Re: [RFC PATCH] rerere: fix overeager gc

SZEDER Gábor [off-list ref] writes:

...  But I'm not sure I
can rely on that when gc'ing.

Looking at the timestamp of "thisimage" would probaboly be more sensible
than "preimage" alone, _if_ "thisimage" still exists.  It is rewritten
every time this particular conflict is observed; this is not necessarily
when the recorded resolution is _used_, but it may be close enough in
practice.

You probably would want to rename the helper as "last_checked_at", though.

After rerere does its work, however, "thisimage" does not have to stay
around (the user can remove it, or we could enhance "gc" to do so).

Here you already know that you have resolution (i.e. "postimage"), but
your new function cannot stat a corresponding "thisimage", so you err
on the safer side---but that means you may keep pre/post image pairs
forever if somebody removes otherwise unused "thisimage" from a distant
past.  Perhaps we should apply cutoff_noresolve to the entry here?

One possibility is to look at the timestamp of the directory itself
instead.  Then we can safely gc otherwise-unused "thisimage" file when
rerere is not in use.  I wonder if directory m_time timestamps are usable
for this purpose on non-POSIX platforms?