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

From: Jeff King <peff@peff.net>
Date: Dec 9, 2022, 01:37 UTC
Message-ID: <Y5KRQMcUlepwNlor@coredump.intra.peff.net>
In-Reply-To: <20221209004918.GI28810@kitsune.suse.cz>

On Fri, Dec 09, 2022 at 01:49:18AM +0100, Michal Suchánek wrote:

Show 6 quoted lines

> > 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.

Show 6 quoted lines

> 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).

-Peff