[FIX] fastapi: Ensure thread safety of the FastAPI app cache

defaultdict in python is not thread safe. Since this data structure
is used to store the cache of FastAPI apps, we must ensure that the
access to this cache is thread safe. This is done by using a lock
to protect the access to the cache.

fastapi/pools/fastapi_app.py

@@ -2,6 +2,7 @@
 # License LGPL-3.0 or later (http://www.gnu.org/licenses/LGPL).
 
 import queue
+import threading
 from collections import defaultdict
 from contextlib import contextmanager
 from typing import Generator
@@ -16,18 +17,25 @@ def __init__(self):
         self._queue_by_db_by_root_path: dict[
             str, dict[str, queue.Queue[FastAPI]]
         ] = defaultdict(lambda: defaultdict(queue.Queue))
+        self._lock = threading.Lock()
 
-    def __get_app(self, env: Environment, root_path: str) -> FastAPI:
+    def __get_pool(self, env: Environment, root_path: str) -> queue.Queue[FastAPI]:
         db_name = env.cr.dbname
+        with self._lock:
+            # default dict is not thread safe but the use
+            return self._queue_by_db_by_root_path[db_name][root_path]
+
+    def __get_app(self, env: Environment, root_path: str) -> FastAPI:
+        pool = self.__get_pool(env, root_path)
         try:
-            return self._queue_by_db_by_root_path[db_name][root_path].get_nowait()
+            return pool.get_nowait()
         except queue.Empty:
             env["fastapi.endpoint"].sudo()
             return env["fastapi.endpoint"].sudo().get_app(root_path)
 
     def __return_app(self, env: Environment, app: FastAPI, root_path: str) -> None:
-        db_name = env.cr.dbname
-        self._queue_by_db_by_root_path[db_name][root_path].put(app)
+        pool = self.__get_pool(env, root_path)
+        pool.put(app)
 
     @contextmanager
     def get_app(