If parsing a commit yields a valid tree oid, but we've seen that same
oid as a non-tree in the same process, the resulting commit struct will
end up with a NULL tree pointer, but not otherwise report a parsing

That's perhaps convenient for callers which want to operate on even
partially corrupt commits (e.g., by still looking at the parents). But
it leaves a potential trap for most callers, who now have to manually
check for a NULL tree. Most do not, and it's likely that there are
possible segfaults lurking. I say "possible" because there are many
candidates, and I don't think it's worth following through on
reproducing them when we can just fix them all in one spot. And
certainly we _have_ seen real-world cases, such as the one fixed by
806278dead (commit-graph.c: handle corrupt/missing trees, 2019-09-05).

Note that we can't quite drop the check in the caller added by that
commit yet, as there's some subtlety with repeated parsings (which will
be addressed in a future commit).

Signed-off-by: Jeff King <p...@peff.net>
 commit.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/commit.c b/commit.c
index 6467c9e175..810419a168 100644
--- a/commit.c
+++ b/commit.c
@@ -401,6 +401,7 @@ int parse_commit_buffer(struct repository *r, struct commit 
*item, const void *b
        struct commit_graft *graft;
        const int tree_entry_len = the_hash_algo->hexsz + 5;
        const int parent_entry_len = the_hash_algo->hexsz + 7;
+       struct tree *tree;
        if (item->object.parsed)
                return 0;
@@ -412,7 +413,12 @@ int parse_commit_buffer(struct repository *r, struct 
commit *item, const void *b
        if (get_oid_hex(bufptr + 5, &parent) < 0)
                return error("bad tree pointer in commit %s",
-       set_commit_tree(item, lookup_tree(r, &parent));
+       tree = lookup_tree(r, &parent);
+       if (!tree)
+               return error("bad tree pointer %s in commit %s",
+                            oid_to_hex(&parent),
+                            oid_to_hex(&item->object.oid));
+       set_commit_tree(item, tree);
        bufptr += tree_entry_len + 1; /* "tree " + "hex sha1" + "\n" */
        pptr = &item->parents;

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