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

[Jeff King <peff@peff.net>]

On Thu, Sep 05, 2019 at 06:04:57PM -0400, Taylor Blau wrote:

> @@ -846,7 +847,11 @@ static void write_graph_chunk_data(struct hashfile *f, int hash_len,
>  		if (parse_commit_no_graph(*list))
>  			die(_("unable to parse commit %s"),
>  				oid_to_hex(&(*list)->object.oid));
> -		hashwrite(f, get_commit_tree_oid(*list)->hash, hash_len);
> +		tree = get_commit_tree_oid(*list);
> +		if (!tree)
> +			die(_("unable to get tree for %s"),
> +				oid_to_hex(&(*list)->object.oid));
> +		hashwrite(f, tree->hash, hash_len);

Yeah, I think this is a good stop-gap to protect ourselves, until a time
when parse_commit() and friends consistently warn us about the breakage.

> diff --git a/commit.c b/commit.c
> index a98de16e3d..fab22cb740 100644
> --- a/commit.c
> +++ b/commit.c
> @@ -358,7 +358,8 @@ struct tree *repo_get_commit_tree(struct repository *r,
>  
>  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;
>  }

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.

This is sort-of attributable to my 834876630b (get_commit_tree(): return
NULL for broken tree, 2019-04-09). Before then it was a BUG(). However,
that state was relatively short-lived. Before 7b8a21dba1 (commit-graph:
lazy-load trees for commits, 2018-04-06), we'd have similarly returned
NULL (and anyway, BUG() is clearly wrong since it's a data error).

None of which argues against your patches, but it's kind of sad that the
issue is present in so many code paths. I wonder if we could be handling
this in a more central way, but I don't see how short of dying.

-Peff