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From: Taylor Blau <me@ttaylorr.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: git@vger.kernel.org, vdye@github.com, jonathantanmy@google.com
Subject: Re: [PATCH v2 3/4] builtin/pack-objects.c: ensure included `--stdin-packs` exist
Date: Tue, 24 May 2022 20:14:52 -0400	[thread overview]
Message-ID: <Yo10/KlseGJeY6uQ@nand.local> (raw)
In-Reply-To: <xmqq1qwifv1c.fsf@gitster.g>

On Tue, May 24, 2022 at 03:03:11PM -0700, Junio C Hamano wrote:
> Taylor Blau <me@ttaylorr.com> writes:
>
> > Calling `is_pack_valid()` early on makes it substantially less likely
> > that we will have to deal with a pack going away, since we'll have an
> > open file descriptor on its contents much earlier.
>
> Sorry for asking a stupid question (or two), but I am confused.

No such thing as a stupid question, so your apology is not necessary in
the slightest :).

> This does make sure that we can read and use the contents of the
> packfile even when somebody else removes it from the disk by
> ensuring that
>
>  (1) we have an open file descriptor to it, so that we could open
>      mmap window into it at will; or
>
>  (2) we have a mmap window that covers all of it (this should be the
>      norm on platforms with vast address space); or
>
>  (3) we are in the same state as (1) by opening the packfile to
>      validate the pack right now.
>
> and during the pack-object we are running (aka "repack"), we can
> continue to read from that pack that may have already disappeared
> from the disk.
>
> But is that sufficient?  Are we writing the resulting new pack(s)
> out in such a way that the repository is healthy without the pack
> we noticed is disappearing?  How do we ensure that?

It's sufficient in the sense that we're writing out all of the objects
we were asked to (from pack-objects's perspective). Of course, if the
"simultaneous writer" is just removing packs right after they are
opened, that will produce an obviously-broken state. But assuming that
repack isn't removing objects it shouldn't (which I think is a safe
assumption from pack-objects' perspective, since all it cares about is
writing packs that contain the desired set of objects), then we are OK.

Thanks,
Taylor

  reply	other threads:[~2022-05-25  0:15 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-13 16:23 [PATCH 0/2] pack-objects: fix a pair of MIDX bitmap-related races Taylor Blau
2022-05-13 16:23 ` [PATCH 1/2] pack-bitmap: check preferred pack validity when opening MIDX bitmap Taylor Blau
2022-05-13 18:19   ` Junio C Hamano
2022-05-13 19:55     ` Taylor Blau
2022-05-13 16:23 ` [PATCH 2/2] builtin/pack-objects.c: ensure pack validity from MIDX bitmap objects Taylor Blau
2022-05-13 23:06   ` Jonathan Tan
2022-05-14 13:17     ` Taylor Blau
2022-05-16  6:07       ` Jonathan Tan
2022-05-14 13:34     ` Taylor Blau
2022-05-16  6:11       ` Jonathan Tan
2022-05-24 18:54 ` [PATCH v2 0/4] pack-objects: fix a pair of MIDX bitmap-related races Taylor Blau
2022-05-24 18:54   ` [PATCH v2 1/4] pack-bitmap.c: check preferred pack validity when opening MIDX bitmap Taylor Blau
2022-05-24 19:36     ` Ævar Arnfjörð Bjarmason
2022-05-24 21:38       ` Taylor Blau
2022-05-24 21:51         ` Ævar Arnfjörð Bjarmason
2022-05-24 18:54   ` [PATCH v2 2/4] builtin/pack-objects.c: avoid redundant NULL check Taylor Blau
2022-05-24 21:44     ` Junio C Hamano
2022-05-25  0:11       ` Taylor Blau
2022-05-24 18:54   ` [PATCH v2 3/4] builtin/pack-objects.c: ensure included `--stdin-packs` exist Taylor Blau
2022-05-24 19:46     ` Ævar Arnfjörð Bjarmason
2022-05-24 21:33       ` Taylor Blau
2022-05-24 21:49         ` Ævar Arnfjörð Bjarmason
2022-05-24 22:03     ` Junio C Hamano
2022-05-25  0:14       ` Taylor Blau [this message]
2022-05-26 19:21     ` Victoria Dye
2022-05-26 20:05       ` Taylor Blau
2022-05-24 18:54   ` [PATCH v2 4/4] builtin/pack-objects.c: ensure pack validity from MIDX bitmap objects Taylor Blau
2022-05-24 21:38   ` [PATCH v2 0/4] pack-objects: fix a pair of MIDX bitmap-related races Junio C Hamano
2022-05-25  0:16     ` Taylor Blau

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