Re: Bad objects error since upgrading GitHub servers to 1.6.1

From: Shawn O. Pearce <spearce@spearce.org>
Date: Jan 28, 2009, 16:16 UTC
Message-ID: <20090128161652.GK1321@spearce.org>
In-Reply-To: <20090128081745.GA2172@coredump.intra.peff.net>

Jeff King <peff@peff.net> wrote:

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> On Wed, Jan 28, 2009 at 12:05:33AM -0800, Junio C Hamano wrote:
> > Jeff King <peff@peff.net> writes:
> > >
> > >        C--D
> > >       /
> > >   A--B
> > >       \
> > >        E--F
> 
> Don't the other changes have similar parallel use cases? [2/2] also deals
> with tag lookup. Wouldn't you also expect, if you had a tag "T" pointing
> to "E" in the above scenario that "git rev-list T..D" would barf? I
> really think you don't want to ignore missing negations _ever_ unless
> the caller knows that such a miss is really only about optimization and
> not correctness.

Exactly what I just said in my other message.

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> Side note:
> 
> As you described, we expect to reach this situation from a partial
> transfer. Which means that you don't actually have a _ref_ for "T" (or
> "F"). So it is unlikely to come up in normal use (you would have to
> manually specify the sha1 of a broken portion of the graph).

True, but in the send-pack case we are discussing the remote side has specified the SHA-1 of broken portions of the graph to us, and we've taken that into consideration. So we have to fix that assumption we've made.

> But what is more important is that your repository _is_ corrupted,

Depends. If the SHA-1 came from the remote side during send-pack, it doesn't matter that we have a broken chain along that path, it may have been a dumb transport fetch that was interrupted. Our local repository isn't corrupt, it just has some extra crap laying around that hasn't gc'd yet.

If the SHA-1 came from the user, then it depends on the context of why the user is giving it to us. In pretty much every case, yes, its a corruption and we should be aborting. :-)

Actually, the only time where it *isn't* a corruption is when its input to "git bundle create A.bdl ... -not $SOMEBADID" as that is the exact same thing as coming from the other side via send-pack.

> I
> think we are losing an important method by which the user finds out. Git
> is usually very good at informing you of a problem in the repo early,
> and I think unconditionally ignoring missing objects would lose that.

Yup, I agree. But as you and Junio have already pointed out, C Git can miss some types of corruption because the revision machinary has some gaps. *sigh*

I'd really like to see those gaps closed. But I don't have a good enough handle on the code structure of the C Git revision machinary to do that myself in a short period of time. I know JGit's well... but that's only because I wrote it. ;-)

Its now on my wish list of things I wish I had time for in C Git. But perhaps someone who is more familiar with the revision machinary will get to it first.

-- 
Shawn.