64
+		writeErr          error
+	)
 
-	var reclaimable int64
+	records, err := loadDupeRows(ctx, db,
+		func(first, path string, size int64) error {
+			// A group's first path is its first row; every other
+			// path is a dupe.
+			if path == first {
+				groups++
 
-	for _, g := range dupes {
-		for _, p := range g.paths[1:] {
-			_, err = fmt.Fprintf(out, "%s\t%s\t%d\n",
-				g.paths[0], p, g.size)
-			if err != nil {
-				return fmt.Errorf("write stdout: %w", err)
+				return nil
 			}
 
+			_, writeErr = fmt.Fprintf(out, "%s\t%s\t%d\n",
+				escapePath(first), escapePath(path), size)
 			dupeFiles++
-			reclaimable += g.size
+			reclaimable += size
+
+			return writeErr
+		})
+
+	if writeErr != nil {
+		return fmt.Errorf("write stdout: %w", writeErr)
 	}
+
+	if err != nil {
+		return fmt.Errorf("database %s: %w", dbPath, err)
 	}
 
 	err = out.Flush()
@@ -103,54 +91,24 @@ func runReport(ctx context.Context) error {
 	fmt.Fprintf(os.Stderr,
 		"report: %d records read, %d duplicate groups, %d dupe files, "+
 			"%s reclaimable\n",
-		len(recs), len(dupes), dupeFiles, humanBytes(reclaimable))
+		records, groups, dupeFiles, humanBytes(reclaimable))
 
 	return nil
 }
 
-// collectDupeGroups groups records by signature and returns every group
-// with two or more paths, each group's paths sorted lexicographically,
-// groups ordered by size descending then by first path ascending.
-func collectDupeGroups(recs []scanRec) []dupeGroup {
-	groups := make(map[fileSig][]string)
-
-	for _, r := range recs {
-		// A record without hashes (its size was unique when last
-		// scanned) has unknown content and is never reported as a
-		// duplicate.
-		if r.head == "" {
-			continue
+// escapePath returns a path as it is written in a report column (README
+// "Report output format"): a backslash, tab, newline or carriage return
+// becomes \\, \t, \n or \r, and every other byte is kept as it is.
+// Grouping and sorting use the raw path, never this form.
+func escapePath(p string) string {
+	// Most paths need no escaping; skip building a replacer for them.
+	if !strings.ContainsAny(p, "\\\t\n\r") {
+		return p
 	}
 
-		k := fileSig{size: r.size, head: r.head, tail: r.tail}
-		groups[k] = append(groups[k], r.path)
-	}
-
-	var dupes []dupeGroup
-
-	for k, paths := range groups {
-		if len(paths) < minGroupSize {
-			continue
-		}
-
-		slices.Sort(paths)
-		dupes = append(dupes, dupeGroup{size: k.size, paths: paths})
-	}
-
-	// Biggest reclaimable space first; ties broken by first path.
-	slices.SortFunc(dupes, func(a, b dupeGroup) int {
-		if a.size != b.size {
-			if a.size > b.size {
-				return -1
-			}
-
-			return 1
-		}
-
-		return strings.Compare(a.paths[0], b.paths[0])
-	})
-
-	return dupes
+	return strings.NewReplacer(
+		`\`, `\\`, "\t", `\t`, "\n", `\n`, "\r", `\r`,
+	).Replace(p)
 }
 
 // humanBytes formats a byte count in human units (binary prefixes).

report_test.go

@@ -1,26 +1,269 @@
 package main
 
 import (
+	"bytes"
+	"database/sql"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
 	"slices"
+	"strings"
 	"testing"
 )
 
-func TestCollectDupeGroups(t *testing.T) {
+// awkwardDir is a directory name holding every byte the reports escape.
+const awkwardDir = "/d/\tone\ntwo\rthree\\four"
+
+// awkwardPairRecs is a duplicate pair in sibling directories /d/A and
+// awkwardDir. A raw tab sorts before "A" but its escaped form `\t`
+// sorts after it, so awkwardDir coming first shows that sorting uses
+// the raw path.
+func awkwardPairRecs() []scanRec {
+	return []scanRec{
+		{size: 5, head: "h", tail: "t", content: "c", path: "/d/A/f"},
+		{size: 5, head: "h", tail: "t", content: "c", path: awkwardDir + "/f"},
+	}
+}
+
+// seedDatabase writes recs into a fresh database and returns its path.
+func seedDatabase(t *testing.T, recs []scanRec) string {
+	t.Helper()
+
+	path := testDBPath(t)
+
+	db, err := openScanDatabase(t.Context(), path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = applyChanges(t.Context(), db, recs, nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = db.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	return path
+}
+
+// dupeGroup is one duplicate group as report reads it: the size, and
+// the paths in report order, first path first.
+type dupeGroup struct {
+	size  int64
+	paths []string
+}
+
+// dupeGroups returns the duplicate groups report reads from db, in
+// report order.
+func dupeGroups(t *testing.T, db *sql.DB) []dupeGroup {
+	t.Helper()
+
+	var groups []dupeGroup
+
+	_, err := loadDupeRows(t.Context(), db,
+		func(first, path string, size int64) error {
+			if path == first {
+				groups = append(groups, dupeGroup{size: size})
+			}
+
+			g := &groups[len(groups)-1]
+			g.paths = append(g.paths, path)
+
+			return nil
+		})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	return groups
+}
+
+// dupeGroupsOf writes recs into a fresh database and returns the
+// duplicate groups report reads from it.
+func dupeGroupsOf(t *testing.T, recs []scanRec) []dupeGroup {
+	t.Helper()
+
+	db := openTestDB(t)
+
+	err := applyChanges(t.Context(), db, recs, nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	return dupeGroups(t, db)
+}
+
+func TestRunReportEscapesPaths(t *testing.T) {
+	t.Setenv(databaseEnv, seedDatabase(t, awkwardPairRecs()))
+
+	var stdout, stderr bytes.Buffer
+
+	code := run([]string{cmdReport}, &stdout, &stderr)
+	if code != exitOK {
+		t.Fatalf("run(report) = %d, want %d; stderr: %s",
+			code, exitOK, stderr.String())
+	}
+
+	want := "first\tdupe\tsize\n" +
+		`/d/\tone\ntwo\rthree\\four/f` + "\t/d/A/f\t5\n"
+	if got := stdout.String(); got != want {
+		t.Errorf("stdout = %q, want %q", got, want)
+	}
+}
+
+func TestReportStdoutFailsWhileReading(t *testing.T) {
+	// Each row holds two paths longer than dir, so the report is more
+	// than twice the stdout buffer and stdout fails while rows are
+	// still being read, not at the final flush.
+	dir := "/" + strings.Repeat("d", 4096)
+
+	recs := make([]scanRec, ioBufSize/len(dir))
+	for i := range recs {
+		recs[i] = scanRec{
+			size: 1, head: "h", tail: "t", content: "c",
+			path: fmt.Sprintf("%s/%d", dir, i),
+		}
+	}
+
+	t.Setenv(databaseEnv, seedDatabase(t, recs))
+
+	err := runReport(t.Context(), failingWriter{})
+	if !errors.Is(err, errWriteFailed) ||
+		!strings.HasPrefix(err.Error(), "write stdout: ") {
+		t.Errorf("error = %v, want write stdout: %v", err, errWriteFailed)
+	}
+}
+
+func TestRunReportsIgnoreInsertionOrder(t *testing.T) {
+	// README §Constraints: identical database contents give identical
+	// output, whatever order the records were inserted in.
+	recs := append(smokeTreeRecs(), awkwardPairRecs()...)
+	recs = append(recs,
+		scanRec{size: 50, head: "b", tail: "b", content: "b", path: "/y/2"},
+		scanRec{size: 50, head: "b", tail: "b", content: "b", path: "/y/1"},
+		scanRec{size: 50, head: "a", tail: "a", content: "a", path: "/x/2"},
+		scanRec{size: 50, head: "a", tail: "a", content: "a", path: "/x/1"},
+		scanRec{size: 50, path: "/x/unhashed"},
+	)
+
+	reversed := slices.Clone(recs)
+	slices.Reverse(reversed)
+
+	for _, name := range []string{cmdReport, cmdTrees} {
+		t.Run(name, func(t *testing.T) {
+			t.Setenv(databaseEnv, seedDatabase(t, recs))
+
+			forward := runStdout(t, name)
+
+			t.Setenv(databaseEnv, seedDatabase(t, reversed))
+
+			backward := runStdout(t, name)
+
+			if strings.Count(forward, "\n") < 3 {
+				t.Errorf("stdout = %q, want at least two rows", forward)
+			}
+
+			if forward != backward {
+				t.Errorf("stdout depends on insertion order: %q vs %q",
+					forward, backward)
+			}
+		})
+	}
+}
+
+// runStdout runs the subcommand name and returns its stdout, failing
+// the test unless it succeeds.
+func runStdout(t *testing.T, name string) string {
+	t.Helper()
+
+	var stdout, stderr bytes.Buffer
+
+	code := run([]string{name}, &stdout, &stderr)
+	if code != exitOK {
+		t.Fatalf("run(%s) = %d, want %d; stderr: %s",
+			name, code, exitOK, stderr.String())
+	}
+
+	return stdout.String()
+}
+
+func TestEscapePath(t *testing.T) {
+	t.Parallel()
+
+	cases := map[string]string{
+		"/srv/plain":    "/srv/plain",
+		"/a\tb":         `/a\tb`,
+		"/a\nb":         `/a\nb`,
+		"/a\rb":         `/a\rb`,
+		`/a\b`:          `/a\\b`,
+		`/a\tb`:         `/a\\tb`,
+		"/not-utf8\xff": "/not-utf8\xff",
+	}
+	for in, want := range cases {
+		if got := escapePath(in); got != want {
+			t.Errorf("escapePath(%q) = %q, want %q", in, got, want)
+		}
+	}
+}
+
+// TestWarnfEscapes checks that a warning naming a path that holds a
+// newline is still one line.
+//
+//nolint:paralleltest // replaces the process-wide os.Stderr
+func TestWarnfEscapes(t *testing.T) {
+	f, err := os.Create(filepath.Join(t.TempDir(), "stderr"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	saved := os.Stderr
+	os.Stderr = f
+
+	t.Cleanup(func() {
+		os.Stderr = saved
+
+		_ = f.Close()
+	})
+
+	(&progress{}).warnf("stat %s: %s", "/d/a\nb", "gone")
+
+	_, err = f.Seek(0, io.SeekStart)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := io.ReadAll(f)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	want := `stat /d/a\nb: gone` + "\n"
+	if string(got) != want {
+		t.Errorf("warning = %q, want %q", got, want)
+	}
+}
+
+func TestDupeGroups(t *testing.T) {
 	t.Parallel()
 
 	recs := []scanRec{
-		{size: 100, head: "h", tail: "t", path: "/z/b"},
-		{size: 100, head: "h", tail: "t", path: "/z/a"},
-		{size: 100, head: "h", tail: "t", path: "/z/c"},
-		{size: 4000, head: "H", tail: "T", path: "/big/2"},
-		{size: 4000, head: "H", tail: "T", path: "/big/1"},
+		{size: 100, head: "h", tail: "t", content: "c", path: "/z/b"},
+		{size: 100, head: "h", tail: "t", content: "c", path: "/z/a"},
+		{size: 100, head: "h", tail: "t", content: "c", path: "/z/c"},
+		{size: 4000, head: "H", tail: "T", content: "C", path: "/big/2"},
+		{size: 4000, head: "H", tail: "T", content: "C", path: "/big/1"},
 		// Same size as the /z group but a different head hash.
-		{size: 100, head: "other", tail: "t", path: "/z/d"},
+		{size: 100, head: "other", tail: "t", content: "c", path: "/z/d"},
 		// A singleton signature must not form a group.
-		{size: 7, head: "u", tail: "u", path: "/lonely"},
+		{size: 7, head: "u", tail: "u", content: "u", path: "/lonely"},
 	}
 
-	groups := collectDupeGroups(recs)
+	groups := dupeGroupsOf(t, recs)
 	if len(groups) != 2 {
 		t.Fatalf("len(groups) = %d, want 2", len(groups))
 	}
@@ -38,33 +281,61 @@ func TestCollectDupeGroups(t *testing.T) {
 	}
 }
 
-func TestCollectDupeGroupsMtimeExcluded(t *testing.T) {
+func TestDupeGroupsContentSeparates(t *testing.T) {
+	t.Parallel()
+
+	// Same size, head, and tail, but different content hashes: the final
+	// rung keeps them apart, so no group forms. Matching content groups.
+	// Records without a content hash never group, not even with each
+	// other.
+	recs := []scanRec{
+		{size: 100, head: "h", tail: "t", content: "c1", path: "/a"},
+		{size: 100, head: "h", tail: "t", content: "c2", path: "/b"},
+		{size: 100, head: "h", tail: "t", content: "c1", path: "/c"},
+		{size: 100, head: "h", tail: "t", path: "/d"},
+		{size: 100, head: "h", tail: "t", path: "/e"},
+	}
+
+	groups := dupeGroupsOf(t, recs)
+	if len(groups) != 1 {
+		t.Fatalf("len(groups) = %d, want 1 (only the matching content)",
+			len(groups))
+	}
+
+	if !slices.Equal(groups[0].paths, []string{"/a", "/c"}) {
+		t.Errorf("group paths = %q, want /a /c", groups[0].paths)
+	}
+}
+
+func TestDupeGroupsMtimeExcluded(t *testing.T) {
 	t.Parallel()
 
 	// mtime is informational only; records differing only in mtime
 	// still group together.
 	recs := []scanRec{
-		{size: 9, mtime: 100, head: "h", tail: "t", path: "/m/1"},
-		{size: 9, mtime: 200, head: "h", tail: "t", path: "/m/2"},
+		{size: 9, mtime: 100, head: "h", tail: "t", content: "c", path: "/m/1"},
+		{size: 9, mtime: 200, head: "h", tail: "t", content: "c", path: "/m/2"},
 	}
 
-	groups := collectDupeGroups(recs)
+	groups := dupeGroupsOf(t, recs)
 	if len(groups) != 1 {
 		t.Fatalf("len(groups) = %d, want 1", len(groups))
 	}
 }
 
-func TestCollectDupeGroupsTieBreak(t *testing.T) {
+func TestDupeGroupsTieBreak(t *testing.T) {
 	t.Parallel()
 
+	// The hashes sort opposite to the first paths, so ordering the
+	// groups by hash instead of by first path fails this test.
 	recs := []scanRec{
-		{size: 50, head: "b", tail: "b", path: "/beta/2"},
-		{size: 50, head: "b", tail: "b", path: "/beta/1"},
-		{size: 50, head: "a", tail: "a", path: "/alpha/2"},
-		{size: 50, head: "a", tail: "a", path: "/alpha/1"},
+		{size: 50, head: "a", tail: "a", content: "a", path: "/beta/2"},
+		{size: 50, head: "a", tail: "a", content: "a", path: "/beta/1"},
+		{size: 50, head: "b", tail: "b", content: "b", path: "/alpha/2"},
+		{size: 50, head: "b", tail: "b", content: "b", path: "/alpha/1"},
 	}
 
-	groups := collectDupeGroups(recs)
+	groups := dupeGroupsOf(t, recs)
 	if len(groups) != 2 {
 		t.Fatalf("len(groups) = %d, want 2", len(groups))
 	}
@@ -76,22 +347,22 @@ func TestCollectDupeGroupsTieBreak(t *testing.T) {
 	}
 }
 
-func TestCollectDupeGroupsDeterministic(t *testing.T) {
+func TestDupeGroupsDeterministic(t *testing.T) {
 	t.Parallel()
 
 	recs := []scanRec{
-		{size: 1, head: "a", tail: "a", path: "/p/1"},
-		{size: 1, head: "a", tail: "a", path: "/p/2"},
-		{size: 2, head: "b", tail: "b", path: "/q/1"},
-		{size: 2, head: "b", tail: "b", path: "/q/2"},
+		{size: 1, head: "a", tail: "a", content: "a", path: "/p/1"},
+		{size: 1, head: "a", tail: "a", content: "a", path: "/p/2"},
+		{size: 2, head: "b", tail: "b", content: "b", path: "/q/1"},
+		{size: 2, head: "b", tail: "b", content: "b", path: "/q/2"},
 	}
 
-	forward := collectDupeGroups(recs)
+	forward := dupeGroupsOf(t, recs)
 
 	reversed := slices.Clone(recs)
 	slices.Reverse(reversed)
 
-	backward := collectDupeGroups(reversed)
+	backward := dupeGroupsOf(t, reversed)
 	if !slices.EqualFunc(forward, backward, func(a, b dupeGroup) bool {
 		return a.size == b.size && slices.Equal(a.paths, b.paths)
 	}) {

scan.go

@@ -6,9 +6,12 @@ import (
 	"crypto/sha256"
 	"database/sql"
 	"encoding/hex"
+	"errors"
 	"fmt"
+	"io"
 	"io/fs"
 	"os"
+	"os/signal"
 	"path/filepath"
 	"slices"
 	"strings"
@@ -16,13 +19,49 @@ import (
 	"syscall"
 )
 
-// chunk is the number of bytes hashed from each end of a file.
-const chunk = 1024
+// The duplicate ladder (see hashSignature and README "Duplicate
+// detection"). A same-size candidate below headTailMin is hashed in
+// full and compared directly; a larger one is separated first by the
+// hashes of its end windows, then by a content hash that is exact below
+// wholeFileMax and deliberately sampled at or above it. The hash phase
+// reads only the end windows of a larger file; the content phase reads
+// it for its content hash only once its size, head, and tail match
+// another file's.
+
+// headTailMin is the size threshold for the end-window gate. A file
+// smaller than this is hashed in full directly, with no separate head
+// and tail step: its head, tail, and content all carry the whole-file
+// hash. A file this size or larger is separated first by its end
+// windows.
+const headTailMin = 10 * 1024 * 1024
+
+// headTailWindow is the number of bytes hashed from each end of a file
+// at or above headTailMin (the head and tail rungs). Because
+// headTailMin is far larger than two windows, the head and tail windows
+// never overlap.
+const headTailWindow = 64 * 1024
+
+// wholeFileMax is the size boundary between the two content rungs: a
+// file strictly smaller than this is content-hashed in full; a file
+// this size or larger is content-hashed by sampling.
+const wholeFileMax = 50 * 1024 * 1024
+
+// sampleStride is the spacing between content samples for large files:
+// one window is read at each gigabyte-aligned offset (0, 1 GiB, ...).
+const sampleStride = 1024 * 1024 * 1024
+
+// sampleWindow is the number of bytes read at each large-file sample
+// offset, truncated at end of file.
+const sampleWindow = 1024 * 1024
 
 // workQueueDepth bounds the job and result channels feeding the walk
 // and hash worker pools.
 const workQueueDepth = 1024
 
+// errInterrupted reports a scan stopped by SIGINT or SIGTERM. runScan
+// has already printed its line, so run prints nothing more.
+var errInterrupted = errors.New("scan interrupted")
+
 // fileRec carries one statted file between the scan phases. dev and
 // ino identify the underlying inode so hard-linked paths can share
 // one read; both are zero when the platform exposes no inode.
@@ -44,23 +83,26 @@ type fileMeta struct {
 	hashed bool
 }
 
-// runScan implements the scan subcommand: three sequential phases —
-// walk (which stats each file as it is discovered), hash, update —
-// that synchronize the persistent database with the filesystem state
-// under the PATH operands. Only files whose size at least one other
-// file shares are ever hashed: a size-unique file cannot be a
-// duplicate. Flag parsing and the at-least-one-operand check are done
-// by cobra. Errors are returned rather than exiting, so that the
-// deferred close — which checkpoints the SQLite WAL — always runs.
-// Cancelling ctx unwinds the worker pools and aborts the scan with the
-// context's error.
+// runScan implements the scan subcommand: four sequential phases —
+// walk (which stats each file as it is discovered), hash, update,
+// content — that synchronize the persistent database with the
+// filesystem state under the PATH operands. Only files whose size at
+// least one other file shares are ever hashed: a size-unique file
+// cannot be a duplicate. A file of headTailMin or more gets its content
+// hash only when its size, head, and tail match another file's. Flag
+// parsing and the at-least-one-operand check are done by cobra. The
+// scan holds the lock on the database for its whole run, so a second
+// scan fails before it walks the filesystem or opens the database.
+// Errors are returned rather than exiting, so that the deferred close —
+// which takes the database out of WAL mode — always runs, and the lock
+// is released after it. When ctx is cancelled, as by the SIGINT or
+// SIGTERM that interruptContext catches, the scan keeps what it has
+// hashed (see syncScan), prints how many files its walk reached, and
+// returns errInterrupted. workers must be at least 1; the scan command
+// rejects anything less.
 func runScan(ctx context.Context, roots []string, workers int,
 	oneFS bool,
 ) error {
-	if workers < 1 {
-		workers = 1
-	}
-
 	roots, err := resolveRoots(roots)
 	if err != nil {
 		return err
@@ -68,14 +110,31 @@ func runScan(ctx context.Context, roots []string, workers int,
 
 	dbPath := databasePath()
 
-	db, err := openScanDatabase(ctx, dbPath)
+	lock, err := lockScanDatabase(dbPath)
 	if err != nil {
 		return err
 	}
 
-	defer func() { _ = db.Close() }()
+	defer func() { _ = lock.Close() }()
+
+	db, err := openScanDatabase(ctx, dbPath)
+	if err != nil && ctx.Err() != nil {
+		// Interrupted while opening; SQLite may report that with an
+		// error of its own rather than the context's.
+		return interrupted(0)
+	}
+
+	if err != nil {
+		return err
+	}
+
+	defer closeScanDatabase(ctx, db, dbPath)
 
 	st, err := syncScan(ctx, db, roots, workers, oneFS)
+	if errors.Is(err, context.Canceled) {
+		return interrupted(st.walked)
+	}
+
 	if err != nil {
 		return fmt.Errorf("update database %s: %w", dbPath, err)
 	}
@@ -89,6 +148,34 @@ func runScan(ctx context.Context, roots []string, workers int,
 	return nil
 }
 
+// interruptContext returns a copy of ctx that the first SIGINT or
+// SIGTERM cancels; the scan command runs the scan under it. stop
+// releases the signals.
+func interruptContext(ctx context.Context) (context.Context, func()) {
+	// A SIGINT ignored from the start, as by a script's background job,
+	// stays ignored.
+	signals := []os.Signal{syscall.SIGTERM}
+	if !signal.Ignored(syscall.SIGINT) {
+		signals = append(signals, syscall.SIGINT)
+	}
+
+	ctx, stop := signal.NotifyContext(ctx, signals...)
+
+	// Stopping restores the default handling, so a second signal ends
+	// the process at once.
+	context.AfterFunc(ctx, stop)
+
+	return ctx, stop
+}
+
+// interrupted prints the line for a scan stopped by a signal after its
+// walk reached walked files, and returns errInterrupted.
+func interrupted(walked int) error {
+	fmt.Fprintf(os.Stderr, "scan: interrupted after %d files\n", walked)
+
+	return errInterrupted
+}
+
 // resolveRoots converts each PATH operand to an absolute, lexically
 // cleaned path (symlinks are not resolved) and verifies that it
 // exists. Database records are keyed by absolute path, so scan results
@@ -142,8 +229,9 @@ func pruneRoots(roots []string) []string {
 }
 
 // scanStats summarizes one scan's database synchronization for the
-// final stderr summary.
+// final stderr summary, or for the line an interrupted scan prints.
 type scanStats struct {
+	walked    int // files the walk reached
 	added     int
 	updated   int
 	removed   int
@@ -165,49 +253,106 @@ type scanState struct {
 	st        scanStats
 }
 
-// syncScan synchronizes the database with the filesystem under roots
-// in three sequential phases: walk (enumerate and stat every file,
-// building a complete size census), hash (read only the new or
-// changed — or previously unhashed — files whose size at least one
-// other file shares, committing results in batches as they arrive),
-// and update (record the size-unique files without reading them, and
-// delete the records the scan no longer verifies). Records outside
-// the roots are never touched.
+// syncScan synchronizes the database with the filesystem under roots;
+// see runPhases. When ctx is cancelled, as by an interrupt, it commits
+// the hashed records still waiting in the batch, starts no other write
+// or deletion, and returns the cancellation.
 func syncScan(ctx context.Context, db *sql.DB, roots []string,
 	workers int, oneFS bool,
 ) (scanStats, error) {
-	roots = pruneRoots(roots)
-
 	s := &scanState{db: db}
 
+	err := s.runPhases(ctx, roots, workers, oneFS)
+	if err == nil || ctx.Err() == nil {
+		return s.st, err
+	}
+
+	// The one write made after the cancellation, so it cannot use ctx.
+	err = applyChanges(context.WithoutCancel(ctx), db, s.batch, nil, nil)
+	if err != nil {
+		return s.st, err
+	}
+
+	return s.st, ctx.Err()
+}
+
+// runPhases synchronizes the database with the filesystem under roots
+// in four sequential phases: walk (enumerate and stat every file,
+// building a complete size census), hash (read only the new or
+// changed — or previously unhashed — files whose size at least one
+// other file shares, committing results in batches as they arrive),
+// update (record the size-unique files without reading them, and
+// delete the records the scan no longer verifies), and content (fill
+// in the content hash of every record of headTailMin or more whose
+// size, head, and tail match another record's). Records outside the
+// roots are never touched, except that the content phase fills in
+// their content hash. Operands the walk cannot start from are dropped
+// first, so the records beneath them count as outside the roots unless
+// they lie under another root.
+func (s *scanState) runPhases(ctx context.Context, roots []string,
+	workers int, oneFS bool,
+) error {
+	// Types are checked before pruning so that an operand under a
+	// dropped one is still scanned, not dropped as lying under it.
+	roots = pruneRoots(s.walkableRoots(roots))
+
 	err := s.loadIndex(ctx, roots)
 	if err != nil {
-		return s.st, err
+		return err
 	}
 
 	changed, unhashed := s.walkPhase(startWalk(ctx, roots, oneFS, workers))
 
 	// A cancelled walk stops early, so its size census covers only part
-	// of the roots, and every file it never reached looks vanished to
-	// the update phase. Defence in depth rather than the only barrier:
-	// that phase would today fail on its first BeginTx with the same
-	// cancelled context before deleting anything. But it is the barrier
-	// that survives a later decision to let an interrupted scan commit
-	// what it has, and it turns a confusing failure deep in the update
-	// phase into a clean abort at the phase boundary.
+	// of the roots, and every file it never reached would look vanished
+	// to the update phase. Stop before anything is written or deleted.
 	err = ctx.Err()
 	if err != nil {
-		return s.st, err
+		return err
 	}
 
 	s.partition(changed, unhashed)
 
 	err = s.hashPhase(ctx, workers)
 	if err != nil {
-		return s.st, err
+		return err
 	}
 
-	return s.st, s.updatePhase(ctx)
+	err = s.updatePhase(ctx)
+	if err != nil {
+		return err
+	}
+
+	return s.contentPhase(ctx, workers)
+}
+
+// walkableRoots returns the operands the walk can start from: regular
+// files, and directories not named .zfs. Every other operand is warned
+// about, counted as skipped, and dropped. A dropped operand is no
+// longer a root, so the records stored beneath it count as outside the
+// roots and are not deleted as unverified, unless it lies under another
+// root. An operand that fails lstat here is kept, and the walk warns
+// about it.
+func (s *scanState) walkableRoots(roots []string) []string {
+	kept := make([]string, 0, len(roots))
+
+	for _, root := range roots {
+		fi, err := os.Lstat(root)
+		if err == nil {
+			warn := operandWarning(root, fi)
+			if warn != "" {
+				s.st.skipped++
+
+				fmt.Fprintln(os.Stderr, escapePath(warn))
+
+				continue
+			}
+		}
+
+		kept = append(kept, root)
+	}
+
+	return kept
 }
 
 // loadIndex indexes the database records under the scan roots for
@@ -241,7 +386,8 @@ func (s *scanState) loadIndex(ctx context.Context, roots []string) error {
 
 // walkPhase drains the walk, appending every walked file's size to
 // the census and resolving what it can immediately: an unchanged file
-// whose record already has hashes needs nothing further. It returns
+// whose record already has hashes needs nothing from the hash phase
+// (the content phase may still fill in its content hash). It returns
 // the new-or-changed files and the unchanged files whose records lack
 // hashes; both remain candidates until the census decides whether
 // their sizes are shared.
@@ -262,6 +408,7 @@ func (s *scanState) walkPhase(
 		}
 
 		s.sizes = append(s.sizes, ev.rec.size)
+		s.st.walked++
 
 		prog.increment()
 
@@ -380,27 +527,40 @@ func sameInode(a, b fileRec) bool {
 	return (a.dev != 0 || a.ino != 0) && a.dev == b.dev && a.ino == b.ino
 }
 
-// hashPhase hashes every queued file with the worker pool — one read
-// per inode run, in inode order — committing completed records to the
-// database in batches as results arrive, so a long scan persists its
-// progress as it goes (an interrupted scan resumes cheaply: the next
-// run skips everything already recorded). The total counts actual
-// reads, so the bar shows a real ETA. A run that fails to hash is
-// warned about and skipped; stale records for its paths, if any, are
-// deleted by the update phase.
+// hashPhase hashes every queued file with hashSignature — the head and
+// tail of a file of headTailMin or more, the whole file below that —
+// committing completed records to the database in batches as results
+// arrive, so a long scan persists its progress as it goes (an
+// interrupted scan resumes cheaply: the next run skips everything
+// already recorded). A run that fails to hash is warned about and
+// skipped; stale records for its paths, if any, are deleted by the
+// update phase.
+func (s *scanState) hashPhase(ctx context.Context, workers int) error {
+	runs := hashRuns(s.toHash)
+	s.toHash = nil
+
+	return s.readRuns(ctx, workers, "hash", runs, hashSignature, s.recordRun)
+}
+
+// readRuns reads runs with the worker pool, one read per inode run, in
+// the order given, under a progress display named label. The workers
+// compute each run's hashes with hash, and each result goes to record;
+// a run that fails to read is warned about and counted as skipped
+// instead. The total counts actual reads, so the bar shows a real ETA.
 //
 // Returning early — a failed database write, or a cancelled scan — must
 // not strand the pool: the feeder would park forever on a full jobs
 // channel and every worker on a full results channel. The deferred stop
 // is what prevents that.
-func (s *scanState) hashPhase(ctx context.Context, workers int) error {
-	runs := hashRuns(s.toHash)
-	s.toHash = nil
-
-	pool := startHashPool(ctx, runs, workers)
+func (s *scanState) readRuns(ctx context.Context, workers int,
+	label string, runs [][]fileRec,
+	hash func(path string, size int64) (string, string, string, error),
+	record func(ctx context.Context, r hashResult) error,
+) error {
+	pool := startHashPool(ctx, runs, workers, hash)
 	defer pool.stop()
 
-	prog := newProgress("hash", int64(len(runs)))
+	prog := newProgress(label, int64(len(runs)))
 	defer prog.finish()
 
 	for range runs {
@@ -417,12 +577,12 @@ func (s *scanState) hashPhase(ctx context.Context, workers int) error {
 		if r.err != nil {
 			s.st.skipped += len(r.run)
 
-			prog.warnf("hash %s: %v", r.run[0].path, r.err)
+			prog.warnf("%s %s: %v", label, r.run[0].path, r.err)
 
 			continue
 		}
 
-		err := s.recordRun(ctx, r)
+		err := record(ctx, r)
 		if err != nil {
 			return err
 		}
@@ -443,18 +603,31 @@ func (s *scanState) recordRun(ctx context.Context, r hashResult) error {
 			mtime:   rec.mtime,
 			head:    r.head,
 			tail:    r.tail,
+			content: r.content,
 			path:    rec.path,
 		})
 	}
 
+	return s.commitFullBatch(ctx)
+}
+
+// commitFullBatch commits the running batch once it holds
+// updateBatchSize records. A batch that fails to commit is kept: the
+// commit fails when the scan is interrupted, and syncScan then commits
+// the batch itself.
+func (s *scanState) commitFullBatch(ctx context.Context) error {
 	if len(s.batch) < updateBatchSize {
 		return nil
 	}
 
 	err := applyBatch(ctx, s.db, s.batch, nil, nil)
+	if err != nil {
+		return err
+	}
+
 	s.batch = s.batch[:0]
 
-	return err
+	return nil
 }
 
 // updatePhase writes the scan's tail under one progress display: the
@@ -504,6 +677,147 @@ func (s *scanState) updatePhase(ctx context.Context) error {
 	return applyChanges(ctx, s.db, nil, deletes, prog)
 }
 
+// contentPhase fills in the content hash of every record of headTailMin
+// or more that lacks one and whose size, head, and tail equal another
+// record's, anywhere in the database: records from this scan and
+// records stored by earlier scans, inside or outside the roots. Only
+// such a file can still be a duplicate, so no other file of headTailMin
+// or more is read beyond its end windows. The files are read with the
+// hash phase's worker pool and their records written back in batches. A
+// failed read is warned about and counted as skipped; the record keeps
+// its empty content, so it is never grouped, and a later scan tries
+// again.
+func (s *scanState) contentPhase(ctx context.Context, workers int) error {
+	toRead, recs, err := s.contentCandidates(ctx)
+	if err != nil {
+		return err
+	}
+
+	err = s.readRuns(ctx, workers, "content", hashRuns(toRead),
+		hashContentOnly, func(ctx context.Context, r hashResult) error {
+			// Every path in the run keeps its record's head and tail
+			// and gains the one content hash read for the run.
+			for _, f := range r.run {
+				rec := recs[f.path]
+				rec.content = r.content
+				s.batch = append(s.batch, rec)
+			}
+
+			return s.commitFullBatch(ctx)
+		})
+	if err != nil {
+		return err
+	}
+
+	return applyChanges(ctx, s.db, s.batch, nil, nil)
+}
+
+// contentCandidates returns the files the content phase reads, and
+// their records by path. Every record contentCandidatesSQL returns has
+// its file checked with lstat, whether or not it already has a content
+// hash: a file that is gone, is no longer a regular file, or has
+// changed by the walk's rule keeps its record as it is and does not
+// count as a match for the others, and any other lstat error is warned
+// about and counted as skipped, with the same result. If such a record
+// has no content hash, it stays out of duplicate groups; if it has one,
+// it is still reported until a scan covering its own tree updates or
+// removes it. The files of a group that pass and have no content hash
+// are read only if at least minGroupSize of the group's files pass, so
+// a group whose other members are all stale costs no reads. Only the
+// records to be read are kept.
+func (s *scanState) contentCandidates(
+	ctx context.Context,
+) ([]fileRec, map[string]scanRec, error) {
+	// The query and the checks take real time on a large database;
+	// without a display the scan looks hung before the reads begin.
+	prog := newProgress("content", -1)
+	defer prog.finish()
+
+	var (
+		toRead []fileRec
+		first  scanRec   // the current group's first record
+		passed int       // the current group's files that passed the check
+		unread []fileRec // those of them without a content hash
+	)
+
+	recs := make(map[string]scanRec)
+
+	// endGroup queues the current group's files to read if at least
+	// minGroupSize of its files passed, and drops their records if not.
+	endGroup := func() {
+		if passed >= minGroupSize {
+			toRead = append(toRead, unread...)
+		} else {
+			for _, f := range unread {
+				delete(recs, f.path)
+			}
+		}
+
+		passed, unread = 0, nil
+	}
+
+	err := loadContentCandidates(ctx, s.db, func(r scanRec, hashed bool) {
+		prog.increment()
+
+		if r.size != first.size || r.head != first.head || r.tail != first.tail {
+			endGroup()
+
+			first = r
+		}
+
+		f, ok, err := unchangedFile(r)
+		if err != nil {
+			s.st.skipped++
+
+			prog.warnf("content %s: %v", r.path, err)
+		}
+
+		if !ok {
+			return
+		}
+
+		passed++
+
+		if !hashed {
+			unread = append(unread, f)
+			recs[r.path] = r
+		}
+	})
+	if err != nil {
+		return nil, nil, err
+	}
+
+	endGroup()
+
+	return toRead, recs, nil
+}
+
+// unchangedFile lstats the file r names and returns it for reading if
+// it is still the regular file r records: the same size, and an mtime
+// no newer than recorded (the walk's change rule). A file that is gone
+// or has changed reports false; any other lstat error is returned.
+func unchangedFile(r scanRec) (fileRec, bool, error) {
+	fi, err := os.Lstat(r.path)
+	if errors.Is(err, fs.ErrNotExist) {
+		return fileRec{}, false, nil
+	}
+
+	if err != nil {
+		return fileRec{}, false, err
+	}
+
+	if !fi.Mode().IsRegular() || fi.Size() != r.size ||
+		fi.ModTime().Unix() > r.mtime {
+		return fileRec{}, false, nil
+	}
+
+	dev, ino := inodeOfInfo(fi)
+
+	return fileRec{
+		path: r.path, size: r.size, mtime: r.mtime, dev: dev, ino: ino,
+	}, true, nil
+}
+
 // underAnyRoot reports whether path is any of the roots or lies under
 // one of them.
 func underAnyRoot(path string, roots []string) bool {
@@ -584,11 +898,43 @@ func sendEvent(ctx context.Context, events chan<- walkEvent,
 	}
 }
 
+// operandWarning returns the one-line warning for an operand the walk
+// does not start from, naming the path and what it is, or "" for one it
+// does: a regular file, or a directory not named .zfs. Symlinks are
+// never followed, including as operands.
+func operandWarning(root string, fi fs.FileInfo) string {
+	var kind string
+
+	switch mode := fi.Mode(); {
+	case mode.IsRegular():
+		return ""
+	case mode.IsDir():
+		if filepath.Base(root) != ".zfs" {
+			return ""
+		}
+
+		kind = ".zfs directory"
+	case mode&fs.ModeSymlink != 0:
+		kind = "symlink"
+	case mode&fs.ModeSocket != 0:
+		kind = "socket"
+	case mode&fs.ModeNamedPipe != 0:
+		kind = "FIFO"
+	case mode&fs.ModeDevice != 0:
+		kind = "device node"
+	default:
+		kind = "non-regular file"
+	}
+
+	return fmt.Sprintf("walk %s: skipping %s operand", root, kind)
+}
+
 // seedRoot turns one PATH operand into the walk's starting state: a
-// regular-file operand is statted and emitted directly, a directory
-// operand becomes an initial job, and a symlink or other non-regular
-// operand yields nothing (symlinks are never followed, including as
-// operands).
+// regular-file operand is statted and emitted directly, and a directory
+// operand becomes an initial job. walkableRoots has already dropped
+// every other operand. One that has changed into something else since
+// is warned about and skipped here; it is still a root, so the records
+// stored beneath it are deleted as unverified.
 func seedRoot(ctx context.Context, root string,
 	events chan<- walkEvent,
 ) []dirJob {
@@ -602,16 +948,19 @@ func seedRoot(ctx context.Context, root string,
 		return nil
 	}
 
-	switch {
-	case fi.IsDir():
-		if filepath.Base(root) == ".zfs" {
+	warn := operandWarning(root, fi)
+	if warn != "" {
+		sendEvent(ctx, events, walkEvent{warn: warn, fail: true})
+
 		return nil
 	}
 
+	if fi.IsDir() {
 		dev, ok := deviceOfInfo(fi)
 
 		return []dirJob{{path: root, rootDev: dev, rootDevOK: ok}}
-	case fi.Mode().IsRegular():
+	}
+
 	dev, ino := inodeOfInfo(fi)
 
 	sendEvent(ctx, events, walkEvent{rec: fileRec{
@@ -623,9 +972,6 @@ func seedRoot(ctx context.Context, root string,
 	}})
 
 	return nil
-	default:
-		return nil
-	}
 }
 
 // startWalkWorkers starts the walk worker pool. Each worker processes
@@ -724,6 +1070,12 @@ func walkOneDir(ctx context.Context, job dirJob, oneFS bool,
 	var subs []dirJob
 
 	for _, e := range entries {
+		// A cancelled scan wants nothing more from this directory: stop
+		// rather than lstat the rest of a large one.
+		if ctx.Err() != nil {
+			return nil
+		}
+
 		p := filepath.Join(job.path, e.Name())
 
 		if e.IsDir() {
@@ -834,12 +1186,15 @@ func inodeOfInfo(fi fs.FileInfo) (uint64, uint64) {
 	return statDev(st), st.Ino
 }
 
-// hashResult carries one inode run's head/tail hashes (or the error
-// that prevented hashing it) from the hash workers to the hash phase.
+// hashResult carries the hashes computed for one inode run (or the
+// error that prevented computing them) from the pool's workers to the
+// phase that started the pool: head, tail, and content from
+// hashSignature, content alone from hashContentOnly.
 type hashResult struct {
 	run     []fileRec
 	head    string
 	tail    string
+	content string
 	err     error
 }
 
@@ -856,10 +1211,10 @@ type hashPool struct {
 }
 
 // startHashPool starts the feeder and the workers over runs. Workers
-// hash each run's first path (all paths in a run are hard links to the
-// same inode) and write one result per run.
-func startHashPool(ctx context.Context, runs [][]fileRec,
-	workers int,
+// hash each run's first path with hash (all paths in a run are hard
+// links to the same inode) and write one result per run.
+func startHashPool(ctx context.Context, runs [][]fileRec, workers int,
+	hash func(path string, size int64) (string, string, string, error),
 ) *hashPool {
 	ctx, cancel := context.WithCancel(ctx)
 
@@ -871,7 +1226,7 @@ func startHashPool(ctx context.Context, runs [][]fileRec,
 	wg.Go(func() { feedHashJobs(ctx, runs, jobs) })
 
 	for range workers {
-		wg.Go(func() { hashWorker(ctx, jobs, results) })
+		wg.Go(func() { hashWorker(ctx, jobs, results, hash) })
 	}
 
 	done := make(chan struct{})
@@ -917,24 +1272,25 @@ func feedHashJobs(ctx context.Context, runs [][]fileRec,
 	}
 }
 
-// hashWorker hashes one inode run at a time until jobs is closed or the
-// scan is cancelled. A cancelled worker drops the runs still queued
-// instead of stopping its reads of jobs: the range must run out for the
-// pool to tear down, and reading a file nobody wants the hash of only
-// delays that.
+// hashWorker hashes one inode run at a time with hash until jobs is
+// closed or the scan is cancelled. A cancelled worker drops the runs
+// still queued instead of stopping its reads of jobs: the range must
+// run out for the pool to tear down, and reading a file nobody wants
+// the hash of only delays that.
 func hashWorker(ctx context.Context, jobs <-chan []fileRec,
 	results chan<- hashResult,
+	hash func(path string, size int64) (string, string, string, error),
 ) {
 	for run := range jobs {
 		if ctx.Err() != nil {
 			continue
 		}
 
-		head, tail, err := hashHeadTail(run[0].path, run[0].size)
+		head, tail, content, err := hash(run[0].path, run[0].size)
 
 		select {
 		case results <- hashResult{
-			run: run, head: head, tail: tail, err: err,
+			run: run, head: head, tail: tail, content: content, err: err,
 		}:
 		case <-ctx.Done():
 			return
@@ -942,55 +1298,151 @@ func hashWorker(ctx context.Context, jobs <-chan []fileRec,
 	}
 }
 
-// emptyHash is the lowercase-hex SHA-256 of the empty input: the head
-// and tail hash of every zero-length file.
+// emptyHash is the lowercase-hex SHA-256 of the empty input: the head,
+// tail, and content hash of every zero-length file.
 const emptyHash = "e3b0c44298fc1c149afbf4c8996fb924" +
 	"27ae41e4649b934ca495991b7852b855"
 
-// hashHeadTail returns the lowercase-hex SHA-256 of the first
-// min(chunk, size) bytes and of the last min(chunk, size) bytes of the
-// file at path. The two reads overlap when size < 2*chunk. size is the
-// value recorded when the file was statted; a zero-length file's
-// hashes are constant, so it is never even opened.
-func hashHeadTail(path string, size int64) (string, string, error) {
+// hashSignature computes the hashes the hash phase records for a file
+// whose size is shared; with the file size they form its duplicate
+// signature. A file below headTailMin is hashed in full and its
+// whole-file SHA-256 is returned as head, tail, and content alike —
+// that range takes no separate end-window step. For a file at or above
+// headTailMin only the head and tail are computed, the SHA-256 of its
+// first and last headTailWindow bytes, and content is returned empty:
+// the content phase computes it with hashContentOnly once the file's
+// size, head, and tail match another file's. Two files are duplicates
+// only when all four agree; any mismatch means not a duplicate. size
+// is the value recorded when the file was statted; a zero-length file
+// has constant hashes and is never opened.
+func hashSignature(path string, size int64) (string, string, string, error) {
 	if size == 0 {
-		return emptyHash, emptyHash, nil
+		return emptyHash, emptyHash, emptyHash, nil
 	}
 
 	//nolint:gosec // hashing operator-supplied paths is the tool's purpose
 	f, err := os.Open(path)
 	if err != nil {
-		return "", "", err
+		return "", "", "", err
 	}
 
 	defer func() { _ = f.Close() }()
 
-	n := min(int64(chunk), size)
+	// Below the threshold the whole file is hashed directly, with no
+	// end-window step: head and tail both carry the whole-file hash.
+	if size < int64(headTailMin) {
+		content, err := hashWhole(f, size)
+		if err != nil {
+			return "", "", "", err
+		}
 
-	buf := make([]byte, n)
+		return content, content, content, nil
+	}
 
-	_, err = f.ReadAt(buf, 0)
+	head, tail, err := hashEnds(f, size)
+	if err != nil {
+		return "", "", "", err
+	}
+
+	return head, tail, "", nil
+}
+
+// hashContentOnly returns the content hash of the file at path, which
+// is at least headTailMin bytes: the content phase's read. head and
+// tail are returned empty, because the content phase keeps the ones its
+// records already hold.
+func hashContentOnly(path string, size int64) (string, string, string, error) {
+	//nolint:gosec // hashing operator-supplied paths is the tool's purpose
+	f, err := os.Open(path)
+	if err != nil {
+		return "", "", "", err
+	}
+
+	defer func() { _ = f.Close() }()
+
+	content, err := hashContent(f, size)
+
+	return "", "", content, err
+}
+
+// hashEnds returns the SHA-256 of the first and last headTailWindow
+// bytes of f. It is called only for files at least headTailMin, which
+// is far larger than two windows, so the windows never overlap and both
+// reads are always full.
+func hashEnds(f *os.File, size int64) (string, string, error) {
+	buf := make([]byte, headTailWindow)
+
+	_, err := f.ReadAt(buf, 0)
 	if err != nil {
 		return "", "", err
 	}
 
 	h := sha256.Sum256(buf)
+	head := hex.EncodeToString(h[:])
 
-	// When the whole file fits in one chunk the tail window is exactly
-	// the bytes just read: reuse the head hash instead of issuing a
-	// second read for every small file.
-	if size <= int64(chunk) {
-		hh := hex.EncodeToString(h[:])
-
-		return hh, hh, nil
-	}
-
-	_, err = f.ReadAt(buf, size-n)
+	_, err = f.ReadAt(buf, size-int64(headTailWindow))
 	if err != nil {
 		return "", "", err
 	}
 
 	t := sha256.Sum256(buf)
 
-	return hex.EncodeToString(h[:]), hex.EncodeToString(t[:]), nil
+	return head, hex.EncodeToString(t[:]), nil
+}
+
+// hashContent returns the content-rung hash of f: the SHA-256 of the
+// whole file when it is smaller than wholeFileMax, or of sampled
+// windows when it is that size or larger.
+func hashContent(f *os.File, size int64) (string, error) {
+	if size >= int64(wholeFileMax) {
+		return hashSamples(f, size)
+	}
+
+	return hashWhole(f, size)
+}
+
+// hashWhole returns the SHA-256 of the entire file. A SectionReader is
+// used so the read is independent of the offset left by any end-window
+// reads. Reading fewer than size bytes means the file shrank between
+// the stat and the hash; that is an error rather than a hash of content
+// that no longer matches the recorded size.
+func hashWhole(f *os.File, size int64) (string, error) {
+	h := sha256.New()
+
+	n, err := io.Copy(h, io.NewSectionReader(f, 0, size))
+	if err != nil {
+		return "", err
+	}
+
+	if n != size {
+		return "", fmt.Errorf("read %d of %d bytes: %w", n, size,
+			io.ErrUnexpectedEOF)
+	}
+
+	return hex.EncodeToString(h.Sum(nil)), nil
+}
+
+// hashSamples feeds sampleWindow bytes at each gigabyte-aligned offset
+// (0, sampleStride, 2*sampleStride, ... while inside the file), in
+// order, into one hash, each window truncated at end of file. This is
+// the probabilistic large-file rung: two files of equal size agreeing
+// on every sample are reported as duplicates without every byte being
+// read. Because size is part of the signature, files of different sizes
+// never reach this comparison, so the sample boundaries always align.
+func hashSamples(f *os.File, size int64) (string, error) {
+	h := sha256.New()
+	buf := make([]byte, sampleWindow)
+
+	for off := int64(0); off < size; off += int64(sampleStride) {
+		n := min(int64(sampleWindow), size-off)
+
+		_, err := f.ReadAt(buf[:n], off)
+		if err != nil {
+			return "", err
+		}
+
+		h.Write(buf[:n])
+	}
+
+	return hex.EncodeToString(h.Sum(nil)), nil
 }

script/bootstrap

@@ -3,20 +3,16 @@
 # this repo. Idempotent: every install is guarded by a check so already
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
 # or apk (detected in that order); assumes nothing is present (not git,
-# make, or go). The linter is NOT installed: golangci-lint runs via
-# docker only (script/lint), pinned by image digest, so the only lint
+# make, or go). Neither the linter nor the Markdown formatter is
+# installed: golangci-lint (script/lint) and prettier (script/fmt,
+# script/fmt-check) run via docker only, pinned by hash, so their only
 # prerequisite is a working docker — which is warned about, not
-# installed, because everything except linting works without it.
+# installed, because everything except linting and formatting works
+# without it.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# yarn provides prettier, which formats Markdown. yarn is a tool, like
-# node/git/make/go below; the reference that governs formatting output is
-# prettier, pinned by yarn.lock's integrity hash and installed by
-# `yarn install --frozen-lockfile`.
-YARN_VERSION="1.22.22"
-
 PKGMGR=""
 SUDO=""
 APT_UPDATED=""
@@ -64,21 +60,6 @@ missing() {
     ! command -v "$1" >/dev/null 2>&1
 }
 
-ensure_node() {
-    if ! missing node; then return 0; fi
-    pkg_install nodejs nodejs node nodejs
-}
-
-ensure_yarn() {
-    if ! missing yarn; then return 0; fi
-    if ! missing corepack; then
-        corepack enable >/dev/null 2>&1 || true
-        corepack prepare "yarn@$YARN_VERSION" --activate
-    else
-        pkg_install yarn yarn yarn yarn
-    fi
-}
-
 main() {
     cd "$ROOT"
 
@@ -92,25 +73,15 @@ main() {
     if missing make; then pkg_install gnumake make make make; fi
     if missing go; then pkg_install go golang go go; fi
 
-    # node runs prettier and is an unpinned host tool for the same reason
-    # git/make/go are: it comes from the host package manager, whatever
-    # version it ships. It is not installed via nvm the way the canonical
-    # template does, because nvm's prebuilt node is glibc-linked and does
-    # not run on this repo's musl/Alpine build image. prettier — the tool
-    # whose version affects formatting output — is pinned by yarn.lock.
-    ensure_node
-    ensure_yarn
-    yarn install --frozen-lockfile
-
-    # Linting runs via docker only (script/lint), so docker is a lint
-    # prerequisite rather than something bootstrap installs. Warn, do
-    # not fail: everything except `make lint` — and, through it,
-    # `make check`, `make docker` and the pre-commit hook — works
-    # without it.
+    # Linting and Markdown formatting run via docker only, so docker is
+    # their prerequisite rather than something bootstrap installs. Warn,
+    # do not fail: everything except `make lint`, `make fmt` and
+    # `make fmt-check` — and, through them, `make check`, `make docker`
+    # and the pre-commit hook — works without it.
     if missing docker; then
-        echo "bootstrap: WARNING: docker not found; make lint, make check" >&2
-        echo "bootstrap: and make docker require it. Install docker to" >&2
-        echo "bootstrap: run the linter." >&2
+        echo "bootstrap: WARNING: docker not found; make lint, make fmt," >&2
+        echo "bootstrap: make fmt-check, make check and make docker" >&2
+        echo "bootstrap: require it." >&2
     fi
 
     go mod download

script/cibuild

@@ -3,23 +3,23 @@
 # push.
 #
 # The Dockerfile runs the gates individually as build steps, not the
-# make check aggregate: the lint stage runs make fmt-check,
+# make check aggregate: the lint stage runs the gofmt check,
 # script/verify-lint-image-pin, golangci-lint config verify and
-# golangci-lint run; the build stage, dropped to an unprivileged user,
-# runs make test and make fmt-check. Neither make lint nor make check
-# appears, because both reach script/lint, which is itself a docker
-# build, and a docker build cannot run inside one. Lint is not skipped
-# by that — the linter is invoked directly in the lint stage, and the
-# build stage's COPY --from=lint makes that stage a prerequisite, so
-# BuildKit must finish it first. Between the two stages everything
-# make check would run has run, which is why a successful build here
-# implies the repo is green.
+# golangci-lint run; the markdown stage runs the prettier check; the
+# build stage, dropped to an unprivileged user, runs make test. None of
+# make lint, make fmt-check or make check appears, because each runs
+# docker, and docker cannot run inside a docker build. Nothing is
+# skipped by that — the linter, gofmt and prettier are invoked directly
+# in their stages, and the build stage's COPY --from lines make those
+# stages prerequisites, so BuildKit must finish them first. Between the
+# three stages everything make check would run has run, which is why a
+# successful build here implies the repo is green.
 #
 # That implication holds only because of CHECK_EPOCH. A COPY layer is
-# invalidated by changed content, and a merge commit's tree is
-# byte-identical to the branch head it merges, so without a fresh value
-# here Docker serves the gate layers from cache and the build reports a
-# green it never earned. Passing the current epoch invalidates the gate
+# invalidated only by changed content, and a rebuild of an unchanged
+# checkout sends the same content, so without a fresh value here Docker
+# serves the gate layers from cache and the build reports a green it
+# never earned. Passing the current epoch invalidates the gate
 # layers on every run while leaving the pinned base images and
 # go mod download cached; see the Dockerfile for the placement.
 set -eu

script/docker

@@ -4,11 +4,11 @@
 #
 # CHECK_EPOCH is passed for the same reason script/cibuild passes it:
 # without it Docker serves the Dockerfile's gate layers from cache on an
-# unchanged tree and this exits 0 having run neither the lint stage's
-# gates nor the builder stage's test and fmt-check gates. This is the
-# set of gates a developer or reviewer runs by hand, so a cached pass
-# here is the most misleading result the repo can produce. Dependency
-# layers sit above the ARG and stay cached.
+# unchanged tree and this exits 0 having run none of the lint stage's
+# gates, the markdown stage's prettier gate or the builder stage's test
+# gate. This is the set of gates a developer or reviewer runs by hand,
+# so a cached pass here is the most misleading result the repo can
+# produce. Dependency layers sit above the ARG and stay cached.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"

script/fmt

@@ -1,23 +1,22 @@
 #!/bin/sh
-# script/fmt: format all files (writes). gofmt for Go, prettier for
-# Markdown. prettier is the pinned devDependency in package.json/
-# yarn.lock; script/bootstrap installs it (see run_prettier).
+# script/fmt: format all files (writes): the Go sources with gofmt, the
+# Markdown with prettier. prettier is never installed on the host: it
+# runs from the Dockerfile's prettier stage with the repository mounted,
+# as the calling user so the files it rewrites keep their owner. The tag
+# makes each build replace the previous image instead of leaving another
+# one behind.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
-
-run_prettier() {
-    if ! command -v yarn >/dev/null 2>&1; then
-        echo "fmt: yarn not found; run script/bootstrap first" >&2
-        exit 1
-    fi
-    yarn run prettier "$@"
-}
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
     gofmt -s -w .
-    run_prettier --write '**/*.md' --tab-width 4 --prose-wrap always
+    image="$("$SCRIPT_DIR/projectname")-prettier"
+    docker build -q --target prettier -t "$image" . >/dev/null
+    docker run --rm --user "$(id -u):$(id -g)" -v "$ROOT:/src" "$image" \
+        prettier --write '**/*.md' --tab-width 4 --prose-wrap always
 }
 
 main "$@"

script/fmt-check

@@ -1,37 +1,34 @@
 #!/bin/sh
 # script/fmt-check: check formatting (read-only). Same scope as
-# script/fmt: gofmt for Go, prettier for Markdown. Both run every time
-# and each reports independently, so a failure names which formatter is
-# unhappy; the script exits non-zero if either found unformatted files.
+# script/fmt, but fails instead of writing. gofmt and prettier both run
+# every time and each reports its own failure, so the output says which
+# one failed.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
-
-run_prettier() {
-    if ! command -v yarn >/dev/null 2>&1; then
-        echo "fmt-check: yarn not found; run script/bootstrap first" >&2
-        exit 1
-    fi
-    yarn run prettier "$@"
-}
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    rc=0
+    status=0
 
     files="$(gofmt -s -l .)"
     if [ -n "$files" ]; then
         echo "gofmt: files not formatted:" >&2
         echo "$files" >&2
-        rc=1
+        status=1
     fi
 
-    if ! run_prettier --check '**/*.md' --tab-width 4 --prose-wrap always; then
+    # Same image as script/fmt; see there.
+    image="$("$SCRIPT_DIR/projectname")-prettier"
+    docker build -q --target prettier -t "$image" . >/dev/null
+    if ! docker run --rm -v "$ROOT:/src:ro" "$image" \
+        prettier --check '**/*.md' --tab-width 4 --prose-wrap always; then
         echo "prettier: Markdown not formatted; run make fmt" >&2
-        rc=1
+        status=1
     fi
 
-    exit "$rc"
+    exit "$status"
 }
 
 main "$@"

trees.go

@@ -5,48 +5,59 @@ import (
 	"context"
 	"crypto/sha256"
 	"fmt"
+	"io"
 	"os"
 	"slices"
 	"strconv"
 	"strings"
 )
 
-// fileSig is a file's duplicate signature; mtime is excluded.
-type fileSig struct {
-	size int64
-	head string
-	tail string
-}
-
-// treeNode is one directory reconstructed from the scan stream.
+// treeNode is one directory reconstructed from the record paths.
 type treeNode struct {
 	path   string
 	parent *treeNode
-	dirs      map[string]*treeNode
-	files     map[string]fileSig
+	// entries holds the serialized child entries until the digest is
+	// computed from them, and is then dropped.
+	entries   []string
 	digest    [sha256.Size]byte
 	fileCount int64
 	totalSize int64
 }
 
 // runTrees implements the trees subcommand: it reads every record from
-// the database, reconstructs the directory hierarchy from the record
-// paths, computes a Merkle-style digest per directory, and prints
-// maximal duplicate-tree groups as TSV on stdout. It never touches the
-// scanned filesystem; its only I/O is the database, stdout, and
-// stderr.
-func runTrees(ctx context.Context) error {
-	recs, err := loadRecords(ctx)
+// the database in path order, reconstructs the directory hierarchy from
+// the record paths, computes a Merkle-style digest per directory, and
+// prints maximal duplicate-tree groups as TSV on stdout. It never
+// touches the scanned filesystem; its only I/O is the database, stdout,
+// and stderr. Any database problem, including a missing database, is
+// fatal.
+func runTrees(ctx context.Context, stdout io.Writer) error {
+	dbPath := databasePath()
+
+	db, err := openReportDatabase(ctx, dbPath)
 	if err != nil {
 		return err
 	}
 
-	super, allDirs := buildHierarchy(recs)
-	super.compute()
+	defer func() { _ = db.Close() }()
+
+	records := 0
+	tree := newTreeBuilder()
+
+	err = loadFileRows(ctx, db, func(r scanRec) {
+		records++
+
+		tree.add(r)
+	})
+	if err != nil {
+		return fmt.Errorf("database %s: %w", dbPath, err)
+	}
+
+	super, allDirs := tree.finish()
 
 	dupes := collectTreeGroups(allDirs, super)
 
-	out := bufio.NewWriterSize(os.Stdout, ioBufSize)
+	out := bufio.NewWriterSize(stdout, ioBufSize)
 
 	_, err = fmt.Fprintln(out, "first\tdupe\tfiles\tsize")
 	if err != nil {
@@ -61,7 +72,8 @@ func runTrees(ctx context.Context) error {
 		first := g[0]
 		for _, n := range g[1:] {
 			_, err = fmt.Fprintf(out, "%s\t%s\t%d\t%d\n",
-				first.path, n.path, first.fileCount, first.totalSize)
+				escapePath(first.path), escapePath(n.path),
+				first.fileCount, first.totalSize)
 			if err != nil {
 				return fmt.Errorf("write stdout: %w", err)
 			}
@@ -79,63 +91,128 @@ func runTrees(ctx context.Context) error {
 	fmt.Fprintf(os.Stderr,
 		"trees: %d records read, %d duplicate tree groups, %d dupe trees, "+
 			"%s reclaimable\n",
-		len(recs), len(dupes), dupeTrees, humanBytes(reclaimable))
+		records, len(dupes), dupeTrees, humanBytes(reclaimable))
 
 	return nil
 }
 
-// buildHierarchy reconstructs the directory hierarchy from the record
-// paths under a synthetic super-root. Paths are split on "/"; for
-// absolute paths the first component is empty, which simply becomes a
-// top-level node representing "/". It returns the super-root and every
-// directory node created.
-func buildHierarchy(recs []scanRec) (*treeNode, []*treeNode) {
+// treeBuilder reconstructs the directory hierarchy from records added
+// in path order, under a synthetic super-root. Paths are split on "/";
+// for absolute paths the first component is empty, which becomes the
+// top-level directory with path "/". In path order all the paths under
+// one directory come together, so a directory is complete once a path
+// outside it is added: its digest is computed then and its entries are
+// dropped. Only the directories holding the latest path keep entries.
+type treeBuilder struct {
+	super *treeNode
+	// open lists the directories holding the latest path, outermost
+	// first, starting with the super-root; names[i] is open[i]'s name.
+	open  []*treeNode
+	names []string
+	// dirs lists every completed directory.
+	dirs []*treeNode
+}
+
+func newTreeBuilder() *treeBuilder {
 	super := &treeNode{}
 
-	var allDirs []*treeNode
+	return &treeBuilder{
+		super: super,
+		open:  []*treeNode{super},
+		names: []string{""},
+	}
+}
 
-	for _, r := range recs {
+// add adds one record. Each record must come after the previous one in
+// path order (byte order); otherwise a completed directory would be
+// started again as a second directory with the same path.
+func (b *treeBuilder) add(r scanRec) {
 	comps := strings.Split(r.path, "/")
+	dirNames, name := comps[:len(comps)-1], comps[len(comps)-1]
 
-		node := super
-		for _, c := range comps[:len(comps)-1] {
-			child := node.dirs[c]
-			if child == nil {
-				childPath := c
-				if node != super {
-					childPath = node.path + "/" + c
+	// Keep the open directories that hold this path; complete the rest.
+	depth := 1
+	for depth < len(b.open) && depth <= len(dirNames) &&
+		b.names[depth] == dirNames[depth-1] {
+		depth++
 	}
 
-				child = &treeNode{path: childPath, parent: node}
-				if node.dirs == nil {
-					node.dirs = make(map[string]*treeNode)
+	b.closeTo(depth)
+
+	for _, c := range dirNames[depth-1:] {
+		b.openDir(c)
 	}
 
-				node.dirs[c] = child
-				allDirs = append(allDirs, child)
+	dir := b.open[len(b.open)-1]
+	dir.entries = append(dir.entries, fileEntry(name, r))
+	dir.fileCount++
+	dir.totalSize += r.size
 }
 
-			node = child
+// openDir starts the directory called name inside the innermost open
+// one.
+func (b *treeBuilder) openDir(name string) {
+	parent := b.open[len(b.open)-1]
+	path := parent.path + "/" + name
+
+	// The root directory's path is "/", not empty, and its children's
+	// paths start with one slash, not two.
+	switch {
+	case parent == b.super && name == "":
+		path = "/"
+	case parent == b.super:
+		path = name
+	case parent.path == "/":
+		path = "/" + name
 	}
 
-		if node.files == nil {
-			node.files = make(map[string]fileSig)
+	b.open = append(b.open, &treeNode{path: path, parent: parent})
+	b.names = append(b.names, name)
 }
 
-		sig := fileSig{size: r.size, head: r.head, tail: r.tail}
+// closeTo completes the open directories after the first n, innermost
+// first: each one's digest is computed and entered in its parent along
+// with its totals.
+func (b *treeBuilder) closeTo(n int) {
+	for len(b.open) > n {
+		last := len(b.open) - 1
+		dir, name := b.open[last], b.names[last]
+		b.open, b.names = b.open[:last], b.names[:last]
 
-		// An unhashed record (its size was unique when last scanned)
-		// has unknown content: give it a signature no other file can
-		// share, so trees containing it never compare equal. Real
-		// heads are hex, so the NUL-prefixed form cannot collide.
-		if sig.head == "" {
-			sig.head = "unhashed\x00" + r.path
+		dir.computeDigest()
+
+		dir.parent.entries = append(dir.parent.entries,
+			"d\x00"+name+"\x00"+string(dir.digest[:]))
+		dir.parent.fileCount += dir.fileCount
+		dir.parent.totalSize += dir.totalSize
+		b.dirs = append(b.dirs, dir)
+	}
 }
 
-		node.files[comps[len(comps)-1]] = sig
+// finish completes every open directory and returns the super-root and
+// every directory.
+func (b *treeBuilder) finish() (*treeNode, []*treeNode) {
+	b.closeTo(1)
+
+	return b.super, b.dirs
 }
 
-	return super, allDirs
+// fileEntry serializes a file child for its directory's digest: its
+// name and its signature (size, head, tail, content); mtime is
+// excluded.
+func fileEntry(name string, r scanRec) string {
+	content := r.content
+
+	// A record without a content hash has unknown content (README
+	// "Database"): give it a signature no other file can share, so
+	// trees containing it never compare equal. Real hashes are hex, so
+	// the NUL-prefixed form cannot collide.
+	if content == "" {
+		content = "unhashed\x00" + r.path
+	}
+
+	return "f\x00" + name + "\x00" + strconv.FormatInt(r.size, 10) +
+		"\x00" + r.head + "\x00" + r.tail + "\x00" + content
 }
 
 // collectTreeGroups groups directories by digest and returns every
@@ -177,38 +254,22 @@ func collectTreeGroups(allDirs []*treeNode, super *treeNode) [][]*treeNode {
 	return dupes
 }
 
-// compute fills in digest, fileCount, and totalSize for n and all of
-// its descendants. A directory's digest is the SHA-256 of its child
-// entries — files serialized with name and signature, subdirectories
-// with name and recursive digest — sorted byte-lexicographically.
-// Filenames cannot contain NUL or "/", so NUL delimiters are
-// unambiguous.
-func (n *treeNode) compute() {
-	entries := make([]string, 0, len(n.dirs)+len(n.files))
-	for name, sig := range n.files {
-		entries = append(entries,
-			"f\x00"+name+"\x00"+strconv.FormatInt(sig.size, 10)+
-				"\x00"+sig.head+"\x00"+sig.tail)
-		n.fileCount++
-		n.totalSize += sig.size
-	}
-
-	for name, child := range n.dirs {
-		child.compute()
-		entries = append(entries, "d\x00"+name+"\x00"+string(child.digest[:]))
-		n.fileCount += child.fileCount
-		n.totalSize += child.totalSize
-	}
-
-	slices.Sort(entries)
+// computeDigest sets n's digest and drops its entries. A directory's
+// digest is the SHA-256 of its child entries — files serialized with
+// name and signature, subdirectories with name and recursive digest —
+// sorted byte-lexicographically. Filenames cannot contain NUL or "/",
+// so NUL delimiters are unambiguous.
+func (n *treeNode) computeDigest() {
+	slices.Sort(n.entries)
 
 	h := sha256.New()
-	for _, e := range entries {
+	for _, e := range n.entries {
 		h.Write([]byte(e))
 		h.Write([]byte{0})
 	}
 
 	copy(n.digest[:], h.Sum(nil))
+	n.entries = nil
 }
 
 // suppressed reports whether a duplicate-tree group is non-maximal: its

trees_test.go

@@ -1,6 +1,8 @@
 package main
 
 import (
+	"bytes"
+	"database/sql"
 	"slices"
 	"testing"
 )
@@ -9,23 +11,56 @@ import (
 const (
 	f1Head    = "f1h"
 	f1Tail    = "f1t"
+	f1Content = "f1c"
 	f2Head    = "f2h"
 	f2Tail    = "f2t"
+	f2Content = "f2c"
 )
 
 // smokeTreeRecs mirrors the README smoke-test tree layout: /d/t1 and
 // /d/t2 are identical, /d/t3 differs from them only by one filename.
 func smokeTreeRecs() []scanRec {
 	return []scanRec{
-		{size: 3000, head: f1Head, tail: f1Tail, path: "/d/t1/f1"},
-		{size: 100, head: f2Head, tail: f2Tail, path: "/d/t1/sub/f2"},
-		{size: 3000, head: f1Head, tail: f1Tail, path: "/d/t2/f1"},
-		{size: 100, head: f2Head, tail: f2Tail, path: "/d/t2/sub/f2"},
-		{size: 3000, head: f1Head, tail: f1Tail, path: "/d/t3/f1"},
-		{size: 100, head: f2Head, tail: f2Tail, path: "/d/t3/sub/f2renamed"},
+		{size: 3000, head: f1Head, tail: f1Tail, content: f1Content, path: "/d/t1/f1"},
+		{size: 100, head: f2Head, tail: f2Tail, content: f2Content, path: "/d/t1/sub/f2"},
+		{size: 3000, head: f1Head, tail: f1Tail, content: f1Content, path: "/d/t2/f1"},
+		{size: 100, head: f2Head, tail: f2Tail, content: f2Content, path: "/d/t2/sub/f2"},
+		{size: 3000, head: f1Head, tail: f1Tail, content: f1Content, path: "/d/t3/f1"},
+		{size: 100, head: f2Head, tail: f2Tail, content: f2Content,
+			path: "/d/t3/sub/f2renamed"},
 	}
 }
 
+// dbTree builds the directory hierarchy from the records in db the way
+// trees does, and returns the super-root and every directory.
+func dbTree(t *testing.T, db *sql.DB) (*treeNode, []*treeNode) {
+	t.Helper()
+
+	tree := newTreeBuilder()
+
+	err := loadFileRows(t.Context(), db, tree.add)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	return tree.finish()
+}
+
+// treeOf writes recs into a fresh database and builds the directory
+// hierarchy from it the way trees does.
+func treeOf(t *testing.T, recs []scanRec) (*treeNode, []*treeNode) {
+	t.Helper()
+
+	db := openTestDB(t)
+
+	err := applyChanges(t.Context(), db, recs, nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	return dbTree(t, db)
+}
+
 // nodeByPath finds the directory node with the given path.
 func nodeByPath(t *testing.T, dirs []*treeNode, path string) *treeNode {
 	t.Helper()
@@ -56,11 +91,10 @@ func groupPaths(groups [][]*treeNode) [][]string {
 	return out
 }
 
-func TestBuildHierarchyCounts(t *testing.T) {
+func TestTreeCounts(t *testing.T) {
 	t.Parallel()
 
-	super, dirs := buildHierarchy(smokeTreeRecs())
-	super.compute()
+	_, dirs := treeOf(t, smokeTreeRecs())
 
 	d := nodeByPath(t, dirs, "/d")
 	if d.fileCount != 6 || d.totalSize != 9300 {
@@ -81,11 +115,98 @@ func TestBuildHierarchyCounts(t *testing.T) {
 	}
 }
 
+func TestTreeRootPath(t *testing.T) {
+	t.Parallel()
+
+	// The root directory's path is "/", never empty, and its
+	// children's paths start with a single slash.
+	_, dirs := treeOf(t, []scanRec{{path: "/f"}, {path: "/srv/g"}})
+
+	got := make([]string, 0, len(dirs))
+	for _, d := range dirs {
+		got = append(got, d.path)
+	}
+
+	slices.Sort(got)
+
+	want := []string{"/", "/srv"}
+	if !slices.Equal(got, want) {
+		t.Fatalf("directory paths = %q, want %q", got, want)
+	}
+}
+
+func TestTreeNamesSortingBeforeSlash(t *testing.T) {
+	t.Parallel()
+
+	// In path order "/a/b-x/f" and "/a/b.txt" come between the file
+	// "/a/b" and "/a/b/f", because "-" and "." sort before "/". Each
+	// directory must still be built once, whole, so /a matches /c.
+	recs := make([]scanRec, 0, 8)
+
+	for _, top := range []string{"/a", "/c"} {
+		for _, p := range []string{"/b", "/b-x/f", "/b.txt", "/b/f"} {
+			content := "c"
+			if p == "/b-x/f" {
+				content = "other"
+			}
+
+			recs = append(recs, scanRec{
+				size: 1, head: "h", tail: "t", content: content, path: top + p,
+			})
+		}
+	}
+
+	super, dirs := treeOf(t, recs)
+
+	got := make([]string, 0, len(dirs))
+	for _, d := range dirs {
+		got = append(got, d.path)
+	}
+
+	slices.Sort(got)
+
+	want := []string{"/", "/a", "/a/b", "/a/b-x", "/c", "/c/b", "/c/b-x"}
+	if !slices.Equal(got, want) {
+		t.Fatalf("directory paths = %q, want %q", got, want)
+	}
+
+	groups := collectTreeGroups(dirs, super)
+
+	gotGroups := groupPaths(groups)
+	wantGroups := [][]string{{"/a", "/c"}}
+
+	if !slices.EqualFunc(gotGroups, wantGroups, slices.Equal) {
+		t.Fatalf("groups = %v, want %v", gotGroups, wantGroups)
+	}
+
+	if groups[0][0].fileCount != 4 || groups[0][0].totalSize != 4 {
+		t.Errorf("group totals: %d files %d bytes, want 4 4",
+			groups[0][0].fileCount, groups[0][0].totalSize)
+	}
+}
+
+func TestRunTreesEscapesPaths(t *testing.T) {
+	t.Setenv(databaseEnv, seedDatabase(t, awkwardPairRecs()))
+
+	var stdout, stderr bytes.Buffer
+
+	code := run([]string{cmdTrees}, &stdout, &stderr)
+	if code != exitOK {
+		t.Fatalf("run(trees) = %d, want %d; stderr: %s",
+			code, exitOK, stderr.String())
+	}
+
+	want := "first\tdupe\tfiles\tsize\n" +
+		`/d/\tone\ntwo\rthree\\four` + "\t/d/A\t1\t5\n"
+	if got := stdout.String(); got != want {
+		t.Errorf("stdout = %q, want %q", got, want)
+	}
+}
+
 func TestTreeDigests(t *testing.T) {
 	t.Parallel()
 
-	super, dirs := buildHierarchy(smokeTreeRecs())
-	super.compute()
+	_, dirs := treeOf(t, smokeTreeRecs())
 
 	t1 := nodeByPath(t, dirs, "/d/t1")
 	t2 := nodeByPath(t, dirs, "/d/t2")
@@ -114,12 +235,11 @@ func TestTreeDigestContentSensitivity(t *testing.T) {
 	const sharedTail = "same"
 
 	recs := []scanRec{
-		{size: 10, head: sharedTail, tail: sharedTail, path: "/r/a/f"},
-		{size: 10, head: "DIFF", tail: sharedTail, path: "/r/b/f"},
+		{size: 10, head: sharedTail, tail: sharedTail, content: "c", path: "/r/a/f"},
+		{size: 10, head: "DIFF", tail: sharedTail, content: "c", path: "/r/b/f"},
 	}
 
-	super, dirs := buildHierarchy(recs)
-	super.compute()
+	_, dirs := treeOf(t, recs)
 
 	a := nodeByPath(t, dirs, "/r/a")
 	b := nodeByPath(t, dirs, "/r/b")
@@ -132,8 +252,7 @@ func TestTreeDigestContentSensitivity(t *testing.T) {
 func TestCollectTreeGroupsMaximal(t *testing.T) {
 	t.Parallel()
 
-	super, dirs := buildHierarchy(smokeTreeRecs())
-	super.compute()
+	super, dirs := treeOf(t, smokeTreeRecs())
 
 	groups := collectTreeGroups(dirs, super)
 
@@ -157,16 +276,14 @@ func TestCollectTreeGroupsDeterministic(t *testing.T) {
 
 	recs := smokeTreeRecs()
 
-	super, dirs := buildHierarchy(recs)
-	super.compute()
+	super, dirs := treeOf(t, recs)
 
 	forward := groupPaths(collectTreeGroups(dirs, super))
 
 	reversed := slices.Clone(recs)
 	slices.Reverse(reversed)
 
-	superR, dirsR := buildHierarchy(reversed)
-	superR.compute()
+	superR, dirsR := treeOf(t, reversed)
 
 	backward := groupPaths(collectTreeGroups(dirsR, superR))
 	if !slices.EqualFunc(forward, backward, slices.Equal) {
@@ -181,12 +298,11 @@ func TestCollectTreeGroupsSiblings(t *testing.T) {
 	// Identical sibling dirs share a parent, so their group cannot be
 	// implied by a parent group and must be reported.
 	recs := []scanRec{
-		{size: 10, head: "h", tail: "t", path: "/p/x1/f"},
-		{size: 10, head: "h", tail: "t", path: "/p/x2/f"},
+		{size: 10, head: "h", tail: "t", content: "c", path: "/p/x1/f"},
+		{size: 10, head: "h", tail: "t", content: "c", path: "/p/x2/f"},
 	}
 
-	super, dirs := buildHierarchy(recs)
-	super.compute()
+	super, dirs := treeOf(t, recs)
 
 	got := groupPaths(collectTreeGroups(dirs, super))
 
@@ -203,13 +319,12 @@ func TestCollectTreeGroupsDifferingParents(t *testing.T) {
 	// extra file, so the parents' digests differ and the x group must
 	// be reported.
 	recs := []scanRec{
-		{size: 10, head: "h", tail: "t", path: "/p/a/x/f"},
-		{size: 99, head: "e", tail: "e", path: "/p/a/extra"},
-		{size: 10, head: "h", tail: "t", path: "/q/b/x/f"},
+		{size: 10, head: "h", tail: "t", content: "c", path: "/p/a/x/f"},
+		{size: 99, head: "e", tail: "e", content: "e", path: "/p/a/extra"},
+		{size: 10, head: "h", tail: "t", content: "c", path: "/q/b/x/f"},
 	}
 
-	super, dirs := buildHierarchy(recs)
-	super.compute()
+	super, dirs := treeOf(t, recs)
 
 	got := groupPaths(collectTreeGroups(dirs, super))