Re: [PATCH] update-server-info: avoid needless overwrites

[Jeff King <peff@peff.net>]

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
> sequence:
> 
>  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.

-Peff