Re: [PATCH 1/2] credential-cache--daemon: allow a concurrently created socket dir

From: Jeff King <peff@peff.net>
Date: Nov 4, 2026, 02:58 UTC
Message-ID: <20261104025835.GB1492379@coredump.intra.peff.net>
In-Reply-To: <20261101-fz-credential-cache-race-v1-1-28f0bac51e11@gmail.com>

On Sun, Nov 01, 2026 at 03:29:25PM -0800, Farid Zakaria wrote:

Show 10 quoted lines

> When no daemon is running, two "git credential-cache store" run at the
> same time each try to start one. If the socket directory does not exist
> yet, both daemons try to create it. The one that comes second dies:
> 
>     fatal: unable to mkdir '<home>/.cache/git/credential': File exists
> 
> That client then fails with "cache daemon did not start".
> 
> The fix is to ignore EEXIST from mkdir(), and check the directory's
> permissions the same way as for one that already existed.

Makes sense. This is what safe_create_leading_directories() does for all of the other path components. So of course I wondered why we bothered to do this last one ourselves, but the comment (which is just out of the context of your patch) explains it:

                /*
                 * We must be sure to create the directory with the correct mode,
                 * not just chmod it after the fact; otherwise, there is a race
                 * condition in which somebody can chdir to it, sleep, then try to open
                 * our protected socket.
                 */

If we find the directory already created, then we are subject to the same race, but it is reasonable to assume that whoever created it did so in the correct non-racy way.

-Peff