Dereference run tree (#30377)

libs/core/langchain_core/tracers/langchain.py

@@ -173,7 +173,13 @@ class LangChainTracer(BaseTracer):
         return chat_model_run
 
     def _persist_run(self, run: Run) -> None:
-        self.latest_run = run
+        # We want to free up more memory by avoiding keeping a reference to the
+        # whole nested run tree.
+        self.latest_run = Run.construct(
+            **run.dict(exclude={"child_runs", "inputs", "outputs"}),
+            inputs=run.inputs,
+            outputs=run.outputs,
+        )
 
     def get_run_url(self) -> str:
         """Get the LangSmith root run URL.

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

@@ -445,12 +445,13 @@ def test_tree_is_constructed(parent_type: Literal["ls", "lc"]) -> None:
         metadata={"some_foo": "some_bar"},
         tags=["afoo"],
     ):
-        if parent_type == "ls":
         collected: dict[str, RunTree] = {}  # noqa
 
         def collect_run(run: RunTree) -> None:
             collected[str(run.id)] = run
 
+        if parent_type == "ls":
+
             @traceable
             def parent() -> str:
                 return child.invoke("foo")
@@ -459,7 +460,6 @@ def test_tree_is_constructed(parent_type: Literal["ls", "lc"]) -> None:
                 parent(langsmith_extra={"on_end": collect_run, "run_id": rid}) == "foo"
             )
             assert collected
-            run = collected.get(str(rid))
 
         else:
 
@@ -468,8 +468,10 @@ def test_tree_is_constructed(parent_type: Literal["ls", "lc"]) -> None:
                 return child.invoke("foo")
 
             tracer = LangChainTracer()
+            tracer._persist_run = collect_run  # type: ignore
+
             assert parent.invoke(..., {"run_id": rid, "callbacks": [tracer]}) == "foo"  # type: ignore
-            run = tracer.latest_run
+    run = collected.get(str(rid))
 
     assert run is not None
     assert run.name == "parent"