Re: [PATCH v2 3/4] builtin/pack-objects.c: ensure included `--stdin-packs` exist

From: Junio C Hamano <gitster@pobox.com>
Date: May 24, 2022, 22:03 UTC
Message-ID: <xmqq1qwifv1c.fsf@gitster.g>
In-Reply-To: <cdc3265ec27f04accc433d9e4e54ac0edc3b3746.1653418457.git.me@ttaylorr.com>

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.

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?

Thanks.