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

[Jeff King <peff@peff.net>]

In interactive mode, "git am -i --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 oid; 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 name).

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>
---
 builtin/am.c              | 14 ++++++++---
 t/t4257-am-interactive.sh | 49 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+), 3 deletions(-)
 create mode 100755 t/t4257-am-interactive.sh

diff --git a/builtin/am.c b/builtin/am.c
index ea16b844f1..33bd7a6eab 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);
 
diff --git a/t/t4257-am-interactive.sh b/t/t4257-am-interactive.sh
new file mode 100755
index 0000000000..6989bf7aba
--- /dev/null
+++ b/t/t4257-am-interactive.sh
@@ -0,0 +1,49 @@
+#!/bin/sh
+
+test_description='am --interactive tests'
+. ./test-lib.sh
+
+test_expect_success 'set up patches to apply' '
+	test_commit unrelated &&
+	test_commit no-conflict &&
+	test_commit conflict-patch file patch &&
+	git format-patch --stdout -2 >mbox &&
+
+	git reset --hard unrelated &&
+	test_commit conflict-master file master base
+'
+
+# Sanity check our setup.
+test_expect_success 'applying all patches generates conflict' '
+	test_must_fail git am mbox &&
+	echo resolved >file &&
+	git add -u &&
+	git am --resolved
+'
+
+test_expect_success 'interactive am can apply a single patch' '
+	git reset --hard base &&
+	printf "%s\n" y n | git am -i mbox &&
+
+	echo no-conflict >expect &&
+	git log -1 --format=%s >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'interactive am can resolve conflict' '
+	git reset --hard base &&
+	printf "%s\n" y y | test_must_fail git am -i mbox &&
+	echo resolved >file &&
+	git add -u &&
+	printf "%s\n" v y | git am -i --resolved &&
+
+	echo conflict-patch >expect &&
+	git log -1 --format=%s >actual &&
+	test_cmp expect actual &&
+
+	echo resolved >expect &&
+	git cat-file blob HEAD:file >actual &&
+	test_cmp expect actual
+'
+
+test_done
-- 
2.22.0.rc1.539.g7bfcdfe86d