core[patch]: Correctly order parent ids in astream events (from root to immediate parent), add defensive check for cycles (#22637)

This PR makes two changes:

1. Fixes the order of parent IDs to be from root to immediate parent
2. Adds a simple defensive check for cycles

libs/core/langchain_core/tracers/event_stream.py

@@ -118,13 +118,20 @@ class _AstreamEventsCallbackHandler(AsyncCallbackHandler, _StreamingCallbackHand
     def _get_parent_ids(self, run_id: UUID) -> List[str]:
         """Get the parent IDs of a run (non-recursively) cast to strings."""
         parent_ids = []
-        parent_id = self.parent_map[run_id]
 
-        while parent_id is not None:
-            parent_ids.append(str(parent_id))
-            parent_id = self.parent_map[parent_id]
+        while parent_id := self.parent_map.get(run_id):
+            str_parent_id = str(parent_id)
+            if str_parent_id in parent_ids:
+                raise AssertionError(
+                    f"Parent ID {parent_id} is already in the parent_ids list. "
+                    f"This should never happen."
+                )
+            parent_ids.append(str_parent_id)
+            run_id = parent_id
 
-        return parent_ids
+        # Return the parent IDs in reverse order, so that the first
+        # parent ID is the root and the last ID is the immediate parent.
+        return parent_ids[::-1]
 
     def _send(self, event: StreamEvent, event_type: str) -> None:
         """Send an event to the stream."""

libs/core/tests/unit_tests/runnables/test_runnable_events_v2.py

@@ -2033,8 +2033,8 @@ async def test_parent_run_id_assignment() -> None:
             "metadata": {},
             "name": "grandchild",
             "parent_ids": [
-                "00000000-0000-0000-0000-000000000008",
                 "00000000-0000-0000-0000-000000000007",
+                "00000000-0000-0000-0000-000000000008",
             ],
             "run_id": "00000000-0000-0000-0000-000000000009",
             "tags": [],
@@ -2045,8 +2045,8 @@ async def test_parent_run_id_assignment() -> None:
             "metadata": {},
             "name": "grandchild",
             "parent_ids": [
-                "00000000-0000-0000-0000-000000000008",
                 "00000000-0000-0000-0000-000000000007",
+                "00000000-0000-0000-0000-000000000008",
             ],
             "run_id": "00000000-0000-0000-0000-000000000009",
             "tags": [],
@@ -2081,6 +2081,42 @@ async def test_parent_run_id_assignment() -> None:
     ]
 
 
+async def test_bad_parent_ids() -> None:
+    """Test handling of situation where a run id is duplicated in the run tree."""
+
+    # Type ignores in the code below need to be investigated.
+    # Looks like a typing issue when using RunnableLambda as a decorator
+    # with async functions.
+    @RunnableLambda  # type: ignore
+    async def child(x: str) -> str:
+        return x
+
+    @RunnableLambda  # type: ignore
+    async def parent(x: str, config: RunnableConfig) -> str:
+        config["run_id"] = uuid.UUID(int=7)
+        return await child.ainvoke(x, config)  # type: ignore
+
+    bond = uuid.UUID(int=7)
+    events = await _collect_events(
+        parent.astream_events("hello", {"run_id": bond}, version="v2"),
+        with_nulled_ids=False,
+    )
+    # Includes only a partial list of events since the run ID gets duplicated
+    # between parent and child run ID and the callback handler throws an exception.
+    # The exception does not get bubbled up to the user.
+    assert events == [
+        {
+            "data": {"input": "hello"},
+            "event": "on_chain_start",
+            "metadata": {},
+            "name": "parent",
+            "parent_ids": [],
+            "run_id": "00000000-0000-0000-0000-000000000007",
+            "tags": [],
+        }
+    ]
+
+
 async def test_runnable_generator() -> None:
     """Test async events from sync lambda."""