Re: Distinguishing FF vs non-FF updates in the reflog?

[Han-Wen Nienhuys <hanwen@google.com>]

On Thu, Mar 18, 2021 at 11:24 PM Martin Fick <mfick@codeaurora.org> wrote:
>
> On Thursday, March 18, 2021 9:58:56 AM MDT Han-Wen Nienhuys wrote:
> > On Wed, Mar 17, 2021 at 10:22 PM Martin Fick <mfick@codeaurora.org> wrote:
> > > On Wednesday, March 17, 2021 9:06:06 PM MDT Han-Wen Nienhuys wrote:
> > > > I'm working on some extensions to Gerrit for which it would be very
> > > > beneficial if we could tell from the reflog if an update is a
> > > > fast-forward or not: if we find a SHA1 in the reflog, and see there
> > > > were only FF updates since, we can be sure that the SHA1 is reachable
> > > > from the branch, without having to open packfiles and decode commits.
> > >
> > > I don't think this would be reliable.
> > >
> > > 1) Not all updates make it to the reflogs
> > > 2) Reflogs can be edited or mucked with
> > > 3) On NFS reflogs can outright be wrong even when used properly as their
> > > are caching issues. We specifically have seen entries that appear to be
> > > FFs that were not.
> >
> > Can you tell a little more about 3) ? SInce we don't annotate non-FF
> > vs FF today, what does "appear to be FFs" mean?
>
> To be honest I don't recall for sure, but I will describe what I think has
> happened. I think that we have seen a server(A) update a branch from
> C1 to C2A, and then later another server(B) update the same branch from C1 to
> C2B. Obviously the move from C2A to C2B is not a FF, but that move is not what
> is recorded. Each of those updates was a FF when viewed as separate entries,

I think those would fail with the way that Gerrit uses JGit, because
C1 -> C2B would fail with LOCK_ERROR. I guess there are code paths in
Git (?) that will execute force-push without checking if the update is
FF or not.

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Han-Wen Nienhuys - Google Munich
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