feat: Make InMemoryMemoryService thread-safe

Even though InMemoryMemoryService is intended only for testing and local development, we eliminate a potential source of bugs during prototyping by providing a thread-safe InMemoryMemoryService.

PiperOrigin-RevId: 781554006

src/google/adk/memory/in_memory_memory_service.py

@@ -14,6 +14,7 @@
 from __future__ import annotations
 
 import re
+import threading
 from typing import TYPE_CHECKING
 
 from typing_extensions import override
@@ -42,20 +43,24 @@ class InMemoryMemoryService(BaseMemoryService):
 
   Uses keyword matching instead of semantic search.
 
-  It is not suitable for multi-threaded production environments. Use it for
-  testing and development only.
+  This class is thread-safe, however, it should be used for testing and
+  development only.
   """
 
   def __init__(self):
+    self._lock = threading.Lock()
+
     self._session_events: dict[str, dict[str, list[Event]]] = {}
-    """Keys are app_name/user_id, session_id. Values are session event lists."""
+    """Keys are "{app_name}/{user_id}". Values are dicts of session_id to
+    session event lists.
+    """
 
   @override
   async def add_session_to_memory(self, session: Session):
     user_key = _user_key(session.app_name, session.user_id)
-    self._session_events[user_key] = self._session_events.get(
-        _user_key(session.app_name, session.user_id), {}
-    )
+
+    with self._lock:
+      self._session_events[user_key] = self._session_events.get(user_key, {})
       self._session_events[user_key][session.id] = [
           event
           for event in session.events
@@ -67,13 +72,14 @@ class InMemoryMemoryService(BaseMemoryService):
       self, *, app_name: str, user_id: str, query: str
   ) -> SearchMemoryResponse:
     user_key = _user_key(app_name, user_id)
-    if user_key not in self._session_events:
-      return SearchMemoryResponse()
+
+    with self._lock:
+      session_event_lists = self._session_events.get(user_key, {})
 
     words_in_query = set(query.lower().split())
     response = SearchMemoryResponse()
 
-    for session_events in self._session_events[user_key].values():
+    for session_events in session_event_lists.values():
       for event in session_events:
         if not event.content or not event.content.parts:
           continue