Re: Concurrent fetch commands

From: Junio C Hamano <gitster@pobox.com>
Date: Dec 31, 2023, 17:27 UTC
Message-ID: <xmqqy1daffk8.fsf@gitster.g>
In-Reply-To: <c11ca0b3-aaf4-4a8d-80a1-3832954aa7aa@haller-berlin.de>

Stefan Haller <lists@haller-berlin.de> writes:

Show 6 quoted lines

> I can reliably reproduce this by doing
>
>    $ git fetch&; sleep 0.1; git pull
>    [1] 42160
>    [1]  + done       git fetch
>    fatal: Cannot rebase onto multiple branches.

I see a bug here.

How this _ought_ to work is

 - The first "git fetch" wants to report what it fetched by writing
   into the $GIT_DIR/FETCH_HEAD file ("git merge FETCH_HEAD" after
   the fetch finishes can consume its contents).

 - The second "git pull" runs "git fetch" under the hood.  Because
   it also wants to write to $GIT_DIR/FETCH_HEAD, and because there
   is already somebody writing to the file, it should notice and
   barf, saying "fatal: a 'git fetch' is already working" or
   something.

But because there is no "Do not overwrite FETCH_HEAD somebody else is using" protection, "git merge" or "git rebase" that is run as the second half of the "git pull" ends up working on the contents of FETCH_HEAD that is undefined, and GIGO result follows.

The "bug" that the second "git fetch" does not notice an already running one (who is in possession of FETCH_HEAD) and refrain from starting is not easy to design a fix for---we cannot just abort by opening it with O_CREAT|O_EXCL because it is a normal thing for $GIT_DIR/FETCH_HEAD to exist after the "last" fetch. We truncate its contents before starting to avoid getting affected by contents leftover by the last fetch, but when there is a "git fetch" that is actively running, and it finishes _after_ the second one starts and truncates the file, the second one will end up seeing the contents the first one left. We have the "--no-write-fetch-head" option for users to explicitly tell which invocation of "git fetch" should not write FETCH_HEAD.

Running "background/priming" fetches (the one before "sleep 0.1" you have) is not a crime by itself, but it is a crime to run them without the "--no-fetch-head" option. Since you have *NO* intention of using its contents to feed a "git merge" (or equivalent) yourself, you are breaking your "git pull" step in your example reproduction yourself.