Re: Question: How to execute git-gc correctly on the git server

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Dec 9, 2022, 13:48 UTC
Message-ID: <221209.86bkoc7kgi.gmgdl@evledraar.gmail.com>
In-Reply-To: <CAOLTT8SR6JWX6mRLbyq4keb4JCfJP6Vq07LzHpb_f+e1jMnsZQ@mail.gmail.com>

On Fri, Dec 09 2022, ZheNing Hu wrote:

Show 31 quoted lines

> Jeff King <peff@peff.net> 于2022年12月9日周五 09:37写道：
>>
>> On Fri, Dec 09, 2022 at 01:49:18AM +0100, Michal Suchánek wrote:
>>
>> > > In this case it's the mtime on the object file (or the pack containing
>> > > it). But yes, it is far from a complete race-free solution.
>> >
>> > So if you are pushing a branch that happens to reuse commits or other
>> > objects from an earlier branh that might have been collected ín the
>> > meantime you are basically doomed.
>>
>> Basically yes. We do "freshen" the mtimes on object files when we omit
>> an object write (e.g., your index state ends up at the same tree as an
>> old one). But for a push, there is no freshening. We check the graph at
>> the time of the push and decide if we have everything we need (either
>> newly pushed, or from what we already had in the repo). And that is
>> what's racy; somebody might be deleting as that check is happening.
>>
>> > People deleting a branch and then pushing another variant in which many
>> > objects are the same is a risk.
>> >
>> > People exporting files from somewhere and adding them to the repo which
>> > are bit-identical when independently exported by multiple people and
>> > sometimes deleting branches is a risk.
>>
>> Yes, both of those are risky (along with many other variants).
>>
>
> I'm wondering if there's an easy and poor performance way to do
> gc safely? For example, add a file lock to the repository during
> git push and git gc?

We don't have any "easy" way to do it, but we probably should. The root cause of the race is tricky to fix, and we don't have any "global ref lock".

But in the context of a client<->server and wanting to do gc on the server a good enough and easy solution would be e.g.:

 1. Have a {pre,post}-receive hook logging attempted/finished pushes
 2. Have the pre-receive hook able to reject (or better yet, hang with
    sleep()) incoming deletions
 3. Do a gc with a small wrapper script, which:
    - Flips the "no deletion ops now" (or "delay deletion ops") switch
    - Polls until it's sure there's no relevant in-progress operations
    - Do a full gc
    - Unlock

You'd need to be certain that all relevant repo operations are going through git-receive-pack etc., i.e. a local "git branch -d" or the like won't run {pre,post}-receive.