[FIX] fastapi: Avoid zombie threads

Each time a fastapi app is created, a new event loop thread is created by the ASGIMiddleware. Unfortunately, every time the cache is cleared, a new app is created with a new even loop thread. This leads to an increase in the number of threads created to manage the asyncio event loop, even though many of them are no longer in use. To avoid this problem, the thread in charge of the event loop is now created only once per thread / process and the result is stored in the thread's local storage. If a new instance of an app needs to be created following a cache reset, this ensures that the same event loop is reused.

refs #484

fastapi/models/fastapi_endpoint.py

@@ -1,7 +1,9 @@
 # Copyright 2022 ACSONE SA/NV
 # License LGPL-3.0 or later (http://www.gnu.org/licenses/LGPL).
 
+import asyncio
 import logging
+import threading
 from functools import partial
 from itertools import chain
 from typing import Any, Callable, Dict, List, Tuple
@@ -19,6 +21,26 @@
 _logger = logging.getLogger(__name__)
 
 
+# Thread-local storage for event loops
+# Using a thread-local storage allows to have a dedicated event loop per thread
+# and avoid the need to create a new event loop for each request. It's also
+# compatible with the multi-worker mode of Odoo.
+_event_loop_storage = threading.local()
+
+
+def get_or_create_event_loop() -> asyncio.AbstractEventLoop:
+    """
+    Get or create a reusable event loop for the current thread.
+    """
+    if not hasattr(_event_loop_storage, "loop"):
+        loop = asyncio.new_event_loop()
+        loop_thread = threading.Thread(target=loop.run_forever, daemon=True)
+        loop_thread.start()
+        _event_loop_storage.loop = loop
+        _event_loop_storage.thread = loop_thread
+    return _event_loop_storage.loop
+
+
 class FastapiEndpoint(models.Model):
 
     _name = "fastapi.endpoint"
@@ -213,7 +235,8 @@ def get_app(self, root_path):
         app = FastAPI()
         app.mount(record.root_path, record._get_app())
         self._clear_fastapi_exception_handlers(app)
-        return ASGIMiddleware(app)
+        event_loop = get_or_create_event_loop()
+        return ASGIMiddleware(app, loop=event_loop)
 
     def _clear_fastapi_exception_handlers(self, app: FastAPI) -> None:
         """