From: Patrick Steinhardt <ps@pks.im>
Date: Mon, 23 Feb 2026 11:30:35 GMT
Subject: Re: [PATCH 4/4] pack-check: fix verification of large objects
Message-ID: <aZw6W_BHoYiC9RYl@pks.im>
In-Reply-To: <20260223111120.GC215364@coredump.intra.peff.net>

On Mon, Feb 23, 2026 at 06:11:20AM -0500, Jeff King wrote:
> On Mon, Feb 23, 2026 at 10:50:43AM +0100, Patrick Steinhardt wrote:
> 
> > diff --git a/pack-check.c b/pack-check.c
> > index 46782a29d5..6149567060 100644
> > --- a/pack-check.c
> > +++ b/pack-check.c
> > @@ -155,7 +155,7 @@ static int verify_packfile(struct repository *r,
> >  			err = error("packed %s from %s is corrupt",
> >  				    oid_to_hex(&oid), p->pack_name);
> >  		else if (!data &&
> > -			 (!(stream = odb_read_stream_open(r->objects, &oid, NULL)) ||
> > +			 (packfile_read_object_stream(&stream, p, entries[i].offset) < 0 ||
> 
> And now this change is delightfully simple.
> 
> > +test_expect_success 'fsck handles multiple packfiles with big blobs' '
> > +	test_when_finished "rm -rf repo" &&
> > +	git init repo &&
> > +	(
> > +		cd repo &&
> > +		blob_one=$(test-tool genrandom one 200k | git hash-object -t blob -w --stdin) &&
> > +		blob_two=$(test-tool genrandom two 200k | git hash-object -t blob -w --stdin) &&
> > +		printf "%s\n" "$blob_one" | git pack-objects .git/objects/pack/pack &&
> > +		printf "%s\n" "$blob_two" | git pack-objects .git/objects/pack/pack &&
> > +		remove_object "$blob_one" &&
> > +		remove_object "$blob_two" &&
> > +		git -c core.bigFileThreshold=100k fsck
> > +	)
> > +'
> 
> I like seeing this much-more-specific test case. It does sort of become
> a noop if we fix the iteration problem, though.
> 
> A more concrete test would probably be something like:
> 
>    1. Two packs, $X and $Y, both contain the same object.
> 
>    2. The object is corrupt in $X but not in $Y.
> 
>    3. Running fsck detects that one copy is corrupt but the other is
>       not.
> 
> Right now it may or may not fail depending on the ordering of the packs
> in the MRU list (which we might be able to tweak via mtimes). But
> hopefully in the "after" state it should deterministically complain
> about $X.

Yeah. The problem I had here is that I'm not sure whether we have any
tools to reliably create a corrupted object, e.g. with a hash mismatch.
I'll have a look for v2.

Thanks!

Patrick