Refuse an unversioned database that already has a files table (closes #11)

An unversioned database (user_version 0) that already contains a files
table was not created by this build; it is a foreign or partially
initialized file. Adopting it silently could corrupt unrelated data, so
createSchema now checks for a files table first and, when one exists,
returns the schema-version error telling the operator to remove the file
and rescan. A genuinely empty database is still created and stamped as
before.

Model: opus-4-8 (implementation); opus-5-5 (rebase)

README.md

@@ -113,9 +113,8 @@ Goals, in order:
     `sfdupes`.
 -   Dependencies: standard library, `github.com/spf13/cobra` for the
     CLI, **one progress-bar library**
-    (`github.com/schollz/progressbar/v3`), **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/schollz/progressbar/v3`), and **one SQLite driver**
+    (`modernc.org/sqlite`, pure Go, so builds keep cgo disabled).
     `github.com/spf13/viper` is permitted if configuration-file support
     is ever needed, but is not currently used. No other third-party
     deps.
@@ -153,42 +152,16 @@ All three subcommands operate on a single SQLite database file:
     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.
--   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
-    everything hashed so far; 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` 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.
+-   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.
 -   Schema (`PRAGMA user_version` is the schema version, currently 1; a
     database with any other version is a fatal error):
 
@@ -582,25 +555,12 @@ Additional requirements:
 ### 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, 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).
+-   `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).
 
-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`.
-
 ## Entrypoints
 
 This repository adheres to the
@@ -738,7 +698,7 @@ All of the following, run in this directory, must pass:
 
     ```sh
     d=$(mktemp -d)
-    export SFDUPES_DATABASE="$(mktemp -d)/db.sqlite"
+    export SFDUPES_DATABASE="$d/db.sqlite"
     mkdir -p "$d/a" "$d/b"
     head -c 2000 /dev/urandom > "$d/a/one.bin"
     cp "$d/a/one.bin" "$d/b/copy.bin"
@@ -766,9 +726,9 @@ All of the following, run in this directory, must pass:
     ./sfdupes report
     ```
 
-    (The database lives in a temp directory of its own: inside `$d`,
-    the scan would record it, and its empty lock file would join the
-    `empty1`/`empty2` group.)
+    (The scan database lives inside `$d` here purely for test hygiene;
+    scanning `$d` therefore also records the SQLite file itself, which
+    is harmless.)
 
     Expected from the first `report`: `one.bin`/`copy.bin`/`copy2.bin`
     form one group (two dupe rows, `first` is the lexicographically

TODO.md

@@ -29,17 +29,9 @@
 
 # Completed Steps
 
-- `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)
+- refuse an unversioned database that already has a `files` table with
+  a clear schema-version error (2026-10-03,
+  https://git.eeqj.de/sneak/sfdupes/issues/11)
 
 - escape tabs, newlines, carriage returns and backslashes in report,
   trees and warning paths; the root directory's path is `/`

db.go

@@ -11,7 +11,6 @@ import (
 	"slices"
 	"strconv"
 
-	"golang.org/x/sys/unix"
 	// The pure-Go SQLite driver, registered as "sqlite"; keeps cgo
 	// disabled.
 	_ "modernc.org/sqlite"
@@ -33,11 +32,6 @@ 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 = `
@@ -70,10 +64,6 @@ 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 {
@@ -84,24 +74,16 @@ func databasePath() string {
 	return defaultDatabasePath
 }
 
-// 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)"
+// 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)" +
+		"&_pragma=journal_mode(WAL)" +
+		"&_pragma=synchronous(NORMAL)"
 
-// 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)
+	db, err := sql.Open("sqlite", dsn)
 	if err != nil {
 		return nil, fmt.Errorf("open database %s: %w", path, err)
 	}
@@ -114,43 +96,6 @@ func openDB(path, params 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) {
@@ -159,7 +104,7 @@ func openScanDatabase(ctx context.Context, path string) (*sql.DB, error) {
 		return nil, fmt.Errorf("create database directory: %w", err)
 	}
 
-	db, err := openDB(path, scanParams)
+	db, err := openDB(path)
 	if err != nil {
 		return nil, err
 	}
@@ -174,24 +119,6 @@ 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.
@@ -207,7 +134,7 @@ func openReportDatabase(ctx context.Context,
 		return nil, fmt.Errorf("database: %w", err)
 	}
 
-	db, err := openDB(path, reportParams)
+	db, err := openDB(path)
 	if err != nil {
 		return nil, err
 	}
@@ -249,9 +176,30 @@ func initSchema(ctx context.Context, db *sql.DB) error {
 }
 
 // createSchema applies the schema to a fresh database and stamps the
-// schema version.
+// schema version. A database with user_version 0 that already has a
+// files table was not created by this build — a foreign or partially
+// initialized file. Adopting it silently could corrupt unrelated data,
+// so that is a fatal schema-version error telling the operator to
+// remove the file and rescan.
 func createSchema(ctx context.Context, db *sql.DB) error {
-	_, err := db.ExecContext(ctx, createTableSQL)
+	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):
+		// Genuinely empty: create the schema below.
+	default:
+		return fmt.Errorf("check for files table: %w", err)
+	}
+
+	_, err = db.ExecContext(ctx, createTableSQL)
 	if err != nil {
 		return fmt.Errorf("create schema: %w", err)
 	}

db_test.go

@@ -5,7 +5,6 @@ import (
 	"database/sql"
 	"errors"
 	"fmt"
-	"os"
 	"path/filepath"
 	"slices"
 	"strings"
@@ -79,6 +78,38 @@ func TestOpenScanDatabaseCreates(t *testing.T) {
 	}
 }
 
+func TestOpenScanDatabaseUnversionedForeign(t *testing.T) {
+	t.Parallel()
+
+	path := testDBPath(t)
+
+	// A database that has a files table but user_version 0 — a foreign
+	// or partially initialized file. scan must refuse it with a clear
+	// schema-version error, not adopt it and not emit a raw SQLite
+	// "table files already exists".
+	db, err := openDB(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 = openScanDatabase(t.Context(), path)
+	if !errors.Is(err, errSchemaVersion) {
+		t.Fatalf("err = %v, want errSchemaVersion", err)
+	}
+
+	if !strings.Contains(err.Error(), "remove the file and rescan") {
+		t.Fatalf("err = %v, want it to tell the operator to remove and rescan",
+			err)
+	}
+}
+
 func TestOpenReportDatabaseMissing(t *testing.T) {
 	t.Parallel()
 
@@ -131,49 +162,6 @@ 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)
-	if err != nil {
-		t.Fatalf("loadFileRows: %v", err)
-	}
-}
-
 func TestApplyChangesRoundTrip(t *testing.T) {
 	t.Parallel()
 

go.mod

@@ -5,7 +5,6 @@ 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
 	modernc.org/sqlite v1.54.0
 )
 
@@ -19,6 +18,7 @@ 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

main.go

@@ -57,27 +57,22 @@ var errNoSubcommand = errors.New("no subcommand")
 var Version = "dev"
 
 func main() {
-	// 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))
+	os.Exit(run(os.Args[1:], 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. The report and trees subcommands write
-// their data to stdout.
-func run(args []string, stdout, stderr io.Writer) int {
+// runs before the process ends.
+func run(args []string, 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(stdout, stderr)
+	root := newRootCommand(stderr)
 	root.SetArgs(args)
 
 	err := root.Execute()
@@ -103,7 +98,7 @@ func run(args []string, stdout, stderr io.Writer) int {
 // newRootCommand builds the command tree. Everything on stdout is
 // machine-readable data; all human-facing output (help, usage, errors)
 // goes to stderr.
-func newRootCommand(stdout, stderr io.Writer) *cobra.Command {
+func newRootCommand(stderr io.Writer) *cobra.Command {
 	root := &cobra.Command{
 		Use:     "sfdupes",
 		Short:   "Find candidate duplicate files by size and head/tail/content SHA-256",
@@ -145,7 +140,7 @@ func newRootCommand(stdout, 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, stdout)
+			return runReport(ctx)
 		}),
 	}
 
@@ -154,7 +149,7 @@ func newRootCommand(stdout, 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, stdout)
+			return runTrees(ctx)
 		}),
 	}
 

main_test.go

@@ -44,54 +44,60 @@ func assertNoSidecars(t *testing.T, path 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) {
+// 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 {
 	t.Helper()
 
-	if os.Geteuid() == 0 {
-		t.Skip("root ignores file permissions")
-	}
-
-	err := os.Chmod(path, 0o400)
+	f, err := os.Create(filepath.Join(t.TempDir(), "stdout"))
 	if err != nil {
 		t.Fatal(err)
 	}
 
-	for _, suffix := range walSuffixes {
-		err = os.Chmod(path+suffix, 0o400)
-		if err != nil && !errors.Is(err, fs.ErrNotExist) {
+	saved := os.Stdout
+	os.Stdout = f
+
+	t.Cleanup(func() {
+		os.Stdout = saved
+
+		_ = f.Close()
+	})
+
+	return func() string {
+		// Read what has been written without disturbing the write
+		// offset, so the capture can be inspected more than once.
+		size, err := f.Seek(0, io.SeekCurrent)
+		if err != nil {
 			t.Fatal(err)
 		}
+
+		if size == 0 {
+			return ""
+		}
+
+		b := make([]byte, size)
+
+		_, err = f.ReadAt(b, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		return string(b)
 	}
-
-	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)
-	})
 }
 
 // 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. It closes the database the way scan does.
+// owns an open database.
 func brokenDatabase(t *testing.T) string {
 	t.Helper()
 
 	path := testDBPath(t)
 
-	db, err := openDB(path, scanParams)
+	db, err := openDB(path)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -102,7 +108,10 @@ func brokenDatabase(t *testing.T) string {
 		t.Fatal(err)
 	}
 
-	closeScanDatabase(t.Context(), db, path)
+	err = db.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
 
 	return path
 }
@@ -135,10 +144,7 @@ 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. 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.
+	// it fails: no os.Exit between the open and the return.
 	cases := map[string][]string{
 		cmdScan:   {cmdScan},
 		cmdReport: {cmdReport},
@@ -154,15 +160,17 @@ func TestRunFatalAfterOpenClosesDatabase(t *testing.T) {
 				args = append(args, t.TempDir())
 			}
 
-			var stdout, stderr bytes.Buffer
+			var stderr bytes.Buffer
 
-			code := run(args, &stdout, &stderr)
+			stdout := captureStdout(t)
+
+			code := run(args, &stderr)
 			if code != exitFatal {
 				t.Errorf("run(%v) = %d, want %d", args, code, exitFatal)
 			}
 
 			assertNoSidecars(t, path)
-			assertFatalOutput(t, stderr.String(), stdout.String())
+			assertFatalOutput(t, stderr.String(), stdout())
 
 			// Proof that the failure happened after the open: only a
 			// query against the opened database can report this.
@@ -180,16 +188,18 @@ func TestRunMissingOperandIsFatalNotUsage(t *testing.T) {
 	// must not dump the usage text.
 	t.Setenv(databaseEnv, testDBPath(t))
 
-	var stdout, stderr bytes.Buffer
+	var stderr bytes.Buffer
+
+	stdout := captureStdout(t)
 
 	missing := filepath.Join(t.TempDir(), "nope")
 
-	code := run([]string{cmdScan, missing}, &stdout, &stderr)
+	code := run([]string{cmdScan, missing}, &stderr)
 	if code != exitFatal {
 		t.Errorf("run(scan %s) = %d, want %d", missing, code, exitFatal)
 	}
 
-	assertFatalOutput(t, stderr.String(), stdout.String())
+	assertFatalOutput(t, stderr.String(), stdout())
 }
 
 // assertFatalOutput checks that a fatal error was reported the way
@@ -233,9 +243,11 @@ func TestRunUsageErrors(t *testing.T) {
 			// path that does not exist.
 			t.Setenv(databaseEnv, testDBPath(t))
 
-			var stdout, stderr bytes.Buffer
+			var stderr bytes.Buffer
 
-			code := run(tc.args, &stdout, &stderr)
+			stdout := captureStdout(t)
+
+			code := run(tc.args, &stderr)
 			if code != exitUsage {
 				t.Errorf("run(%v) = %d, want %d", tc.args, code, exitUsage)
 			}
@@ -244,7 +256,7 @@ func TestRunUsageErrors(t *testing.T) {
 				t.Errorf("stderr = %q, want %q", stderr.String(), tc.want)
 			}
 
-			if got := stdout.String(); got != "" {
+			if got := stdout(); got != "" {
 				t.Errorf("stdout = %q, want nothing (data only)", got)
 			}
 		})
@@ -253,9 +265,9 @@ func TestRunUsageErrors(t *testing.T) {
 
 // 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) {
-	t.Parallel()
-
 	assertHumanOutput(t, "--help")
 	assertHumanOutput(t, "--version")
 }
@@ -266,9 +278,11 @@ func TestRunHelpAndVersionSucceed(t *testing.T) {
 func assertHumanOutput(t *testing.T, arg string) {
 	t.Helper()
 
-	var stdout, stderr bytes.Buffer
+	var stderr bytes.Buffer
 
-	code := run([]string{arg}, &stdout, &stderr)
+	stdout := captureStdout(t)
+
+	code := run([]string{arg}, &stderr)
 	if code != exitOK {
 		t.Errorf("run(%s) = %d, want %d", arg, code, exitOK)
 	}
@@ -277,7 +291,7 @@ func assertHumanOutput(t *testing.T, arg string) {
 		t.Errorf("run(%s) wrote nothing to stderr", arg)
 	}
 
-	if got := stdout.String(); got != "" {
+	if got := stdout(); got != "" {
 		t.Errorf("stdout = %q, want nothing (data only)", got)
 	}
 }
@@ -306,15 +320,17 @@ func scanFixture(t *testing.T) []string {
 		t.Fatal(err)
 	}
 
-	var stdout, stderr bytes.Buffer
+	var stderr bytes.Buffer
 
-	code := run([]string{cmdScan, dir}, &stdout, &stderr)
+	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())
 	}
 
-	if got := stdout.String(); got != "" {
+	if got := stdout(); got != "" {
 		t.Errorf("scan stdout = %q, want nothing (data only)", got)
 	}
 
@@ -335,16 +351,18 @@ func TestRunReportSucceeds(t *testing.T) {
 
 	dupes := scanFixture(t)
 
-	var stdout, stderr bytes.Buffer
+	var stderr bytes.Buffer
 
-	code := run([]string{cmdReport}, &stdout, &stderr)
+	stdout := captureStdout(t)
+
+	code := run([]string{cmdReport}, &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.String(); got != want {
+	if got := stdout(); got != want {
 		t.Errorf("stdout = %q, want %q", got, want)
 	}
 
@@ -357,9 +375,11 @@ func TestRunTreesSucceeds(t *testing.T) {
 
 	dupes := scanFixture(t)
 
-	var stdout, stderr bytes.Buffer
+	var stderr bytes.Buffer
 
-	code := run([]string{cmdTrees}, &stdout, &stderr)
+	stdout := captureStdout(t)
+
+	code := run([]string{cmdTrees}, &stderr)
 	if code != exitOK {
 		t.Fatalf("run(trees) = %d, want %d; stderr: %s",
 			code, exitOK, stderr.String())
@@ -369,197 +389,9 @@ 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.String(); got != want {
+	if got := stdout(); 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)
-		}
-	}
-}

report.go

@@ -4,7 +4,6 @@ import (
 	"bufio"
 	"context"
 	"fmt"
-	"io"
 	"os"
 	"slices"
 	"strings"
@@ -32,9 +31,9 @@ type scanRec struct {
 // 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 always runs; the database is
-// closed before the caller formats its output, so it stays closed even
-// if that output fails.
+// 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) {
 	dbPath := databasePath()
 
@@ -66,7 +65,7 @@ type dupeGroup struct {
 // 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, stdout io.Writer) error {
+func runReport(ctx context.Context) error {
 	recs, err := loadRecords(ctx)
 	if err != nil {
 		return err
@@ -74,7 +73,7 @@ func runReport(ctx context.Context, stdout io.Writer) error {
 
 	dupes := collectDupeGroups(recs)
 
-	out := bufio.NewWriterSize(stdout, ioBufSize)
+	out := bufio.NewWriterSize(os.Stdout, ioBufSize)
 
 	_, err = fmt.Fprintln(out, "first\tdupe\tsize")
 	if err != nil {

report_test.go

@@ -50,9 +50,11 @@ func seedDatabase(t *testing.T, recs []scanRec) string {
 func TestRunReportEscapesPaths(t *testing.T) {
 	t.Setenv(databaseEnv, seedDatabase(t, awkwardPairRecs()))
 
-	var stdout, stderr bytes.Buffer
+	var stderr bytes.Buffer
 
-	code := run([]string{cmdReport}, &stdout, &stderr)
+	stdout := captureStdout(t)
+
+	code := run([]string{cmdReport}, &stderr)
 	if code != exitOK {
 		t.Fatalf("run(report) = %d, want %d; stderr: %s",
 			code, exitOK, stderr.String())
@@ -60,7 +62,7 @@ func TestRunReportEscapesPaths(t *testing.T) {
 
 	want := "first\tdupe\tsize\n" +
 		`/d/\tone\ntwo\rthree\\four/f` + "\t/d/A/f\t5\n"
-	if got := stdout.String(); got != want {
+	if got := stdout(); got != want {
 		t.Errorf("stdout = %q, want %q", got, want)
 	}
 }

scan.go

@@ -85,13 +85,10 @@ type fileMeta struct {
 // 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. Cancelling ctx unwinds the worker pools and
-// aborts the scan with the context's error.
+// 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.
 func runScan(ctx context.Context, roots []string, workers int,
 	oneFS bool,
 ) error {
@@ -106,19 +103,12 @@ func runScan(ctx context.Context, roots []string, workers int,
 
 	dbPath := databasePath()
 
-	lock, err := lockScanDatabase(dbPath)
-	if err != nil {
-		return err
-	}
-
-	defer func() { _ = lock.Close() }()
-
 	db, err := openScanDatabase(ctx, dbPath)
 	if err != nil {
 		return err
 	}
 
-	defer closeScanDatabase(ctx, db, dbPath)
+	defer func() { _ = db.Close() }()
 
 	st, err := syncScan(ctx, db, roots, workers, oneFS)
 	if err != nil {

scan_test.go

@@ -7,7 +7,6 @@ import (
 	"database/sql"
 	"encoding/hex"
 	"fmt"
-	"io"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -1549,7 +1548,7 @@ func TestScanHashWriteFailureUnwindsPool(t *testing.T) {
 
 	code := run([]string{
 		cmdScan, "--workers", strconv.Itoa(hashLeakWorkers), dir,
-	}, io.Discard, &stderr)
+	}, &stderr)
 	if code != exitFatal {
 		t.Fatalf("run(scan) = %d, want %d; stderr: %s",
 			code, exitFatal, stderr.String())

trees.go

@@ -5,7 +5,6 @@ import (
 	"context"
 	"crypto/sha256"
 	"fmt"
-	"io"
 	"os"
 	"slices"
 	"strconv"
@@ -37,7 +36,7 @@ type treeNode struct {
 // 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, stdout io.Writer) error {
+func runTrees(ctx context.Context) error {
 	recs, err := loadRecords(ctx)
 	if err != nil {
 		return err
@@ -48,7 +47,7 @@ func runTrees(ctx context.Context, stdout io.Writer) error {
 
 	dupes := collectTreeGroups(allDirs, super)
 
-	out := bufio.NewWriterSize(stdout, ioBufSize)
+	out := bufio.NewWriterSize(os.Stdout, ioBufSize)
 
 	_, err = fmt.Fprintln(out, "first\tdupe\tfiles\tsize")
 	if err != nil {

trees_test.go

@@ -108,9 +108,11 @@ func TestBuildHierarchyRootPath(t *testing.T) {
 func TestRunTreesEscapesPaths(t *testing.T) {
 	t.Setenv(databaseEnv, seedDatabase(t, awkwardPairRecs()))
 
-	var stdout, stderr bytes.Buffer
+	var stderr bytes.Buffer
 
-	code := run([]string{cmdTrees}, &stdout, &stderr)
+	stdout := captureStdout(t)
+
+	code := run([]string{cmdTrees}, &stderr)
 	if code != exitOK {
 		t.Fatalf("run(trees) = %d, want %d; stderr: %s",
 			code, exitOK, stderr.String())
@@ -118,7 +120,7 @@ func TestRunTreesEscapesPaths(t *testing.T) {
 
 	want := "first\tdupe\tfiles\tsize\n" +
 		`/d/\tone\ntwo\rthree\\four` + "\t/d/A\t1\t5\n"
-	if got := stdout.String(); got != want {
+	if got := stdout(); got != want {
 		t.Errorf("stdout = %q, want %q", got, want)
 	}
 }