Re: What's cooking in git.git (Oct 2021, #02; Wed, 6)

From: Taylor Blau <me@ttaylorr.com>
To: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
Cc: Jeff King <peff@peff.net>, Junio C Hamano <gitster@pobox.com>,
	git@vger.kernel.org
Subject: Re: What's cooking in git.git (Oct 2021, #02; Wed, 6)
Date: Thu, 7 Oct 2021 00:15:44 -0400	[thread overview]
Message-ID: <YV50cEN6hdvNOrpE@nand.local> (raw)
In-Reply-To: <87wnmph73b.fsf@evledraar.gmail.com>

On Thu, Oct 07, 2021 at 04:57:10AM +0200, Ævar Arnfjörð Bjarmason wrote:
> @@ -1927,11 +1929,9 @@ int midx_repack(struct repository *r, const char *object_dir, size_t batch_size,
>  	}
>
>  	result = write_midx_internal(object_dir, NULL, NULL, NULL, NULL, flags);
> -	m = NULL;

This was the only hunk that gave me a little bit of pause. But it is the
right thing to be doing.

When this code was originally written, write_midx_internal took a
pointer to a struct multi_pack_index, and sometimes called close_midx()
on that pointer. Calling close_midx() on the same pointer is UB, since
close_midx() frees the provided pointer.

In other words, when this code was originally written, the 'm' variable
in midx_repack() could become stale.

But since f57a739691 (midx: avoid opening multiple MIDXs when writing,
2021-09-01), we don't pass a pointer into write_midx_internal(), only
the path to an object directory. So we can always call close_midx()
here.

Thanks,
Taylor