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

On Thu, Mar 18, 2021 at 11:24 PM Martin Fick [off-list ref] wrote:

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

quoted

On Wed, Mar 17, 2021 at 10:22 PM Martin Fick [off-list ref] wrote:

quoted

On Wednesday, March 17, 2021 9:06:06 PM MDT Han-Wen Nienhuys wrote:

quoted

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.

-- 
Han-Wen Nienhuys - Google Munich
I work 80%. Don't expect answers from me on Fridays.
--

Google Germany GmbH, Erika-Mann-Strasse 33, 80636 Munich

Registergericht und -nummer: Hamburg, HRB 86891

Sitz der Gesellschaft: Hamburg

Geschäftsführer: Paul Manicle, Halimah DeLaine Prado