Prevent Git GC issue between merger and executor

Since executors can also handle merge jobs and those merges happen in
the executor's repo cache we need to protect temporary merger refs from
being garbage collected.

Because the executor's update jobs might reset the local branch heads in
between merges, we create the refs for the speculative branch state in
'refs/zuul' instead. Those refs are cleaned up when the related branch
no longer exists.

Branch names for the Zuul refs are hashed (SHA1) in order to avoid
issues with empty directories when the branch name contains slashes.
E.g. the speculative state of the master branch will be referenced by
'refs/zuul/4f26aeafdb2367620a393c973eddbe8f8b846ebd'

Change-Id: Idd2b0bd2dfeba22f3961f851f8a463bc5c9d37ff

tests/unit/test_merger_repo.py

@@ -647,7 +647,7 @@ class TestMerger(ZuulTestCase):
 
     def test_merge_temp_refs(self):
         """
-        Test that the merge updates local branches in order to avoid
+        Test that the merge updates local zuul refs in order to avoid
         garbage collection of needed objects.
         """
         merger = self.executor_server.merger
@@ -671,13 +671,26 @@ class TestMerger(ZuulTestCase):
         # Merge A -> B -> C
         result = merger.mergeChanges([item_a, item_b, item_c])
         self.assertIsNotNone(result)
+        merge_state = result[3]
 
         cache_repo = merger.getRepo('gerrit', 'org/project')
         repo = cache_repo.createRepoObject(zuul_event_id="dummy")
 
-        # Make sure local refs are updated
-        self.assertIn("foo/bar", repo.refs)
-        self.assertEqual(repo.refs.master.commit, repo.head.commit)
+        # Make sure zuul refs are updated
+        foobar_zuul_ref = Repo.refNameToZuulRef("foo/bar")
+        master_zuul_ref = Repo.refNameToZuulRef("master")
+        ref_map = {r.path: r for r in repo.refs}
+        self.assertIn(foobar_zuul_ref, ref_map)
+        self.assertIn(master_zuul_ref, ref_map)
+
+        self.assertEqual(
+            ref_map[master_zuul_ref].commit.hexsha,
+            merge_state[("gerrit", "org/project", "master")]
+        )
+        self.assertEqual(
+            ref_map[foobar_zuul_ref].commit.hexsha,
+            merge_state[("gerrit", "org/project", "foo/bar")]
+        )
 
         # Delete the remote branch so a reset cleanes up the local branch
         parent_repo.delete_head('foo/bar', force=True)
@@ -690,7 +703,7 @@ class TestMerger(ZuulTestCase):
             Repo._cleanup_leaked_ref_dirs(parent_path, None, [])
 
         cache_repo.reset()
-        self.assertNotIn("foo/bar", repo.refs)
+        self.assertNotIn(foobar_zuul_ref, [r.path for r in repo.refs])
 
         # Create another head 'foo' that can't be created if the 'foo/bar'
         # branch wasn't cleaned up properly
@@ -703,5 +716,18 @@ class TestMerger(ZuulTestCase):
         # Merge A -> B -> C
         result = merger.mergeChanges([item_a, item_b, item_c])
         self.assertIsNotNone(result)
-        self.assertIn("foo", repo.refs)
-        self.assertEqual(repo.refs.master.commit, repo.head.commit)
+        merge_state = result[3]
+
+        foo_zuul_ref = Repo.refNameToZuulRef("foo")
+        ref_map = {r.path: r for r in repo.refs}
+
+        self.assertIn(foo_zuul_ref, ref_map)
+        self.assertIn(master_zuul_ref, ref_map)
+        self.assertEqual(
+            ref_map[master_zuul_ref].commit.hexsha,
+            merge_state[("gerrit", "org/project", "master")]
+        )
+        self.assertEqual(
+            ref_map[foo_zuul_ref].commit.hexsha,
+            merge_state[("gerrit", "org/project", "foo")]
+        )