fix: Fix pickling lock errors in McpSessionManager

When we added the session_lock_map, it resulted in pickling errors during Agent Engine deployment. To fix, we implemented custom getstate and setstate methods to exclude the lock map lock and session lock map from pickling. Closes https://github.com/google/adk-python/issues/4486.

Co-authored-by: Kathy Wu <wukathy@google.com>
PiperOrigin-RevId: 871056554

src/google/adk/tools/mcp_tool/mcp_session_manager.py

@@ -500,6 +500,30 @@ class MCPSessionManager:
             )
         raise ConnectionError(f'Failed to create MCP session: {e}') from e
 
+  def __getstate__(self):
+    """Custom pickling to exclude non-picklable runtime objects."""
+    state = self.__dict__.copy()
+    # Remove unpicklable entries or those that shouldn't persist across pickle
+    state['_sessions'] = {}
+    state['_session_lock_map'] = {}
+
+    # Locks and file-like objects cannot be pickled
+    state.pop('_lock_map_lock', None)
+    state.pop('_errlog', None)
+
+    return state
+
+  def __setstate__(self, state):
+    """Custom unpickling to restore state."""
+    self.__dict__.update(state)
+    # Re-initialize members that were not pickled
+    self._sessions = {}
+    self._session_lock_map = {}
+    self._lock_map_lock = threading.Lock()
+    # If _errlog was removed during pickling, default to sys.stderr
+    if not hasattr(self, '_errlog') or self._errlog is None:
+      self._errlog = sys.stderr
+
   async def close(self):
     """Closes all sessions and cleans up resources."""
     async with self._session_lock:

tests/unittests/tools/mcp_tool/test_mcp_session_manager.py

@@ -607,6 +607,39 @@ class TestMCPSessionManager:
     exit_stack2.aclose.assert_not_called()
     assert len(manager._sessions) == 0
 
+  @pytest.mark.asyncio
+  async def test_pickle_mcp_session_manager(self):
+    """Verify that MCPSessionManager can be pickled and unpickled."""
+    import pickle
+
+    manager = MCPSessionManager(self.mock_stdio_connection_params)
+
+    # Access the lock to ensure it's initialized
+    lock = manager._session_lock
+    assert isinstance(lock, asyncio.Lock)
+
+    # Add a mock session to verify it's cleared on pickling
+    manager._sessions["test"] = (Mock(), Mock(), asyncio.get_running_loop())
+
+    # Pickle and unpickle
+    pickled = pickle.dumps(manager)
+    unpickled = pickle.loads(pickled)
+
+    # Verify basics are restored
+    assert unpickled._connection_params == manager._connection_params
+
+    # Verify transient/unpicklable members are re-initialized or cleared
+    assert unpickled._sessions == {}
+    assert unpickled._session_lock_map == {}
+    assert isinstance(unpickled._lock_map_lock, type(manager._lock_map_lock))
+    assert unpickled._lock_map_lock is not manager._lock_map_lock
+    assert unpickled._errlog == sys.stderr
+
+    # Verify we can still get a lock in the new instance
+    new_lock = unpickled._session_lock
+    assert isinstance(new_lock, asyncio.Lock)
+    assert new_lock is not lock
+
 
 @pytest.mark.asyncio
 async def test_retry_on_errors_decorator():