core[patch]: Share executor for async callbacks run in sync context (#30779)

To avoid having to create ephemeral threads, grab the thread lock, etc.

libs/core/langchain_core/callbacks/manager.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import asyncio
+import atexit
 import functools
 import logging
 import uuid
@@ -314,12 +315,13 @@ def handle_event(
                 # If we try to submit this coroutine to the running loop
                 # we end up in a deadlock, as we'd have gotten here from a
                 # running coroutine, which we cannot interrupt to run this one.
-                # The solution is to create a new loop in a new thread.
-                with ThreadPoolExecutor(1) as executor:
-                    executor.submit(
+                # The solution is to run the synchronous function on the globally shared
+                # thread pool executor to avoid blocking the main event loop.
+                _executor().submit(
                     cast("Callable", copy_context().run), _run_coros, coros
                 ).result()
             else:
+                # If there's no running loop, we can run the coroutines directly.
                 _run_coros(coros)
 
 
@@ -2618,3 +2620,19 @@ def dispatch_custom_event(
         data,
         run_id=callback_manager.parent_run_id,
     )
+
+
+@functools.lru_cache(maxsize=1)
+def _executor() -> ThreadPoolExecutor:
+    # If the user is specifying ASYNC callback handlers to be run from a
+    # SYNC context, and an event loop is already running,
+    # we cannot submit the coroutine to the running loop, because it
+    # would result in a deadlock. Instead we have to schedule them
+    # on a background thread. To avoid creating & shutting down
+    # a new executor every time, we use a lazily-created, shared
+    # executor. If you're using regular langgchain parallelism (batch, etc.)
+    # you'd only ever need 1 worker, but we permit more for now to reduce the chance
+    # of slowdown if you are mixing with your own executor.
+    cutie = ThreadPoolExecutor(max_workers=10)
+    atexit.register(cutie.shutdown, wait=True)
+    return cutie

libs/core/tests/benchmarks/test_async_callbacks.py

@@ -45,12 +45,12 @@ class MyCustomAsyncHandler(AsyncCallbackHandler):
 
 
 @pytest.mark.benchmark
-async def test_async_callbacks(benchmark: BenchmarkFixture) -> None:
-    infinite_cycle = cycle([AIMessage(content=" ".join(["hello", "goodbye"] * 1000))])
+async def test_async_callbacks_in_sync(benchmark: BenchmarkFixture) -> None:
+    infinite_cycle = cycle([AIMessage(content=" ".join(["hello", "goodbye"] * 500))])
     model = GenericFakeChatModel(messages=infinite_cycle)
 
     @benchmark
-    def async_callbacks() -> None:
-        for _ in range(10):
+    def sync_callbacks() -> None:
+        for _ in range(5):
             for _ in model.stream("meow", {"callbacks": [MyCustomAsyncHandler()]}):
                 pass