Re: [RFC/PATCH 0/6] hash-object: use fsck to check objects

[Jeff King <peff@peff.net>]

On Wed, Jan 18, 2023 at 04:38:40PM -0500, Taylor Blau wrote:

> On Wed, Jan 18, 2023 at 12:59:24PM -0800, Junio C Hamano wrote:
> > The --literally option was invented initially primarily to allow a
> > bogus type of object (e.g. "hash-object -t xyzzy --literally") but I
> > am happy to see that we are finding different uses.  I wonder if
> > these objects of known types but with syntactically bad contents can
> > be "repack"ed from loose into packed?
> >
> > >   [5/6]: fsck: provide a function to fsck buffer without object struct
> 
> It is indeed possible:
> 
> --- >8 ---
> Initialized empty Git repository in /home/ttaylorr/src/git/t/trash directory.t9999-test/.git/
> expecting success of 9999.1 'repacking corrupt loose object into packed':
> 	name=$(echo $ZERO_OID | sed -e "s/00/Q/g") &&
> 	printf "100644 fooQ$name" | q_to_nul |
> 		git hash-object -w --stdin -t tree >in &&
> 
> 	git pack-objects .git/objects/pack/pack <in
> 
> Enumerating objects: 1, done.
> Counting objects: 100% (1/1), done.
> 06146c77fd19c096858d6459d602be0fdf10891b
> Writing objects: 100% (1/1), done.
> Total 1 (delta 0), reused 0 (delta 0), pack-reused 0
> ok 1 - repacking corrupt loose object into packed
> --- 8< ---

Right, we don't do any fsck-ing when packing objects. Nor should we, I
think. We should be checking objects when they come into the repository
(via index-pack/unpack-objects) or when they're created (hash-object),
but there's little need to do so when they migrate between storage
formats.

The fact that "--literally" manually writes a loose object is mostly an
implementation detail. I think if we are not writing an object with an
esoteric type, that it could even just hit the regular index_fd() code
path (and drop the HASH_FORMAT_CHECK flag).

If you do write one with "-t xyzzy", I think pack-objects would barf,
but not because of fsck checks. It just couldn't represent that type
(which really makes such objects pretty pointless; you cannot ever fetch
or push them!).

-Peff