Re: [PATCH] update-server-info: avoid needless overwrites
- Date: Tue, 14 May 2019 07:24:51 -0400
- From: Jeff King <peff@xxxxxxxx>
- Subject: Re: [PATCH] update-server-info: avoid needless overwrites
On Tue, May 14, 2019 at 12:33:11PM +0200, Ævar Arnfjörð Bjarmason wrote:
> > I think it would work because any update-server-info, whether from A or
> > B, will take into account the full current repo state (and we don't look
> > at that state until we take the lock). So you might get an interleaved
> > "A-push, B-push, B-maint, A-maint", but that's OK. A-maint will
> > represent B's state when it runs.
> Maybe we're talking about different things. I mean the following
> 1. Refs "X" and "Y" are at X=A Y=A
> 2. Concurrent push #1 happens, updating X from A..F
> 3. Concurrent push #2 happens, updating Y from A..F
> 4. Concurrent push #1 succeeds
> 5. Concurrent push #1 starts update-server-info. Reads X=F Y=A
> 5. Concurrent push #2 succeeds
> 6. Concurrent push #2 starts update-server-info. Reads X=F Y=F
> 7. Concurrent push #2's update-server-info finishes, X=F Y=F written to "info"
> 8. Concurrent push #1's update-server-info finishes, X=A Y=F written to "info"
> I.e. because we have per-ref locks and no lock at all on
> update-server-info (but that would need to be a global ref lock, not
> just on the "info" files) we can have a push that's already read "X"'s
> value as "A" while updating "Y" win the race against an
> update-server-info that updated "X"'s value to "F".
> It will get fixed on the next push (at least as far as "X"'s value
> goes), but until that time dumb clients will falsely see that "X" hasn't
> been updated.
That's the same situation. But I thought we were talking about having an
update-server-info lock. In which case the #2 update-server-info or the
#1 update-server-info runs in its entirety, and cannot have their read
and write steps interleaved (that's what I meant by "don't look at the
state until we take the lock"). Then that gives us a strict ordering: we
know that _some_ update-server-info (be it #1 or #2's) will run after
any given update.