[PATCH 3/4] reachable.c: mark reachable detached HEAD from all worktrees

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

Same reason as the previous commit, this prevents potential object
deletion that other worktrees need.

There is a slight change in behavior. The current implementation always
adds HEAD. The new one only adds _detached_ HEAD. But that's fine because
all normal refs should be already covered by for_each_ref() in
mark_reachable_objects().

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 reachable.c      | 14 +++++++++++---
 t/t5304-prune.sh | 12 ++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/reachable.c b/reachable.c
index 8f67242..e5f9170 100644
--- a/reachable.c
+++ b/reachable.c
@@ -170,6 +170,13 @@ static void add_objects_from_worktree(struct rev_info *revs)
 				    worktree_git_path(wt, "index")) > 0)
 			add_index_objects_to_pending(revs, 0, &istate);
 		discard_index(&istate);
+
+		if (wt->is_detached) {
+			struct object *o;
+			o = parse_object_or_die(wt->head_sha1, "HEAD");
+			add_pending_object(revs, o, "");
+		}
+
 	}
 	free_worktrees(worktrees);
 
@@ -199,13 +206,14 @@ void mark_reachable_objects(struct rev_info *revs, int mark_reflog,
 	/* Add all external refs */
 	for_each_ref(add_one_ref, revs);
 
-	/* detached HEAD is not included in the list above */
-	head_ref(add_one_ref, revs);
-
 	/* Add all reflog info */
 	if (mark_reflog)
 		add_reflogs_to_pending(revs, 0);
 
+	/*
+	 * Add all objects from the in-core index file and detached
+	 * HEAD which is not included in the list above
+	 */
 	add_objects_from_worktree(revs);
 
 	cp.progress = progress;
diff --git a/t/t5304-prune.sh b/t/t5304-prune.sh
index cba45c7..683bdb0 100755
--- a/t/t5304-prune.sh
+++ b/t/t5304-prune.sh
@@ -292,4 +292,16 @@ test_expect_success 'prune: handle index in multiple worktrees' '
 	test_cmp second-worktree/blob actual
 '
 
+test_expect_success 'prune: handle HEAD in multiple worktrees' '
+	git worktree add --detach third-worktree &&
+	echo "new blob for third-worktree" >third-worktree/blob &&
+	git -C third-worktree add blob &&
+	git -C third-worktree commit -m "third" &&
+	rm .git/worktrees/third-worktree/index &&
+	test_must_fail git -C third-worktree show :blob &&
+	git prune --expire=now &&
+	git -C third-worktree show HEAD:blob >actual &&
+	test_cmp third-worktree/blob actual
+'
+
 test_done
-- 
2.8.2.524.g6ff3d78