fix: resolve active_branch correctly for reftable refs

reviewed-by: Sebastian Thiel <sebastian.thiel@icloud.com>

Author: codex

git/repo/base.py

@@ -1045,8 +1045,13 @@ def active_branch(self) -> Head:
         :return:
             :class:`~git.refs.head.Head` to the active branch
         """
-        # reveal_type(self.head.reference)  # => Reference
-        return self.head.reference
+        active_branch = self.head.reference
+        if active_branch.name == ".invalid":
+            raise ValueError(
+                "HEAD points to 'refs/heads/.invalid', which Git uses to mark refs as incompatible "
+                "with older clients"
+            )
+        return active_branch
 
     def blame_incremental(self, rev: str | HEAD | None, file: str, **kwargs: Any) -> Iterator["BlameEntry"]:
         """Iterator for blame information for the given file at the given revision.

test/test_repo.py

@@ -962,6 +962,48 @@ def test_empty_repo(self, rw_dir):
 
         assert "BAD MESSAGE" not in contents, "log is corrupt"
 
+    @with_rw_directory
+    def test_active_branch_raises_value_error_when_head_ref_is_invalid(self, rw_dir):
+        repo = Repo.init(rw_dir)
+        with open(osp.join(rw_dir, ".git", "HEAD"), "w") as f:
+            f.write("ref: refs/heads/.invalid\n")
+
+        self.assertRaisesRegex(
+            ValueError,
+            r"refs/heads/\.invalid.*older clients",
+            lambda: repo.active_branch,
+        )
+
+    @with_rw_directory
+    def test_active_branch_raises_value_error_for_reftable_repo(self, rw_dir):
+        git = Git(rw_dir)
+        try:
+            git.init(ref_format="reftable")
+        except GitCommandError as err:
+            if err.status == 129:
+                pytest.skip("git init --ref-format is not supported by this git version")
+            raise
+
+        with open(osp.join(rw_dir, ".git", "HEAD")) as f:
+            self.assertEqual(f.read(), "ref: refs/heads/.invalid\n")
+
+        repo = Repo(rw_dir)
+        self.assertRaisesRegex(
+            ValueError,
+            r"refs/heads/\.invalid.*older clients",
+            lambda: repo.active_branch,
+        )
+
+    @with_rw_directory
+    def test_active_branch_raises_type_error_when_head_is_detached(self, rw_dir):
+        repo = Repo.init(rw_dir)
+        with open(osp.join(rw_dir, "a.txt"), "w") as f:
+            f.write("a")
+        repo.index.add(["a.txt"])
+        repo.index.commit("initial commit")
+        repo.git.checkout(repo.head.commit.hexsha)
+        self.assertRaisesRegex(TypeError, "detached symbolic reference", lambda: repo.active_branch)
+
     def test_merge_base(self):
         repo = self.rorepo
         c1 = "f6aa8d1"