DeRuina / timberjack / 35971776523

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fix(rotation): name collision (#124)

* fix(rotation): never overwrite an existing backup on name collision (#123)

Two rotations that formatted to the same timestamp with the same reason
produced identical backup filenames, and os.Rename silently replaced the
earlier backup. This affected every rotation trigger (size, interval,
scheduled and manual) and was easy to hit with a low-precision
BackupTimeFormat.

openNew now asks uniqueBackupName for the destination. When the natural
name is taken it appends "_1", "_2", ... to the reason segment until a
free name is found, so timeFromName keeps parsing the timestamp and
cleanup keeps working. A compressed variant (".gz"/".zst") also counts
as occupying the base name, otherwise the mill would later mistake the
fresh backup for an already-compressed leftover and delete it. After
maxBackupNameAttempts candidates the rotation fails with an error rather
than overwrite anything.

TestMaxBackups is adjusted accordingly: a pre-existing compressed backup
at the target name now yields a "_1" backup alongside it, and both still
count as one timestamp for MaxBackups.

Fixes #123

* test(rotation): cover compression of suffixed collision backups

24 of 24 new or added lines in 1 file covered. (100.0%)

796 of 886 relevant lines covered (89.84%)

51.79 hits per line