Re: reference-transaction regression in 2.36.0-rc1

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Tue, Apr 12 2022, Bryan Turner wrote:

> On Tue, Apr 12, 2022 at 2:20 AM Bryan Turner <bturner@atlassian.com> wrote:
>>
>> It looks like Git 2.36.0-rc1 includes the changes to eliminate/pare
>> back reference transactions being raised independently for loose and
>> packed refs. It's a great improvement, and one we're very grateful
>> for.
>>
>> It looks like there's a regression, though. When deleting a ref that
>> _only_ exists in packed-refs, by the time the "prepared" callback is
>> raised the ref has already been deleted completely. Normally when
>> "prepared" is raised the ref is still present. The ref still being
>> present is important to us, since the reference-transaction hook is
>> frequently not passed any previous hash; we resolve the ref during
>> "prepared", if the previous hash is the null SHA1, so that we can
>> correctly note what the tip commit was when the ref was deleted.
>
> I've re-tested this with 2.36.0-rc2 and it has the same regression (as
> expected). However, in playing with it more, the regression is more
> serious than I had initially considered. It goes beyond just losing
> access to the SHA of the tip commit for deleted refs. If a ref only
> exists packed, this regression means vetoing the "prepared" callback
> _cannot prevent its deletion_, which violates the contract for the
> reference-transaction as I understand it.
>
> Here's a slightly modified reproduction:
> git init ref-tx
> cd ref-tx
> git commit -m "Initial commit" --allow-empty
> git branch to-delete
> git pack-refs --all
> echo 'exit 1;' > .git/hooks/reference-transaction
> chmod +x .git/hooks/reference-transaction
> git branch -d to-delete
>
> Running this reproduction ends with:
> $ git branch -d to-delete
> fatal: ref updates aborted by hook
> $ git rev-parse to-delete --
> fatal: bad revision 'to-delete'
>
> Even though the reference-transaction vetoed "prepared", the ref was
> still deleted.
>
> In Bitbucket, we use the reference-transaction to perform replication.
> When we get the "prepared" callback on one machine, we dispatch the
> same change(s) to other replicas. Those replicas process the changes
> and arrive at their own "prepared" callbacks (or don't), at which
> point they vote to commit or rollback. The votes are tallied and the
> majority decision wins.
>
> With this regression, that sort of setup no longer works reliably for
> ref deletions. If the ref only exists packed, it's deleted (and
> _visibly_ deleted) before we ever get the "prepared" callback. So if
> coordination fails (i.e. the majority votes to rollback), even if we
> try to abort the change it's already too late.

This does look lik a series regression.

I haven't had time to bisect this, but I suspect that it'll come down to
something in the 991b4d47f0a (Merge branch
'ps/avoid-unnecessary-hook-invocation-with-packed-refs', 2022-02-18)
series.

I happen to know that Patrick is OoO until after the final v2.36.0 is
scheduled (but I don't know for sure that we won't spot this thread &
act on it before then).

Is this something you think you'll be able to dig into further and
possibly come up with a patch? It looks like you're way ahead of at
least myself in knowing how this *should* work :)