man-db's manipulation of index.db mtime timestamp interferes with incremental backups

[Oliver O.]

It appears that since version 2.7.0, man-db manipulates the mtime timestamp of /var/cache/man/de/index.db in a way that the content may change, while the timestamp is reset to its previous value (see attachment).

See https://lists.ubuntu.com/archives/foundations-bugs/2014-September/209326.html

This interferes with incremental backups which pick up files via their modification timestamps. Such backups will miss updated files which do not have their mtime timestamps changed accordingly. Automated comparisons between such backups and (btrfs snapshots of) their source data will fail. The integrity of such backups cannot be guaranteed.

Release: Ubuntu 14.10
Package: man-db 2.7.0.2-2

[Colin Watson (cjwatson)]

This is a consequence of the poor layout noted in this comment in purge_missing:

        if (will_run_mandb)
                /* Reset mtime to avoid confusing mandb into not running.
                 * TODO: It would be better to avoid this by only opening
                 * the database once between here and mandb.
                 */
                MYDBM_SET_TIME (dbf, db_mtime);

There are some situations where a later update_db will not actually modify the database and update the mtime, and in this situation purge_missing may have updated the database but reset the mtime to its previous value. The reorganisation implied by this comment would fix this.

[Launchpad Janitor (janitor)]

This bug was fixed in the package man-db - 2.10.0-2

---------------
man-db (2.10.0-2) unstable; urgency=medium

  * Fix occasional mandb-symlink-target-timestamp test failure.

 -- Colin Watson <email address hidden> Fri, 04 Feb 2022 19:21:06 +0000