> Evidently, when we disabled backups in /tmp, we missed the fact that
> lock_file called from write-region will be affected.  When backups are
> not disabled, this works correctly.

To me, the fact that it works with backups enabled seems like an
accident.  The connection between locking and backups isn't very
obvious, and I don't see any commentary about it (which I would expect
for such a subtle interaction if it was done on purpose).