am --skip/--abort: merge HEAD/ORIG_HEAD tree into index

After running "git am --abort", and then running "git reset --hard",
files that were not modified would still be re-checked out.

This is because clean_index() in builtin/am.c mistakenly called the
read_tree() function, which overwrites all entries in the index,
including the stat info.

"git am --skip" did not seem to have this issue because am_skip() called
am_run(), which called refresh_cache() to update the stat info. However,
there's still a performance penalty as the lack of stat info meant that
refresh_cache() would have to scan all files for changes.

Fix this by using unpack_trees() instead to merge the tree into the
index, so that the stat info from the index is kept.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -1939,16 +1939,49 @@ static int fast_forward_to(struct tree *head, struct tree *remote, int reset)
 	return 0;
 }
 
+/**
+ * Merges a tree into the index. The index's stat info will take precedence
+ * over the merged tree's. Returns 0 on success, -1 on failure.
+ */
+static int merge_tree(struct tree *tree)
+{
+	struct lock_file *lock_file;
+	struct unpack_trees_options opts;
+	struct tree_desc t[1];
+
+	if (parse_tree(tree))
+		return -1;
+
+	lock_file = xcalloc(1, sizeof(struct lock_file));
+	hold_locked_index(lock_file, 1);
+
+	memset(&opts, 0, sizeof(opts));
+	opts.head_idx = 1;
+	opts.src_index = &the_index;
+	opts.dst_index = &the_index;
+	opts.merge = 1;
+	opts.fn = oneway_merge;
+	init_tree_desc(&t[0], tree->buffer, tree->size);
+
+	if (unpack_trees(1, t, &opts)) {
+		rollback_lock_file(lock_file);
+		return -1;
+	}
+
+	if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
+		die(_("unable to write new index file"));
+
+	return 0;
+}
+
 /**
  * Clean the index without touching entries that are not modified between
  * `head` and `remote`.
  */
 static int clean_index(const unsigned char *head, const unsigned char *remote)
 {
-	struct lock_file *lock_file;
 	struct tree *head_tree, *remote_tree, *index_tree;
 	unsigned char index[GIT_SHA1_RAWSZ];
-	struct pathspec pathspec;
 
 	head_tree = parse_tree_indirect(head);
 	if (!head_tree)
@@ -1973,18 +2006,8 @@ static int clean_index(const unsigned char *head, const unsigned char *remote)
 	if (fast_forward_to(index_tree, remote_tree, 0))
 		return -1;
 
-	memset(&pathspec, 0, sizeof(pathspec));
-
-	lock_file = xcalloc(1, sizeof(struct lock_file));
-	hold_locked_index(lock_file, 1);
-
-	if (read_tree(remote_tree, 0, &pathspec)) {
-		rollback_lock_file(lock_file);
+	if (merge_tree(remote_tree))
 		return -1;
-	}
-
-	if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
-		die(_("unable to write new index file"));
 
 	remove_branch_state();
 

t/t4151-am-abort.sh

@@ -168,4 +168,28 @@ test_expect_success 'am --abort on unborn branch will keep local commits intact'
 	test_cmp expect actual
 '
 
+test_expect_success 'am --skip leaves index stat info alone' '
+	git checkout -f --orphan skip-stat-info &&
+	git reset &&
+	test_commit skip-should-be-untouched &&
+	test-chmtime =0 skip-should-be-untouched.t &&
+	git update-index --refresh &&
+	git diff-files --exit-code --quiet &&
+	test_must_fail git am 0001-*.patch &&
+	git am --skip &&
+	git diff-files --exit-code --quiet
+'
+
+test_expect_success 'am --abort leaves index stat info alone' '
+	git checkout -f --orphan abort-stat-info &&
+	git reset &&
+	test_commit abort-should-be-untouched &&
+	test-chmtime =0 abort-should-be-untouched.t &&
+	git update-index --refresh &&
+	git diff-files --exit-code --quiet &&
+	test_must_fail git am 0001-*.patch &&
+	git am --abort &&
+	git diff-files --exit-code --quiet
+'
+
 test_done