Re: [PATCH 3/3] commit-graph.c: handle corrupt/missing trees

[Jeff King <peff@peff.net>]

On Fri, Sep 06, 2019 at 11:42:14AM -0400, Taylor Blau wrote:

> > >  struct object_id *get_commit_tree_oid(const struct commit *commit)
> > >  {
> > > -	return &get_commit_tree(commit)->object.oid;
> > > +	struct tree *tree = get_commit_tree(commit);
> > > +	return tree ? &tree->object.oid : NULL;
> > >  }
> 
> You mentioned in the version of this series that is rebased on GitHub's
> fork that it may be worth putting this hunk in a separate commit
> entirely. I don't disagree, so if there are other comments that merit a
> reroll of this, I'm happy to pull this change out as 3/4.

Yeah, I could go either way on that, I think. I was thinking it might be
fixing other callsites, but it seems that nobody else bothers to check
for NULL anyway. But being in its own commit, we could explain that.

> > This one in theory benefits lots of other callsites, too, since it means
> > we'll actually return NULL instead of nonsense like "8". But grepping
> > around for calls to this function, I found literally zero of them
> > actually bother checking for a NULL result. So there are probably dozens
> > of similar segfaults waiting to happen in other code paths.
> > Discouraging.
> 
> Discouraging indeed. I think that you suggest it below, but perhaps the
> right thing to do here is implement 'get_commit_tree_oid()' as follows:
> 
>   struct object_id *get_commit_tree_oid(const struct commit *commit)
>   {
>     struct tree *tree = get_commit_tree(commit);
>     if (!tree)
>       die(_("unable to get tree from commit %s"),
>           oid_to_hex(&commit->object.oid));
>     return &tree->object.oid;
>   }
> 
> Which then puts the onus on the *caller* to check their commit pointer
> to make sure that it has a legit tree in it, unless they're OK with
> dying.

Yeah, I agree that would prevent segfaults (and is similar to what René
proposed for tags with a similar situation). It does feel like a step
backwards in terms of lib-ification. But maybe it's a
belt-and-suspenders on top of trying to catch this case at the parsing
stage, too.

> All of that said, I don't know if I think it's worth holding this series
> up on the above in the meantime. I do think that it (or something like
> it) is generally worth doing, but I'm not sure that now is the time to
> do it.

I'd agree with that, and I think it's sensible to take your patches with
the extra tree check. We can rip it out later if it becomes redundant.

-Peff