NFR-R2 asks for the catalog to be backed up on a schedule and before
schema migrations. Only the second half existed: every backup on disk
was a pre-migration copy, and a library that never migrated was never
backed up at all.
A backup is now also taken at the end of a library sweep when the newest
one is more than a day old — the moment the catalog is quiet and a day's
collection and people edits have just been folded in — on its own
thread and its own connection, so the copy of a 130 MB file is not spent
on the UI. Whether one is due is read from the backup directory, not
the catalog, so the ordinary case costs nothing. An empty catalog is
skipped: there is nothing in it a rescan would not rebuild. Pruning to
KEEP_BACKUPS applies as before.
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