Use the common Git directory for alternates

Author: codex

git/repo/base.py

@@ -961,8 +961,7 @@ def _get_alternates(self) -> List[str]:
         :return:
             List of strings being pathnames of alternates
         """
-        if self.git_dir:
-            alternates_path = osp.join(self.git_dir, "objects", "info", "alternates")
+        alternates_path = osp.join(self.common_dir, "objects", "info", "alternates")
 
         if osp.exists(alternates_path):
             with open(alternates_path, "rb") as f:

test/test_repo.py

@@ -327,13 +327,29 @@ def test_daemon_export(self):
 
     def test_alternates(self):
         cur_alternates = self.rorepo.alternates
-        # empty alternates
-        self.rorepo.alternates = []
-        self.assertEqual(self.rorepo.alternates, [])
+        try:
+            # Empty alternates.
+            self.rorepo.alternates = []
+            self.assertEqual(self.rorepo.alternates, [])
+            alts = ["other/location", "this/location"]
+            self.rorepo.alternates = alts
+            self.assertEqual(alts, self.rorepo.alternates)
+        finally:
+            self.rorepo.alternates = cur_alternates
+
+    @with_rw_directory
+    def test_alternates_use_common_dir(self, rw_dir):
+        common_dir = osp.join(rw_dir, "common")
+        git_dir = osp.join(rw_dir, "worktrees", "linked")
+        os.makedirs(osp.join(common_dir, "objects", "info"))
+        os.makedirs(osp.join(git_dir, "objects", "info"))
+        repo = mock.Mock(common_dir=common_dir, git_dir=git_dir)
+
         alts = ["other/location", "this/location"]
-        self.rorepo.alternates = alts
-        self.assertEqual(alts, self.rorepo.alternates)
-        self.rorepo.alternates = cur_alternates
+        Repo._set_alternates(repo, alts)
+
+        self.assertEqual(Repo._get_alternates(repo), alts)
+        self.assertFalse(osp.exists(osp.join(git_dir, "objects", "info", "alternates")))
 
     def test_repr(self):
         assert repr(self.rorepo).startswith("<git.repo.base.Repo ")