Re: [BUG] Destructive access to an "objects/info/alternates" repository

From: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
To: Stefan Monnier <monnier@iro.umontreal.ca>
Cc: git@vger.kernel.org
Subject: Re: [BUG] Destructive access to an "objects/info/alternates" repository
Date: Mon, 4 Jan 2021 08:54:10 -0500	[thread overview]
Message-ID: <20210104135410.myjaygaulqnxcnsc@chatter.i7.local> (raw)
In-Reply-To: <jwv1rf26k9n.fsf-monnier+gmane.comp.version-control.git@gnu.org>

On Sun, Jan 03, 2021 at 11:50:35AM -0500, Stefan Monnier wrote:
> >> FWIW, I just tried it with
> >> 
> >>     git clone --shared ~otheruser/nongnu
> >> 
> >> and got the same result.
> >> I also tried with another repository, still with the same result (tho
> >> different pack.idx names, of course).
> >
> > I've tried several times to reproduce this, but I am unable to do so. You may
> > need to provide the tarball with ~otheruser/nongnu contents for someone to be
> > able to properly debug this.
> 
> OK, you can try with the following tarball:
> 
>     https://www.iro.umontreal.ca/~monnier/nongnu.tar.gz

Great, thank you for providing that. At this point, I can verify the behaviour
you are seeing:

$ cat .git/objects/info/alternates
/tmp/test/nongnu/.git/objects
$ git gc
Enumerating objects: 119, done.
Nothing new to pack.
warning: unable to unlink '/tmp/test/nongnu/.git/objects/pack/pack-0d85e74ac2f7e51ce26f281e64eb738e8182fa95.idx': Permission denied
warning: unable to unlink '/tmp/test/nongnu/.git/objects/pack/pack-5fe14feff49ccdee5469af9dc94f6784e8464a6b.idx': Permission denied

Notably, if I run repack/prune by themselves, this does not happen:

$ git repack -Adln
Enumerating objects: 119, done.
Nothing new to pack.
$ git prune --expire=now
$

I would guess that this is happening in builtin/gc.c#686 [1], where it calls
clean_pack_garbage(). Running "git count-objects -v" shows:

$ git count-objects -v
warning: no corresponding .pack: /tmp/test/nongnu/.git/objects/pack/pack-0d85e74ac2f7e51ce26f281e64eb738e8182fa95.idx
warning: no corresponding .pack: /tmp/test/nongnu/.git/objects/pack/pack-5fe14feff49ccdee5469af9dc94f6784e8464a6b.idx
count: 0
size: 0
in-pack: 0
packs: 0
size-pack: 0
prune-packable: 0
garbage: 2
size-garbage: 6573
alternate: /tmp/test/nongnu/.git/objects

I agree with Stefan that this is undesired behaviour, even when it only
happens when attempting to clean up garbage -- git-gc (and git-count-objects)
should distinguish between garbage in the parent repository and its own
repository and not attempt any modification of the parent repository during
gc or any other operation.

[1] https://git.kernel.org/pub/scm/git/git.git/tree/builtin/gc.c#n686

-K