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
