Re: concurrent fetches to update same mirror

> On Wed, Jan 05, 2011 at 03:53:25PM -0500, Jeff King wrote:
>
>>> If both fetch processes try to update the same ref at the same time,
>>> one will get the lock and continue, and the other will crash with an
>>> error (because the lock was busy).  If one is slightly slower than the
>>> other, they will probably update the refs twice, with the slower fetch
>>> updating what the faster one had just updated.  :-)
>>
>> I assumed it would take the "old" value at the very beginning of the
>> fetch (before talking with the remote), and then see that the ref was
>> changed under our feet. Or does it simply do it at the end?
>
> Hmm. Weirder even, builtin/fetch.c:s_update_ref takes a "check_old"
> flag, and we do always use it for branch updates. But not for tag
> updates. I can't think of why. The code blames all the way back to the
> original builtin-fetch.
>
> Anyway, when we do check, we check the value from the beginning of the
> fetch. So you can get lock conflicts. For example, doing this:
>
>    mkdir repo&&  cd repo&&  git init
>    echo contents>foo&&  git add .&&  git commit -m one
>    git update-ref refs/remotes/origin/master refs/heads/master
>    git remote add origin some-remote-repo-that-takes-a-few-seconds
>    xterm -e 'git fetch -v; read'&  xterm -e 'git fetch -v; read'
>
> I.e., putting some cruft into the ref and then updating it. One fetch
> will force-write over the ref properly:
>
>     + ac32203...4e64590 master     ->  origin/master  (forced update)
>
> but the other one will barf on the lock:
>
>    error: Ref refs/remotes/origin/master is at 4e6459052ab329914c7712a926773e566b8c821d but expected ac32203727daa3bcb5fc041786aa45adbbe86299
>    ...
>     ! ac32203...4e64590 master     ->  origin/master  (unable to update local ref)
>
> Interestingly, in the case of ref _creation_, not update, like this:
>
>    mkdir repo&&  cd repo&&  git init
>    git remote add origin some-remote-repo-that-takes-a-few-seconds
>    xterm -e 'git fetch -v; read'&  xterm -e 'git fetch -v; read'
>
> then both will happily update, the second one overwriting the results of
> the first. It seems in the case of locking a ref which previously didn't
> exist, we don't enforce that it still doesn't exist.
>
> I wonder if we should, but perhaps there is some corner case I am not
> considering. The code is in lock_ref_sha1_basic, but blaming didn't turn
> up anything helpful.
>
> -Peff