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

From: Martin Fick <mfick@codeaurora.org>
Date: Mar 18, 2021, 22:24 UTC
Message-ID: <4400050.5IlZNYTcJN@mfick-lnx>
In-Reply-To: <CAFQ2z_MavgAGDyJzc9-+j6zTDODP7hCdPHtB5dyx-reLMSLX3Q@mail.gmail.com>

On Thursday, March 18, 2021 9:58:56 AM MDT Han-Wen Nienhuys wrote:

Show 18 quoted lines

> 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, but if you look at both lines you can see that the second entry does not start where the first one left off. This would be detectable, it would appear as if there was a missed entry that did a rewind from C2A to C1, but that rewind presumably actually came in as part of the second update from server (B) as it had a cached version of the branch and believed it still pointed to C1 when it made its update,

-Martin

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