fix: Avoid errors from executor modifying tasks dict during exit routine

- This could happen if a task happened to finish while the exit routine is running

libs/langgraph/langgraph/pregel/executor.py

@@ -86,19 +86,21 @@ def __exit__(
         exc_value: Optional[BaseException],
         traceback: Optional[TracebackType],
     ) -> Optional[bool]:
+        # copy the tasks as done() callback may modify the dict
+        tasks = self.tasks.copy()
         # cancel all tasks that should be cancelled
-        for task, (cancel, _) in self.tasks.items():
+        for task, (cancel, _) in tasks.items():
             if cancel:
                 task.cancel()
         # wait for all tasks to finish
-        if tasks := {t for t in self.tasks if not t.done()}:
+        if tasks := {t for t in tasks if not t.done()}:
             concurrent.futures.wait(tasks)
         # shutdown the executor
         self.stack.__exit__(exc_type, exc_value, traceback)
         # re-raise the first exception that occurred in a task
         if exc_type is None:
             # if there's already an exception being raised, don't raise another one
-            for task, (_, reraise) in self.tasks.items():
+            for task, (_, reraise) in tasks.items():
                 if not reraise:
                     continue
                 try:
@@ -161,17 +163,19 @@ async def __aexit__(
         exc_value: Optional[BaseException],
         traceback: Optional[TracebackType],
     ) -> None:
+        # copy the tasks as done() callback may modify the dict
+        tasks = self.tasks.copy()
         # cancel all tasks that should be cancelled
-        for task, (cancel, _) in self.tasks.items():
+        for task, (cancel, _) in tasks.items():
             if cancel:
                 task.cancel(self.sentinel)
         # wait for all tasks to finish
-        if self.tasks:
-            await asyncio.wait(self.tasks)
+        if tasks:
+            await asyncio.wait(tasks)
         # if there's already an exception being raised, don't raise another one
         if exc_type is None:
             # re-raise the first exception that occurred in a task
-            for task, (_, reraise) in self.tasks.items():
+            for task, (_, reraise) in tasks.items():
                 if not reraise:
                     continue
                 try: