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From: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
To: Jeff King <peff@peff.net>
Cc: Eric Wong <e@80x24.org>,
	git@vger.kernel.org, Junio C Hamano <gitster@pobox.com>
Subject: Re: [PATCH] update-server-info: avoid needless overwrites
Date: Tue, 14 May 2019 13:57:35 +0200	[thread overview]
Message-ID: <87h89xw8nk.fsf@evledraar.gmail.com> (raw)
In-Reply-To: <20190514112451.GB26957@sigill.intra.peff.net>


On Tue, May 14 2019, Jeff King wrote:

> 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.

Yeah you're right. I *thought* in my last E-mail we were talking about
the current state, but re-reading upthread I see that was a fail on my
part.

An update-server-info lock would solve this indeed. We could still end
up with a situation where whatever a naïve version of the lockfile API
would fail for the "new" update since the old one was underway, so we'd
need something similar to core.*Ref*Timeout, but if we ran into a *.lock
or the timeout we could exit non-zero, as opposed to silently failing
like it does now when it races.

  reply	other threads:[~2019-05-14 11:57 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-11  1:34 Eric Wong
2019-05-11  7:35 ` Eric Sunshine
2019-05-11 20:47   ` [PATCH v2] " Eric Wong
2019-05-11 21:17 ` [PATCH] " Eric Wong
2019-05-11 23:37 ` Ævar Arnfjörð Bjarmason
2019-05-12  0:38   ` Eric Wong
2019-05-12  4:08   ` Jeff King
2019-05-12  7:16     ` Ævar Arnfjörð Bjarmason
2019-05-14  9:47       ` Jeff King
2019-05-14 10:33         ` Ævar Arnfjörð Bjarmason
2019-05-14 11:24           ` Jeff King
2019-05-14 11:57             ` Ævar Arnfjörð Bjarmason [this message]
2019-05-14 11:50         ` Eric Wong
2019-05-14 12:13           ` dumb HTTP things I want to do Eric Wong
2019-05-14 12:27             ` Jeff King
2019-05-14 12:19           ` [PATCH] update-server-info: avoid needless overwrites Ævar Arnfjörð Bjarmason
2019-05-14 12:29             ` Jeff King
2019-05-15  0:45             ` [PATCH 2/1] server-info: conditionally update on fetch Eric Wong
2019-05-15 20:38               ` [WIP] repack leaving stale entries in objects/info/packs Eric Wong
2019-05-15 21:48                 ` Jeff King
2019-05-23  8:59                   ` [PATCH] server-info: do not list unlinked packs Eric Wong
2019-05-23 10:24                     ` Jeff King
2019-05-23 17:27                       ` [PATCH v2] " Eric Wong
2019-05-24  6:05                         ` Jeff King
2019-05-24  7:34                         ` Ævar Arnfjörð Bjarmason
2019-05-13 23:17 ` [PATCH v3] update-server-info: avoid needless overwrites Eric Wong

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