feat: add per-PR reviewer leases for parallel review (Closes #407)

Structured PR-thread leases with acquire/heartbeat MCP tools and
mutation-time enforcement so reviewers cannot race the same PR queue.

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

tests/test_reviewer_pr_lease.py

@@ -0,0 +1,193 @@
+"""Tests for per-PR reviewer leases (#407)."""
+
+import sys
+import unittest
+from datetime import datetime, timedelta, timezone
+from unittest.mock import patch
+
+sys.path.insert(0, str(__import__("pathlib").Path(__file__).resolve().parent.parent))
+
+import reviewer_pr_lease as leases
+
+
+def _lease_comment(
+    pr_number: int,
+    session_id: str,
+    *,
+    phase: str = "claimed",
+    minutes_ago: int = 0,
+    candidate_head: str = "a" * 40,
+) -> dict:
+    now = datetime.now(timezone.utc) - timedelta(minutes=minutes_ago)
+    body = leases.format_lease_body(
+        repo="Scaled-Tech-Consulting/Gitea-Tools",
+        pr_number=pr_number,
+        issue_number=295,
+        reviewer_identity="rev1",
+        profile="prgs-reviewer",
+        session_id=session_id,
+        worktree="branches/review-pr382",
+        phase=phase,
+        candidate_head=candidate_head,
+        target_branch="master",
+        target_branch_sha="b" * 40,
+        last_activity=now,
+    )
+    return {"id": 1, "body": body, "user": {"login": "rev1"}}
+
+
+class TestReviewerLeaseAcquire(unittest.TestCase):
+    def setUp(self):
+        leases.clear_session_lease()
+
+    def test_two_reviewers_cannot_lease_same_pr(self):
+        comments = [_lease_comment(382, "session-a")]
+        result = leases.assess_acquire_lease(
+            comments,
+            pr_number=382,
+            reviewer_identity="rev2",
+            profile="prgs-reviewer",
+            session_id="session-b",
+            repo="Scaled-Tech-Consulting/Gitea-Tools",
+            issue_number=295,
+            worktree="branches/review-pr382-b",
+            candidate_head="c" * 40,
+            target_branch="master",
+            target_branch_sha="d" * 40,
+        )
+        self.assertFalse(result["acquire_allowed"])
+        self.assertTrue(any("already has active" in r for r in result["reasons"]))
+
+    def test_two_reviewers_can_lease_different_prs(self):
+        comments = [_lease_comment(382, "session-a")]
+        result = leases.assess_acquire_lease(
+            comments,
+            pr_number=383,
+            reviewer_identity="rev2",
+            profile="prgs-reviewer",
+            session_id="session-b",
+            repo="Scaled-Tech-Consulting/Gitea-Tools",
+            issue_number=296,
+            worktree="branches/review-pr383",
+            candidate_head="c" * 40,
+            target_branch="master",
+            target_branch_sha="d" * 40,
+        )
+        self.assertTrue(result["acquire_allowed"])
+        self.assertIsNotNone(result["lease_body"])
+
+
+class TestReviewerLeaseFreshness(unittest.TestCase):
+    def test_stale_warning_after_30_minutes(self):
+        lease = leases.parse_lease_comment(
+            _lease_comment(382, "session-a", minutes_ago=35)["body"]
+        )
+        self.assertEqual(
+            leases.classify_lease_freshness(lease),
+            "stale_warning",
+        )
+
+    def test_reclaimable_after_60_minutes(self):
+        lease = leases.parse_lease_comment(
+            _lease_comment(382, "session-a", minutes_ago=65)["body"]
+        )
+        self.assertEqual(
+            leases.classify_lease_freshness(lease),
+            "reclaimable",
+        )
+
+
+class TestReviewerLeaseMutationGate(unittest.TestCase):
+    def setUp(self):
+        leases.clear_session_lease()
+
+    def test_reviewer_without_lease_cannot_mutate(self):
+        head = "f" * 40
+        comments = [_lease_comment(382, "other-session", candidate_head=head)]
+        result = leases.assess_mutation_lease_gate(
+            pr_number=382,
+            comments=comments,
+            reviewer_identity="rev1",
+            session_id="my-session",
+            mutation="approve",
+            live_head_sha=head,
+            pinned_head_sha=head,
+        )
+        self.assertTrue(result["block"])
+
+    def test_owned_lease_allows_mutation(self):
+        head = "f" * 40
+        comments = [_lease_comment(382, "my-session", candidate_head=head)]
+        leases.record_session_lease({
+            "pr_number": 382,
+            "session_id": "my-session",
+            "candidate_head": head,
+            "target_branch": "master",
+        })
+        result = leases.assess_mutation_lease_gate(
+            pr_number=382,
+            comments=comments,
+            reviewer_identity="rev1",
+            session_id="my-session",
+            mutation="approve",
+            live_head_sha=head,
+            pinned_head_sha=head,
+        )
+        self.assertFalse(result["block"])
+
+    def test_head_change_invalidates_lease(self):
+        reviewed = "f" * 40
+        live = "e" * 40
+        comments = [_lease_comment(382, "my-session", candidate_head=reviewed)]
+        leases.record_session_lease({
+            "pr_number": 382,
+            "session_id": "my-session",
+            "candidate_head": reviewed,
+        })
+        result = leases.assess_mutation_lease_gate(
+            pr_number=382,
+            comments=comments,
+            reviewer_identity="rev1",
+            session_id="my-session",
+            mutation="merge",
+            live_head_sha=live,
+            pinned_head_sha=reviewed,
+        )
+        self.assertTrue(result["block"])
+        self.assertTrue(any("head" in r.lower() for r in result["reasons"]))
+
+
+class TestReviewerLeaseMcpGate(unittest.TestCase):
+    def setUp(self):
+        leases.clear_session_lease()
+        patch("mcp_server.verify_preflight_purity").start()
+        patch("gitea_audit.audit_enabled", return_value=False).start()
+        mcp_server = __import__("mcp_server")
+        mcp_server._IDENTITY_CACHE.clear()
+        mcp_server.init_review_decision_lock("prgs", "review_pr")
+        mcp_server.record_preflight_check("whoami")
+        mcp_server.record_preflight_check("capability", "reviewer")
+
+    def tearDown(self):
+        patch.stopall()
+        leases.clear_session_lease()
+
+    def test_reviewer_pr_lease_gate_helper_blocks_without_session(self):
+        import mcp_server
+        head = "a" * 40
+        with patch("mcp_server._fetch_pr_comments", return_value=[]):
+            reasons = mcp_server._reviewer_pr_lease_gate(
+                pr_number=382,
+                remote="prgs",
+                host=None,
+                org=None,
+                repo=None,
+                mutation="approve",
+                live_head_sha=head,
+                pinned_head_sha=head,
+            )
+        self.assertTrue(any("lease" in r.lower() for r in reasons))
+
+
+if __name__ == "__main__":
+    unittest.main()