Weakref to tracer (#9954)

Prevent memory/thread leakage

libs/langchain/langchain/callbacks/tracers/langchain.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 
 import logging
 import os
+import weakref
 from concurrent.futures import Future, ThreadPoolExecutor, wait
 from datetime import datetime
 from typing import Any, Callable, Dict, List, Optional, Set, Union
@@ -18,8 +19,10 @@ from langchain.schema.messages import BaseMessage
 
 logger = logging.getLogger(__name__)
 _LOGGED = set()
-_TRACERS: List[LangChainTracer] = []
+_TRACERS: weakref.WeakSet[LangChainTracer] = weakref.WeakSet()
 _CLIENT: Optional[Client] = None
+_MAX_EXECUTORS = 10  # TODO: Remove once write queue is implemented
+_EXECUTORS: List[ThreadPoolExecutor] = []
 
 
 def log_error_once(method: str, exception: Exception) -> None:
@@ -34,8 +37,9 @@ def log_error_once(method: str, exception: Exception) -> None:
 def wait_for_all_tracers() -> None:
     """Wait for all tracers to finish."""
     global _TRACERS
-    for tracer in _TRACERS:
-        tracer.wait_for_futures()
+    for tracer in list(_TRACERS):
+        if tracer is not None:
+            tracer.wait_for_futures()
 
 
 def _get_client() -> Client:
@@ -68,17 +72,22 @@ class LangChainTracer(BaseTracer):
             "LANGCHAIN_PROJECT", os.getenv("LANGCHAIN_SESSION", "default")
         )
         if use_threading:
-            # set max_workers to 1 to process tasks in order
-            self.executor: Optional[ThreadPoolExecutor] = ThreadPoolExecutor(
-                max_workers=1
-            )
+            global _MAX_EXECUTORS
+            if len(_EXECUTORS) < _MAX_EXECUTORS:
+                self.executor: Optional[ThreadPoolExecutor] = ThreadPoolExecutor(
+                    max_workers=1
+                )
+                _EXECUTORS.append(self.executor)
+            else:
+                self.executor = _EXECUTORS.pop(0)
+                _EXECUTORS.append(self.executor)
         else:
             self.executor = None
         self.client = client or _get_client()
         self._futures: Set[Future] = set()
         self.tags = tags or []
         global _TRACERS
-        _TRACERS.append(self)
+        _TRACERS.add(self)
 
     def on_chat_model_start(
         self,