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]>

gitea_mcp_server.py

@@ -543,6 +543,7 @@ import already_landed_reconcile  # noqa: E402
 import author_mutation_worktree  # noqa: E402
 import issue_claim_heartbeat  # noqa: E402
 import issue_work_duplicate_gate  # noqa: E402
+import reviewer_pr_lease  # noqa: E402
 import merged_cleanup_reconcile  # noqa: E402
 import reconciler_profile  # noqa: E402
 import reconciliation_workflow  # noqa: E402
@@ -2016,6 +2017,7 @@ def init_review_decision_lock(remote: str | None, task: str | None):
         (os.environ.get(SESSION_PROFILE_LOCK_ENV) or "").strip()
         or profile_name
     )
+    reviewer_pr_lease.clear_session_lease()
     _save_review_decision_lock({
         "task": task,
         "remote": remote,
@@ -2458,6 +2460,20 @@ def _evaluate_pr_review_submission(
             result["permission_report"] = elig["permission_report"]
         return result
 
+    if live:
+        reasons.extend(_reviewer_pr_lease_gate(
+            pr_number=pr_number,
+            remote=remote,
+            host=host,
+            org=org,
+            repo=repo,
+            mutation=action,
+            live_head_sha=result.get("head_sha"),
+            pinned_head_sha=expected_head_sha,
+        ))
+        if reasons:
+            return result
+
     auth_user = result["authenticated_user"]
     pr_author = result["pr_author"]
     if action == "approve" and auth_user and pr_author and auth_user == pr_author:
@@ -3257,6 +3273,19 @@ def gitea_merge_pr(
             result["permission_report"] = elig["permission_report"]
         return result
 
+    reasons.extend(_reviewer_pr_lease_gate(
+        pr_number=pr_number,
+        remote=remote,
+        host=host,
+        org=org,
+        repo=repo,
+        mutation="merge",
+        live_head_sha=result.get("head_sha"),
+        pinned_head_sha=expected_head_sha,
+    ))
+    if reasons:
+        return result
+
     # Gate 4 — head SHA must match if the caller pinned a reviewed SHA.
     actual_sha = result["head_sha"]
     if expected_head_sha and actual_sha and expected_head_sha != actual_sha:
@@ -4556,6 +4585,261 @@ def _namespace_mutation_block(mutation_task: str, **extra_fields) -> dict | None
     return blocked
 
 
+def _fetch_pr_comments(
+    pr_number: int,
+    *,
+    remote: str,
+    host: str | None,
+    org: str | None,
+    repo: str | None,
+) -> list[dict]:
+    h, o, r = _resolve(remote, host, org, repo)
+    auth = _auth(h)
+    api = f"{repo_api_url(h, o, r)}/issues/{pr_number}/comments"
+    return api_request("GET", api, auth) or []
+
+
+def _reviewer_pr_lease_gate(
+    *,
+    pr_number: int,
+    remote: str,
+    host: str | None,
+    org: str | None,
+    repo: str | None,
+    mutation: str,
+    live_head_sha: str | None,
+    pinned_head_sha: str | None,
+) -> list[str]:
+    """Return block reasons when the session lacks an owned PR reviewer lease."""
+    session = reviewer_pr_lease.get_session_lease()
+    session_id = (session or {}).get("session_id")
+    identity = _authenticated_username(remote) or ""
+    try:
+        comments = _fetch_pr_comments(
+            pr_number, remote=remote, host=host, org=org, repo=repo)
+    except Exception as exc:
+        return [f"cannot fetch PR comments for lease gate: {_redact(str(exc))}"]
+    assessment = reviewer_pr_lease.assess_mutation_lease_gate(
+        pr_number=pr_number,
+        comments=comments,
+        reviewer_identity=identity,
+        session_id=session_id,
+        mutation=mutation,
+        live_head_sha=live_head_sha,
+        pinned_head_sha=pinned_head_sha,
+    )
+    return list(assessment.get("reasons") or []) if assessment.get("block") else []
+
+
+@mcp.tool()
+def gitea_acquire_reviewer_pr_lease(
+    pr_number: int,
+    worktree: str,
+    candidate_head: str | None = None,
+    target_branch: str = "master",
+    target_branch_sha: str | None = None,
+    issue_number: int | None = None,
+    session_id: str | None = None,
+    remote: str = "dadeschools",
+    host: str | None = None,
+    org: str | None = None,
+    repo: str | None = None,
+) -> dict:
+    """Acquire a per-PR reviewer lease before review/merge mutations (#407)."""
+    read_block = _profile_operation_gate("gitea.read")
+    if read_block:
+        return {
+            "success": False,
+            "acquired": False,
+            "reasons": read_block,
+            "permission_report": _permission_block_report("gitea.read"),
+        }
+    comment_block = _profile_operation_gate("gitea.pr.comment")
+    if comment_block:
+        return {
+            "success": False,
+            "acquired": False,
+            "reasons": comment_block,
+            "permission_report": _permission_block_report("gitea.pr.comment"),
+        }
+
+    verify_preflight_purity(remote)
+    h, o, r = _resolve(remote, host, org, repo)
+    auth = _auth(h)
+    profile = get_profile()
+    identity = _authenticated_username(remote) or profile.get("username") or ""
+    sid = (session_id or "").strip() or reviewer_pr_lease.new_session_id()
+    repo_label = f"{o}/{r}"
+
+    comments = _fetch_pr_comments(
+        pr_number, remote=remote, host=host, org=org, repo=repo)
+    assessment = reviewer_pr_lease.assess_acquire_lease(
+        comments,
+        pr_number=pr_number,
+        reviewer_identity=identity,
+        profile=profile.get("profile_name") or "unknown",
+        session_id=sid,
+        repo=repo_label,
+        issue_number=issue_number,
+        worktree=worktree,
+        candidate_head=candidate_head,
+        target_branch=target_branch,
+        target_branch_sha=target_branch_sha,
+    )
+    if not assessment.get("acquire_allowed"):
+        return {
+            "success": False,
+            "acquired": False,
+            "reasons": assessment.get("reasons") or [],
+            "existing_lease": assessment.get("existing_lease"),
+        }
+
+    body = assessment["lease_body"]
+    comment_url = f"{repo_api_url(h, o, r)}/issues/{pr_number}/comments"
+    with _audited(
+        "comment_pr",
+        host=h,
+        remote=remote,
+        org=o,
+        repo=r,
+        pr_number=pr_number,
+        request_metadata={"source": "acquire_reviewer_pr_lease"},
+    ):
+        posted = api_request("POST", comment_url, auth, {"body": body})
+
+    session_lease = reviewer_pr_lease.record_session_lease({
+        "pr_number": pr_number,
+        "issue_number": issue_number,
+        "session_id": sid,
+        "reviewer_identity": identity,
+        "profile": profile.get("profile_name"),
+        "worktree": worktree,
+        "phase": "claimed",
+        "candidate_head": candidate_head,
+        "target_branch": target_branch,
+        "target_branch_sha": target_branch_sha,
+        "repo": repo_label,
+        "comment_id": posted.get("id"),
+    })
+    return {
+        "success": True,
+        "acquired": True,
+        "pr_number": pr_number,
+        "session_id": sid,
+        "comment_id": posted.get("id"),
+        "session_lease": session_lease,
+        "reasons": [],
+    }
+
+
+@mcp.tool()
+def gitea_heartbeat_reviewer_pr_lease(
+    pr_number: int,
+    phase: str,
+    worktree: str | None = None,
+    candidate_head: str | None = None,
+    target_branch_sha: str | None = None,
+    remote: str = "dadeschools",
+    host: str | None = None,
+    org: str | None = None,
+    repo: str | None = None,
+) -> dict:
+    """Post a reviewer lease heartbeat / phase update on the PR thread (#407)."""
+    comment_block = _profile_operation_gate("gitea.pr.comment")
+    if comment_block:
+        return {
+            "success": False,
+            "posted": False,
+            "reasons": comment_block,
+            "permission_report": _permission_block_report("gitea.pr.comment"),
+        }
+    session = reviewer_pr_lease.get_session_lease()
+    if not session or session.get("pr_number") != pr_number:
+        return {
+            "success": False,
+            "posted": False,
+            "reasons": [
+                f"no in-session lease for PR #{pr_number}; acquire first "
+                "(fail closed)"
+            ],
+        }
+
+    verify_preflight_purity(remote)
+    h, o, r = _resolve(remote, host, org, repo)
+    auth = _auth(h)
+    body = reviewer_pr_lease.format_lease_body(
+        repo=f"{o}/{r}",
+        pr_number=pr_number,
+        issue_number=session.get("issue_number"),
+        reviewer_identity=session.get("reviewer_identity") or "",
+        profile=session.get("profile") or "unknown",
+        session_id=session.get("session_id") or reviewer_pr_lease.new_session_id(),
+        worktree=worktree or session.get("worktree") or "",
+        phase=phase,
+        candidate_head=candidate_head or session.get("candidate_head"),
+        target_branch=session.get("target_branch") or "master",
+        target_branch_sha=target_branch_sha or session.get("target_branch_sha"),
+    )
+    comment_url = f"{repo_api_url(h, o, r)}/issues/{pr_number}/comments"
+    with _audited(
+        "comment_pr",
+        host=h,
+        remote=remote,
+        org=o,
+        repo=r,
+        pr_number=pr_number,
+        request_metadata={"source": "heartbeat_reviewer_pr_lease", "phase": phase},
+    ):
+        posted = api_request("POST", comment_url, auth, {"body": body})
+
+    updated = reviewer_pr_lease.record_session_lease({
+        **session,
+        "phase": phase,
+        "worktree": worktree or session.get("worktree"),
+        "candidate_head": candidate_head or session.get("candidate_head"),
+        "target_branch_sha": target_branch_sha or session.get("target_branch_sha"),
+        "last_comment_id": posted.get("id"),
+    })
+    return {
+        "success": True,
+        "posted": True,
+        "pr_number": pr_number,
+        "phase": phase,
+        "comment_id": posted.get("id"),
+        "session_lease": updated,
+        "reasons": [],
+    }
+
+
+@mcp.tool()
+def gitea_assess_reviewer_pr_lease(
+    pr_number: int,
+    remote: str = "dadeschools",
+    host: str | None = None,
+    org: str | None = None,
+    repo: str | None = None,
+) -> dict:
+    """Read-only: assess active reviewer lease state for a PR (#407)."""
+    read_block = _profile_operation_gate("gitea.read")
+    if read_block:
+        return {
+            "success": False,
+            "reasons": read_block,
+            "permission_report": _permission_block_report("gitea.read"),
+        }
+    comments = _fetch_pr_comments(
+        pr_number, remote=remote, host=host, org=org, repo=repo)
+    active = reviewer_pr_lease.find_active_reviewer_lease(
+        comments, pr_number=pr_number)
+    return {
+        "success": True,
+        "pr_number": pr_number,
+        "active_lease": active,
+        "session_lease": reviewer_pr_lease.get_session_lease(),
+        "reasons": [],
+    }
+
+
 @mcp.tool()
 def gitea_list_issue_comments(
     issue_number: int,