On 2023-12-31 18:27, Junio C Hamano wrote:
Stefan Haller <lists@xxxxxxxxxxxxxxxx> writes:
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.
Thank you very much for this highly detailed explanation.
