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

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

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?

But you are right: since the reflog for a branch is in a different
file from the branch head, there is no way to do an update to both of
them at the same time. I guess this will have to be a reftable-only
feature.

I believe that today git can do very fast reachability checks without opening
pack files by using some of its indexes (bitmap code or https://git-scm.com/
docs/commit-graph ?). It probably makes sense to add this ability to jgit if
that is what you need?

The bitmaps are generated by GC, and you can't GC all the time. JGit
has support for bitmaps, and its support actually predates C-Git's
support for it. (It was added to JGit by Colby Ranger who worked in
Shawn's team).

I expect that the commit graph doesn't work for my intended use-case.

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