Re: [PATCH] rerere: keep a background gc from killing a rebase

[Patrick Steinhardt <ps@pks.im>]

On Thu, Sep 03, 2026 at 10:11:05AM +0200, Thomas Bachem wrote:
> Hi Patrick,
> 
> On Thu, Sep 03, 2026 at 09:40:04AM +0200, Patrick Steinhardt wrote:
> > I think this hints that we should tweak the default value of
> > "maintenance.rerere-gc.auto". The way it's currently written we indeed
> > are quite aggressive with spawning `git rerere gc`, and I agree that we
> > should tweak it. And in the best case we'd not only respect whether we
> > have a specific number of entries, but we should also respect whether
> > those would be garbage collected in the first place.
> >
> > I'll send a patch series later today to do this.
> 
> Thanks. Checking whether anything would actually be pruned sounds
> right to me. It takes the frequency away, not the race, so I'd still
> do the sequencer part Phillip asked for.

Yes. Ideally, I'd think that we should both introduce the grace period
for locking the file and adapting the heuristic used by the maintenance
strategy. Whether we should completely disable auto-maintenance when in
the sequencer... I dunno. In any case, that feels like another separate
topic that should probably be discussed in its own series.

> > Having a locking timeout is sensible anyway, I think. It does not only
> > solve races with a concurrent maintenance run, but also with concurrent
> > writers.
> 
> Phillip found the wait unfortunate and I offered to drop it. You would
> keep it. I think the two fit together: wait up to rerere.lockTimeout,
> then warn and return -1 instead of dying, so the caller goes on
> without rerere this once. The gc passes 0 and does not wait. That
> takes the die out, which is what broke the rebase. The wait stays,
> bounded to a second, but skipping rerere is not free either: it can
> mean resolving a conflict again that rerere had already recorded, and
> a second is cheap next to that. With the sequencer no longer spawning
> the gc and your heuristic change, it should rarely come to either.
> Phillip, would that work for you?

I think that having the wait is a sensible thing to do, as the race was
a preexisting one that was only uncovered by the change to the default
maintenance strategy. It can also happen with two concurrent processes
that both happen to write rerere entries. You wouldn't normally see the
wait anyway, so in the happy path nobody will really care. And in the
cases where you would see it the user is probably more happy to wait a
bit than having Git die (or just not write a rerere entry at all).

Patrick