Re: [PATCH v2] packfile: freshen the mtime of packfile by configuration

[Taylor Blau <me@ttaylorr.com>]

On Wed, Jul 14, 2021 at 03:39:18AM +0200, Ævar Arnfjörð Bjarmason wrote:
> Hrm, per my v1 feedback (and I'm not sure if my suggestion is even good
> here, there's others more familiar with this area than I am), I was
> thinking of something like a *.bump file written via:
>
>     core.packUseBumpFiles=bool
>
> Or something like that, anyway, the edge case in allowing the user to
> pick arbitrary suffixes is that we'd get in-the-wild user arbitrary
> configuration squatting on a relatively sensitive part of the object
> store.
>
> E.g. we recently added *.rev files to go with
> *.{pack,idx,bitmap,keep,promisor} (and I'm probably forgetting some
> suffix). What if before that a user had set:
>
>     core.packMtimeSuffix=rev

I think making the suffix configurable is probably a mistake. It seems
like an unnecessary detail to expose, but it also forces us to think
about cases like these where the configured suffix is already used for
some other purpose.

I don't think that a new ".bump" file is a bad idea, but it does seem
like we have a lot of files that represent a relatively little amount of
the state that a pack can be in. The ".promisor" and ".keep" files both
come to mind here. Some thoughts in this direction:

  - Combining *all* of the pack-related files (including the index,
    reverse-index, bitmap, and so on) into a single "pack-meta" file
    seems like a mistake for caching reasons.

  - But a meta file that contains just the small state (like promisor
    information and whether or not the pack is "kept") seems like it
    could be OK. On the other hand, being able to tweak the kept state
    by touching or deleting a file is convenient (and having to rewrite
    a meta file containing other information is much less so).

But a ".bump" file does seem like an awkward way to not rely on the
mtime of the pack itself. And I do think it runs into compatibility
issues like Ævar mentioned. Any version of Git that includes a
hypothetical .bump file (or something like it) needs to also update the
pack's mtime, too, so that old versions of Git can understand it. (Of
course, that could be configurable, but that seems far too obscure to
me).

Stepping back, I'm not sure I understand why freshening a pack is so
slow for you. freshen_file() just calls utime(2), and any sync back to
the disk shouldn't need to update the pack itself, just a couple of
fields in its inode. Maybe you could help explain further.

In any case, I couldn't find a spot in your patch that updates the
packed_git's 'mtime' field, which is used to (a) sort packs in the
linked list of packs, and (b) for determining the least-recently used
pack if it has individual windows mmapped.

Thanks,
Taylor