On Mon, May 20, 2019 at 10:35:53AM +0200, Alejandro Sanchez wrote:

alex@polaris:~/slurm/source$ git add -u
alex@polaris:~/slurm/source$ git am -i --continue
Applying: Handle duplicate archive file names.
error: object 861d3c6f689a3ca5eb5fb5c409d46de0ad5555e1 is a commit, not a tree
BUG: diff-lib.c:526: run_diff_index must be passed exactly one tree
Aborted (core dumped)

Hmm. So I think the interesting error is probably that first line: some
code expects to look up a tree but sees a commit, and then as a result
it probably feeds too few items to run_diff_index(), triggering the
assertion failure.

Just grepping around, this looks quite suspicious:

  $ git grep -hW get_oid_tree builtin/am.c
  static void write_index_patch(const struct am_state *state)
  ...
  if (!get_oid_tree("HEAD", &head))
	tree = lookup_tree(the_repository, &head);
  else
	tree = lookup_tree(the_repository,
			   the_repository->hash_algo->empty_tree);

Using get_oid_tree() does not actually return a tree; it just
prioritizes trees when disambiguating names (which is pointless here,
since we're not feeding an ambiguous oid). HEAD will always be a commit,
and then lookup_tree() similarly does not peel that down to an actual
tree. And this whole function is called only in interactive-mode, so
it's possible that it's simply not used much and nobody noticed.

I haven't tried to reproduce yet. Is the repository (and patch) that you
used to demonstrate this publicly available? Or alternatively, is it
possible to show a backtrace from the coredump?

If my blind guess is right, then something like this probably fixes it:

diff --git a/builtin/am.c b/builtin/am.c
index bdd1bbc35d..93305560c1 100644
--- a/builtin/am.c
+++ b/builtin/am.c

@@ -1339,9 +1339,17 @@ static void write_index_patch(const struct am_state *state)
 	struct rev_info rev_info;
 	FILE *fp;
 
-	if (!get_oid_tree("HEAD", &head))
-		tree = lookup_tree(the_repository, &head);
-	else
+	if (!get_oid("HEAD", &head)) {
+		struct object *obj;
+		struct commit *commit;
+
+		obj = parse_object_or_die(&head, NULL);
+		commit = object_as_type(the_repository, obj, OBJ_COMMIT, 0);
+		if (!commit)
+			die("unable to parse HEAD as a commit");
+
+		tree = get_commit_tree(commit);
+	} else
 		tree = lookup_tree(the_repository,
 				   the_repository->hash_algo->empty_tree);
 

-Peff