[PATCH v2 4/4] am: fix --interactive HEAD tree resolution

In --interactive mode, "git am --resolved" will try to generate a patch based on what is in the index, so that it can prompt "apply this patch?". To do so it needs the tree of HEAD, which it tries to get with get_oid_tree(). However, this doesn't yield a tree object; the "tree" part just means "if you must disambiguate short oids, then prefer trees" (and we do not need to disambiguate at all, since we are feeding a ref).

Instead, we must parse the oid as a commit (which should always be true in a non-corrupt repository), and access its tree pointer manually.

This has been broken since the conversion to C in 7ff2683253 (builtin-am: implement -i/--interactive, 2015-08-04), but there was no test coverage because of interactive-mode's insistence on having a tty. That was lifted in the previous commit, so we can now add a test for this case.

Note that before this patch, the test would result in a BUG() which comes from 3506dc9445 (has_uncommitted_changes(): fall back to empty tree, 2018-07-11). But before that, we'd have simply segfaulted (and in fact this is the exact type of case the BUG() added there was trying to catch!).

Signed-off-by: Jeff King <peff@peff.net>
---
This addresses points raised in Dscho's review (except for the test_cmp
thing). Since there were no changes to patches 1-3, I didn't bother
re-posting. Diff against v1 below.

  diff --git a/builtin/am.c b/builtin/am.c
  index 33bd7a6eab..78389d08b6 100644
  --- a/builtin/am.c
  +++ b/builtin/am.c
  @@ -1340,14 +1340,7 @@ static void write_index_patch(const struct am_state *state)
   	FILE *fp;
   
   	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");
  -
  +		struct commit *commit = lookup_commit_or_die(&head, "HEAD");
   		tree = get_commit_tree(commit);
   	} else
   		tree = lookup_tree(the_repository,
  diff --git a/t/t4257-am-interactive.sh b/t/t4257-am-interactive.sh
  index 6989bf7aba..5344bd248a 100755
  --- a/t/t4257-am-interactive.sh
  +++ b/t/t4257-am-interactive.sh
  @@ -23,7 +23,8 @@ test_expect_success 'applying all patches generates conflict' '
   
   test_expect_success 'interactive am can apply a single patch' '
   	git reset --hard base &&
  -	printf "%s\n" y n | git am -i mbox &&
  +	# apply the first, but not the second
  +	test_write_lines y n | git am -i mbox &&
   
   	echo no-conflict >expect &&
   	git log -1 --format=%s >actual &&
  @@ -32,10 +33,12 @@ test_expect_success 'interactive am can apply a single patch' '
   
   test_expect_success 'interactive am can resolve conflict' '
   	git reset --hard base &&
  -	printf "%s\n" y y | test_must_fail git am -i mbox &&
  +	# apply both; the second one will conflict
  +	test_write_lines y y | test_must_fail git am -i mbox &&
   	echo resolved >file &&
   	git add -u &&
  -	printf "%s\n" v y | git am -i --resolved &&
  +	# interactive "--resolved" will ask us if we want to apply the result
  +	echo y | git am -i --resolved &&
   
   	echo conflict-patch >expect &&
   	git log -1 --format=%s >actual &&

 builtin/am.c              |  7 +++---
 t/t4257-am-interactive.sh | 52 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+), 3 deletions(-)
 create mode 100755 t/t4257-am-interactive.sh