bug#65720: Guile-Git-managed checkouts grow way too much

[Simon Tournier <zimon.toutoune@gmail.com>]

Hi,

On Mon, 04 Sep 2023 at 23:47, Ludovic Courtès <ludo@gnu.org> wrote:

>> It would seem that libgit2 doesn’t do the equivalent of ‘git gc’.
>
> Confirmed: <https://github.com/libgit2/libgit2/issues/3247>.

Ouch!

        The goals of the project haven't changed, and neither have the
        tradeoffs. If one were to rewrite git-gc on top of libgit2, the
        best-case scenario is ending up with what we already had.

        If you want to use regular maintenance on some repostories, use
        git gc, that's what it's there for.

        https://github.com/libgit2/libgit2/issues/3247#issuecomment-152508040

> My inclination for the short term would be to work around this
> limitation by (1) finding a heuristic to determine is a checkout has
> likely accumulated too much cruft, and (2) considering such checkouts
> as expired (thereby forcing a re-clone) or running ‘git gc’ on them if
> ‘git’ is available.

About (1) maybe we could add a “counter” and teach after X updates of
the checkout then let run (2).  Well, I guess the number of crufts is
more or less proportional with the number of checkout updates; that’s
the heuristic I would use.

The most annoying is (2).  Because forcing a re-clone does not appear to
me a solution; I prefer to waste disk space (and probably run myself and
manually ‘git gc’) than re-clone… Somehow this re-clone would always
happen when I am using a poor network.

Moreover, assuming this clean-up (2) would be run once every while, we
could imagine to invoke something like,

    guix shell -C git-minimal
         -- git
         -C ~/.cache/guix/checkouts/pjmkglp4t7znuugeurpurzikxq3tnlaywmisyr27shj7apsnalwq
         gc

when the checkout is updated.  And maybe we could provide another “guix
pull” command-line option for turning off this and mark it as done
(reset the “counter”).

Well, that’s a poor solution but we can assume that git-minimal is at
worse available using “guix shell git-minimal”.  Note that the closure
of git-minimal is far less than re-cloning the full Guix repository.

Cheers,
simon