fix: M2 on_failure assignee 从 tasks 表读取 + infrastructure 防递归（司马懿 Review #65）

M2: on_failure 中 assignee = meta.get('from', '') 读到 'system' 而非实际 Agent
→ 改为 SELECT must_haves, assignee FROM tasks 直接读 tasks.assignee 字段

附带：infrastructure failure 改为直接 DB INSERT，不走 _send_toolchain_task 防递归

tests/unit/test_toolchain_handler_v2.py

@@ -398,12 +398,14 @@ class TestSendToolchainTask:
 
 class TestOnFailureRouting:
     def test_business_failure_creates_gitea_comment(self, handler, tmp_db):
-        """Business failure → Gitea PR comment"""
+        """Business failure → Gitea PR comment @task assignee (not must_hives field)"""
+        # S4: must_hives does NOT contain assignee — production data doesn't have it
         must_haves = json.dumps({
             "action_type": "review_result",
             "context": {"repo": "sanguo/test", "pr_number": 42},
-            "assignee": "zhangfei-dev",
+            "from": "system",
         })
+        # assignee is set on the tasks table row (as production code writes it)
         _insert_task(tmp_db, "t-fail", must_haves)
 
         with patch.object(handler, "_create_gitea_comment") as mock_comment:
@@ -414,9 +416,12 @@ class TestOnFailureRouting:
             call_args = mock_comment.call_args
             assert call_args[0][0] == "sanguo/test"
             assert call_args[0][1] == 42
+            # M2: comment body should @ the task's assignee from tasks table
+            comment_body = call_args[0][2]
+            assert "@zhangfei-dev" in comment_body
 
     def test_infrastructure_failure_creates_task(self, handler, tmp_db):
-        """Infrastructure failure → _send_toolchain_task for jiangwei-infra"""
+        """Infrastructure failure → direct DB task for jiangwei-infra (no reverse dep)"""
         must_haves = json.dumps({
             "action_type": "review_result",
             "context": {"repo": "sanguo/test", "pr_number": 42},
@@ -427,15 +432,22 @@ class TestOnFailureRouting:
             mock_comment.return_value = False  # Gitea API down
             with patch.object(handler, "_create_gitea_issue") as mock_issue:
                 mock_issue.return_value = False  # Gitea API still down
-                with patch("src.api.toolchain_routes._send_toolchain_task") as mock_send:
-                    mock_send.return_value = "tc-infra"
-                    verify = VerifyResult(False, "no_action", "no action_report")
-                    handler.on_failure("t-infra", "zhangfei-dev", tmp_db, verify)
-                    # Should eventually try to create infrastructure_failure task
-                    mock_send.assert_called()
-                    call_kwargs = mock_send.call_args
-                    assert call_kwargs[1]["action_type"] == "infrastructure_failure"
-                    assert call_kwargs[1]["to_agent"] == "jiangwei-infra"
+                verify = VerifyResult(False, "no_action", "no action_report")
+                handler.on_failure("t-infra", "zhangfei-dev", tmp_db, verify)
+
+                # S3: should directly INSERT into DB, not call _send_toolchain_task
+                # Verify a new task was created in DB for jiangwei-infra
+                conn = get_connection(tmp_db)
+                rows = conn.execute(
+                    "SELECT * FROM tasks WHERE assignee=?",
+                    ("jiangwei-infra",)
+                ).fetchall()
+                conn.close()
+                assert len(rows) >= 1, "No infrastructure_failure task created"
+                infra_task = rows[0]
+                assert infra_task["task_type"] == "toolchain"
+                meta = json.loads(infra_task["must_haves"])
+                assert meta["action_type"] == "infrastructure_failure"
 
 
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