Support using async callback handlers with sync callback manager (#10945)

The current behaviour just calls the handler without awaiting the
coroutine, which results in exceptions/warnings, and obviously doesn't
actually execute whatever the callback handler does

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libs/langchain/langchain/callbacks/manager.py

@@ -5,12 +5,14 @@ import functools
 import logging
 import os
 import uuid
+from concurrent.futures import ThreadPoolExecutor
 from contextlib import asynccontextmanager, contextmanager
 from contextvars import ContextVar
 from typing import (
     TYPE_CHECKING,
     Any,
     AsyncGenerator,
+    Coroutine,
     Dict,
     Generator,
     List,
@@ -370,37 +372,84 @@ def _handle_event(
     **kwargs: Any,
 ) -> None:
     """Generic event handler for CallbackManager."""
-    message_strings: Optional[List[str]] = None
-    for handler in handlers:
-        try:
-            if ignore_condition_name is None or not getattr(
-                handler, ignore_condition_name
-            ):
-                getattr(handler, event_name)(*args, **kwargs)
-        except NotImplementedError as e:
-            if event_name == "on_chat_model_start":
-                if message_strings is None:
-                    message_strings = [get_buffer_string(m) for m in args[1]]
-                _handle_event(
-                    [handler],
-                    "on_llm_start",
-                    "ignore_llm",
-                    args[0],
-                    message_strings,
-                    *args[2:],
-                    **kwargs,
-                )
-            else:
+    coros: List[Coroutine[Any, Any, Any]] = []
+
+    try:
+        message_strings: Optional[List[str]] = None
+        for handler in handlers:
+            try:
+                if ignore_condition_name is None or not getattr(
+                    handler, ignore_condition_name
+                ):
+                    event = getattr(handler, event_name)(*args, **kwargs)
+                    if asyncio.iscoroutine(event):
+                        coros.append(event)
+            except NotImplementedError as e:
+                if event_name == "on_chat_model_start":
+                    if message_strings is None:
+                        message_strings = [get_buffer_string(m) for m in args[1]]
+                    _handle_event(
+                        [handler],
+                        "on_llm_start",
+                        "ignore_llm",
+                        args[0],
+                        message_strings,
+                        *args[2:],
+                        **kwargs,
+                    )
+                else:
+                    handler_name = handler.__class__.__name__
+                    logger.warning(
+                        f"NotImplementedError in {handler_name}.{event_name}"
+                        f" callback: {e}"
+                    )
+            except Exception as e:
                 logger.warning(
-                    f"NotImplementedError in {handler.__class__.__name__}.{event_name}"
-                    f" callback: {e}"
+                    f"Error in {handler.__class__.__name__}.{event_name} callback: {e}"
                 )
-        except Exception as e:
-            logger.warning(
-                f"Error in {handler.__class__.__name__}.{event_name} callback: {e}"
-            )
-            if handler.raise_error:
-                raise e
+                if handler.raise_error:
+                    raise e
+    finally:
+        if coros:
+            try:
+                # Raises RuntimeError if there is no current event loop.
+                asyncio.get_running_loop()
+                loop_running = True
+            except RuntimeError:
+                loop_running = False
+
+            if loop_running:
+                # 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(_run_coros, coros).result()
+            else:
+                _run_coros(coros)
+
+
+def _run_coros(coros: List[Coroutine[Any, Any, Any]]) -> None:
+    if hasattr(asyncio, "Runner"):
+        # Python 3.11+
+        # Run the coroutines in a new event loop, taking care to
+        # - install signal handlers
+        # - run pending tasks scheduled by `coros`
+        # - close asyncgens and executors
+        # - close the loop
+        with asyncio.Runner() as runner:
+            # Run the coroutine, get the result
+            for coro in coros:
+                runner.run(coro)
+
+            # Run pending tasks scheduled by coros until they are all done
+            while pending := asyncio.all_tasks(runner.get_loop()):
+                runner.run(asyncio.wait(pending))
+    else:
+        # Before Python 3.11 we need to run each coroutine in a new event loop
+        # as the Runner api is not available.
+        for coro in coros:
+            asyncio.run(coro)
 
 
 async def _ahandle_event_for_handler(

libs/langchain/tests/unit_tests/callbacks/test_callback_manager.py

@@ -92,6 +92,27 @@ def test_callback_manager() -> None:
     _test_callback_manager(manager, handler1, handler2)
 
 
+def test_callback_manager_with_async() -> None:
+    """Test the CallbackManager."""
+    handler1 = FakeCallbackHandler()
+    handler2 = FakeCallbackHandler()
+    handler3 = FakeAsyncCallbackHandler()
+    handler4 = FakeAsyncCallbackHandler()
+    manager = CallbackManager(handlers=[handler1, handler2, handler3, handler4])
+    _test_callback_manager(manager, handler1, handler2, handler3, handler4)
+
+
+@pytest.mark.asyncio
+async def test_callback_manager_with_async_with_running_loop() -> None:
+    """Test the CallbackManager."""
+    handler1 = FakeCallbackHandler()
+    handler2 = FakeCallbackHandler()
+    handler3 = FakeAsyncCallbackHandler()
+    handler4 = FakeAsyncCallbackHandler()
+    manager = CallbackManager(handlers=[handler1, handler2, handler3, handler4])
+    _test_callback_manager(manager, handler1, handler2, handler3, handler4)
+
+
 def test_ignore_llm() -> None:
     """Test ignore llm param for callback handlers."""
     handler1 = FakeCallbackHandler(ignore_llm_=True)