Add sfdupes info: show the database's details (closes #117)

info prints the database's path and source, the sizes of the database
file and its -wal file, when the database file last changed, the schema
version and the record counts, one per line as a name, a tab and the
value. It opens the database as report does and shows an open and a
count phase on stderr. The schema version and the counts come from one
read-only transaction, the counts from one query.

seedDatabase now closes test databases out of WAL mode, as scan does;
otherwise info's own connection creates an empty -wal file. The open
phase's name and text became constants for the linter, and the scan
command moved to newScanCommand to keep newRootCommand short enough.

Judgement call: the lines are written after the last display ends, so they skip the display's stdout writer.

Model: opus-5-5

README.md

@@ -38,8 +38,9 @@ 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
-with no `PATH` operand — prints a usage message and exits 2.
+duplicate-tree report. `info` prints the database's details: its path, the sizes
+of its files and the number of its records. A missing/invalid subcommand — or a
+`scan` invocation with no `PATH` operand — prints a usage message and exits 2.
 
 The database defaults to `/var/lib/sfdupes/db.sqlite` and can be placed anywhere
 by setting `SFDUPES_DATABASE`. The intended deployment is a daily `sfdupes scan`
@@ -109,9 +110,9 @@ As a file `/etc/cron.d/sfdupes`:
 - 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.
+- `report`, `trees` and `info` 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
 
@@ -235,7 +236,7 @@ Goals, in order:
 
 ### Subcommands
 
-Three subcommands, all implemented:
+Four subcommands, all implemented:
 
 1.  `scan` — walk the filesystem and synchronize the database: one signature
     record per regular file.
@@ -243,11 +244,14 @@ Three subcommands, all implemented:
 3.  `trees` — tree-level duplicate report: reconstruct the directory hierarchy
     from the database records, compute a Merkle-style digest per directory, and
     report maximal groups of identical trees.
+4.  `info` — the database's details: its path, the sizes of its files, its
+    schema version and the counts of its records.
 
 ```
 sfdupes scan [--workers N] [-x] PATH...
 sfdupes report > dupes.tsv
 sfdupes trees > dupetrees.tsv
+sfdupes info
 sfdupes --version
 sfdupes [command] --help
 ```
@@ -262,7 +266,7 @@ stdout.
 
 ### Database
 
-All three subcommands operate on a single SQLite database file:
+All four subcommands operate on a single SQLite database file:
 
 - Location: the value of the `SFDUPES_DATABASE` environment variable when set
   and non-empty, otherwise `/var/lib/sfdupes/db.sqlite`. There is no
@@ -285,13 +289,13 @@ All three subcommands operate on a single SQLite database file:
     symlinks resolved. After a `cd` through a symlink to a directory, a path
     starting `../` therefore names a file beside the directory the symlink leads
     to, as it does for any other program. `scan` says `new database` when no
-    file is at that path as it starts. `report` and `trees` print the line
-    before they open the database, so the error for a missing database comes
-    after it.
+    file is at that path as it starts. `report`, `trees` and `info` print the
+    line before they open the database, so the error for a missing database
+    comes after it.
 
-- `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.
+- `scan` creates the database (and its parent directory) on first use. `report`,
+  `trees` and `info` 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's path with symlinks resolved
@@ -307,8 +311,8 @@ All three subcommands operate on a single SQLite database file:
   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.
+  deleting the file while a scan runs would let a second scan start. `report`,
+  `trees` and `info` 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.
@@ -341,10 +345,10 @@ All three subcommands operate on a single SQLite database file:
     - In the content phase: content hashes, again committed 10,000 at a time.
     - At the end, as it closes the database: the `-wal` and `-shm` files go
       away, unless a report still has the database open (see above).
-- `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.
+- `report`, `trees` and `info` 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. `scan` creates the schema and sets
   the version in one transaction, so a first scan stopped while doing so leaves
@@ -727,13 +731,55 @@ 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.
 
+### `info` mode
+
+`info` takes no positional arguments and prints the database's details on
+stdout, one per line: a name, a tab, and the value. It opens the database the
+way `report` does, read-only and without the scan lock, so it runs during a scan
+and needs only read access (see "Database"). Like `report`, it never touches the
+scanned filesystem.
+
+```
+database	/var/lib/sfdupes/db.sqlite (default)
+database file size	2147483648 (2.0 GiB)
+-wal file size	4124152 (3.9 MiB)
+modified	2026-10-16T12:02:50Z
+schema version	1
+records	12345678
+records with head and tail hashes	1234567
+records with content hash	123456
+size of recorded files	150000000000000 (136.4 TiB)
+```
+
+- `database` is the path and where it came from, as on the line every subcommand
+  prints first on stderr (see "Database").
+- `database file size` and `-wal file size` are in bytes, then in human units.
+  The `-wal` line is there only when a `-wal` file is beside the database, as
+  while a scan runs.
+- `modified` is the time the database file was last changed, in UTC. While the
+  database is in WAL mode, the newest changes may be in the `-wal` file alone.
+- `schema version` is the database's `PRAGMA user_version`. A database with
+  another version than this build's is refused, as by `report`.
+- `records` is the number of records. `records with head and tail hashes` and
+  `records with content hash` count the records that have those hashes (see
+  "Database"), and `size of recorded files` adds up the sizes of all records, in
+  bytes, then in human units.
+
+`info` runs in two phases, each with its own progress display on stderr (see
+"Progress"): `open` opens the database; `count` reads the schema version and
+counts the records with one query, a single pass over the table, both in one
+read-only transaction, so the numbers agree with each other while a scan is
+committing. `info` writes its lines once the `count` display has ended, so on a
+terminal they come after the displays.
+
 ### Progress
 
-Use the progress-bar library for all progress of `scan`, `report` and `trees`;
-rendering in the style of `pv` is the model. All progress goes to stderr.
+Use the progress-bar library for all progress of `scan`, `report`, `trees` and
+`info`; rendering in the style of `pv` is the model. All progress goes to
+stderr.
 
-`scan`, `report` and `trees` must always show that they are working and not
-hung. Each phase gets its own display from the moment it starts, naming the
+`scan`, `report`, `trees` and `info` must always show that they are working and
+not hung. Each phase gets its own display from the moment it starts, naming the
 phase and its place among the command's phases and showing its progress, its
 elapsed time and its estimated time remaining (ETA).
 
@@ -773,6 +819,14 @@ A scan has five phases (`[1/5] load`, `[2/5] walk`, `[3/5] hash`,
   the rows to stdout as it goes. While the count runs, the display says it is
   counting the records.
 
+`info` has two phases (`[1/2] open`, `[2/2] count`, see "`info` mode"):
+
+- `open` has no total. While it opens the database, waiting on a busy one
+  included, the display says it is opening the database.
+- `count` has no total either, because one query counts everything: the display
+  says it is counting the records until the query returns, and its ETA is
+  unknown.
+
 Each display is one line holding:
 
 - the phase and its place among the phases
@@ -792,13 +846,14 @@ Example shape (exact layout is flexible, content is not):
 
 Additional requirements:
 
-- On a terminal, a phase with an exact total (every phase but `scan`'s `walk`
-  and the `open` phase of `report` and `trees`, once it has counted what it will
-  do) is drawn as its line followed by the progress-bar library's bar, 10
-  columns wide, which shows the percent in place of the line's own. Any other
-  phase, and a phase whose bar has no room, is drawn as the library's spinner
-  followed by its line. Each display is drawn again several times a second, so
-  its elapsed time keeps moving while a phase waits for its next item.
+- On a terminal, a phase with an exact total (every phase but `scan`'s `walk`,
+  the `open` phase of `report` and `trees`, and both phases of `info`, once it
+  has counted what it will do) is drawn as its line followed by the progress-bar
+  library's bar, 10 columns wide, which shows the percent in place of the line's
+  own. Any other phase, and a phase whose bar has no room, is drawn as the
+  library's spinner followed by its line. Each display is drawn again several
+  times a second, so its elapsed time keeps moving while a phase waits for its
+  next item.
 - On a terminal no line reaches the terminal's last column, however narrow the
   terminal is. The phase and its place, the progress, the elapsed time and the
   ETA are kept whole: a line too wide for the terminal leaves out its rate first
@@ -831,10 +886,12 @@ Additional requirements:
 - `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), or a `scan` stopped by `SIGINT` or `SIGTERM` (see below).
+  created/opened/read/written, a missing database for `report`/`trees`/`info`,
+  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).
+  `--workers` below 1, and `report`/`trees`/`info` 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: