feat: require approval at current PR head for merge (#471)

Add merge_approval_gate assessment so gitea_merge_pr fails closed when
visible APPROVED reviews do not pin the live head SHA. Expose
approval_at_current_head and stale_approval_block_reason in review
feedback. Document re-review requirement in canonical merge workflow.

Closes #471.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

tests/test_audit.py

@@ -313,7 +313,8 @@ class TestGatedToolAudit(_AuditWiringBase):
             {"login": "merger-bot"}, self._pr("author-bot"),
             self._pr("author-bot"),
             [{"id": 1, "user": {"login": "reviewer-bot"}, "state": "APPROVED",
-              "submitted_at": "2026-07-06T10:00:00Z", "dismissed": False}],
+              "commit_id": "abc123", "submitted_at": "2026-07-06T10:00:00Z",
+              "dismissed": False}],
             {}, {"merged_commit_sha": "c1"},
         ]
         env = self._env(GITEA_PROFILE_NAME="gitea-merger",

tests/test_mcp_server.py

@@ -972,6 +972,29 @@ class TestMergePR(unittest.TestCase):
         self.assertIn("[REDACTED]", blob)
         self.assertNotIn("abc-secret-xyz", blob)
 
+    @patch("mcp_server.api_request")
+    @patch("mcp_server.get_auth_header", return_value=FAKE_AUTH)
+    def test_merge_blocked_on_stale_approval_head(self, _auth, mock_api):
+        old_sha = "8b61c4b41f1b49b271ed3b99657431cf06eeda3e"
+        new_sha = "3e4b721d60e97147ba0704773cf57cd0d42cbe31"
+        mock_api.side_effect = [
+            {"login": "merger-bot"}, self._pr("author-bot", sha=new_sha),
+            self._pr("author-bot", sha=new_sha),
+            [_formal_review("reviewer-bot", "APPROVED", sha=old_sha)],
+        ]
+        env = {"GITEA_PROFILE_NAME": "gitea-merger",
+               "GITEA_ALLOWED_OPERATIONS": "read,merge"}
+        with patch.dict(os.environ, env, clear=True):
+            r = gitea_merge_pr(
+                pr_number=8, confirmation=self._confirm(8), remote="prgs")
+        self.assertFalse(r["performed"])
+        self.assertTrue(r.get("approval_visible"))
+        self.assertFalse(r.get("approval_at_current_head"))
+        self.assertTrue(any("stale approval" in x for x in r["reasons"]))
+        self.assertTrue(any(old_sha in x for x in r["reasons"]))
+        self.assertTrue(any(new_sha in x for x in r["reasons"]))
+        self._assert_no_merge_call(mock_api)
+
     @patch("mcp_server.api_request")
     @patch("mcp_server.get_auth_header", return_value=FAKE_AUTH)
     def test_merge_blocked_without_visible_approval(self, _auth, mock_api):

tests/test_merge_approval_gate.py

@@ -0,0 +1,59 @@
+"""Hermetic tests for merge approval head pinning (#471)."""
+import sys
+import unittest
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from merge_approval_gate import assess_merge_approval_head  # noqa: E402
+
+HEAD_OLD = "8b61c4b41f1b49b271ed3b99657431cf06eeda3e"
+HEAD_NEW = "3e4b721d60e97147ba0704773cf57cd0d42cbe31"
+
+
+class TestMergeApprovalGate(unittest.TestCase):
+    def test_fresh_approval_at_current_head(self):
+        result = assess_merge_approval_head(
+            current_head_sha=HEAD_NEW,
+            latest_by_reviewer={
+                "reviewer1": {
+                    "verdict": "APPROVED",
+                    "dismissed": False,
+                    "reviewed_head_sha": HEAD_NEW,
+                    "submitted_at": "2026-07-06T12:00:00Z",
+                }
+            },
+        )
+        self.assertTrue(result["approval_at_current_head"])
+        self.assertIsNone(result["stale_approval_block_reason"])
+
+    def test_stale_approval_after_rebase(self):
+        result = assess_merge_approval_head(
+            current_head_sha=HEAD_NEW,
+            latest_by_reviewer={
+                "reviewer1": {
+                    "verdict": "APPROVED",
+                    "dismissed": False,
+                    "reviewed_head_sha": HEAD_OLD,
+                    "submitted_at": "2026-07-06T10:00:00Z",
+                }
+            },
+        )
+        self.assertFalse(result["approval_at_current_head"])
+        self.assertEqual(result["latest_approved_head_sha"], HEAD_OLD)
+        self.assertIn("stale approval", result["stale_approval_block_reason"])
+        self.assertIn(HEAD_OLD, result["stale_approval_block_reason"])
+        self.assertIn(HEAD_NEW, result["stale_approval_block_reason"])
+        self.assertIn("re-review PR at current head", result["stale_approval_block_reason"])
+
+    def test_no_approval_entries(self):
+        result = assess_merge_approval_head(
+            current_head_sha=HEAD_NEW,
+            latest_by_reviewer={},
+        )
+        self.assertFalse(result["approval_at_current_head"])
+        self.assertIsNone(result["latest_approved_head_sha"])
+
+
+if __name__ == "__main__":
+    unittest.main()

tests/test_review_feedback.py

@@ -115,6 +115,22 @@ class TestPRReviewFeedbackDiscovery(unittest.TestCase):
             result["latest_review_state_by_reviewer"], {"reviewer1": "APPROVED"})
         self.assertFalse(result["has_blocking_change_requests"])
         self.assertTrue(result["approval_visible"])
+        self.assertTrue(result["approval_at_current_head"])
+
+    @patch("mcp_server.api_request")
+    @patch("mcp_server.get_auth_header", return_value=FAKE_AUTH)
+    @patch("mcp_server.get_profile")
+    def test_stale_approval_not_at_current_head(self, mock_get_profile, _auth, mock_api):
+        mock_get_profile.return_value = self._profile()
+        mock_api.side_effect = [
+            _pr_details(head_sha="newhead3"),
+            [_review("reviewer1", "APPROVED", commit_id="oldhead1")],
+        ]
+        result = gitea_get_pr_review_feedback(pr_number=5, remote="prgs")
+        self.assertTrue(result["approval_visible"])
+        self.assertFalse(result["approval_at_current_head"])
+        self.assertEqual(result["latest_approved_head_sha"], "oldhead1")
+        self.assertIn("stale approval", result["stale_approval_block_reason"])
 
     @patch("mcp_server.api_request")
     @patch("mcp_server.get_auth_header", return_value=FAKE_AUTH)