fix/mcp_session_auto_close_when_Mcpworkbench_deleted (#6497)

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## Why are these changes needed?
As-is: Deleting `McpWorkbench` does not close the `McpSession`.  
To-be: Deleting `McpWorkbench` now properly closes the `McpSession`.

<!-- Please give a short summary of the change and the problem this
solves. -->

## Related issue number

<!-- For example: "Closes #1234" -->

## Checks

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<https://microsoft.github.io/autogen/>. See
<https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md> to
build and test documentation locally.
- [x] I've added tests (if relevant) corresponding to the changes
introduced in this PR.
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python/packages/autogen-ext/src/autogen_ext/tools/mcp/_workbench.py

@@ -1,3 +1,4 @@
+import asyncio
 import builtins
 import warnings
 from typing import Any, List, Literal, Mapping
@@ -152,6 +153,7 @@ class McpWorkbench(Workbench, Component[McpWorkbenchConfig]):
         self._server_params = server_params
         # self._session: ClientSession | None = None
         self._actor: McpSessionActor | None = None
+        self._actor_loop: asyncio.AbstractEventLoop | None = None
         self._read = None
         self._write = None
 
@@ -253,6 +255,7 @@ class McpWorkbench(Workbench, Component[McpWorkbenchConfig]):
         if isinstance(self._server_params, (StdioServerParams, SseServerParams)):
             self._actor = McpSessionActor(self._server_params)
             await self._actor.initialize()
+            self._actor_loop = asyncio.get_event_loop()
         else:
             raise ValueError(f"Unsupported server params type: {type(self._server_params)}")
 
@@ -282,4 +285,10 @@ class McpWorkbench(Workbench, Component[McpWorkbenchConfig]):
 
     def __del__(self) -> None:
         # Ensure the actor is stopped when the workbench is deleted
-        pass
+        if self._actor and self._actor_loop:
+            loop = self._actor_loop
+            if loop.is_running() and not loop.is_closed():
+                loop.call_soon_threadsafe(lambda: asyncio.create_task(self.stop()))
+            else:
+                msg = "Cannot safely stop actor at [McpWorkbench.__del__]: loop is closed or not running"
+                warnings.warn(msg, RuntimeWarning, stacklevel=2)

python/packages/autogen-ext/tests/tools/test_mcp_tools.py

@@ -1,12 +1,17 @@
+import asyncio
 import logging
 import os
+import threading
+from typing import cast
 from unittest.mock import AsyncMock, MagicMock
 
 import pytest
+from _pytest.logging import LogCaptureFixture  # type: ignore[import]
 from autogen_core import CancellationToken
 from autogen_core.tools import Workbench
 from autogen_core.utils import schema_to_pydantic_model
 from autogen_ext.tools.mcp import (
+    McpSessionActor,
     McpWorkbench,
     SseMcpToolAdapter,
     SseServerParams,
@@ -594,4 +599,54 @@ async def test_lazy_init_and_finalize_cleanup() -> None:
     assert workbench._actor is not None  # type: ignore[reportPrivateUsage]
     assert workbench._actor._active is True  # type: ignore[reportPrivateUsage]
 
+    actor = workbench._actor  # type: ignore[reportPrivateUsage]
     del workbench
+    await asyncio.sleep(0.1)
+    assert actor._active is False
+
+
+@pytest.mark.asyncio
+async def test_del_to_new_event_loop_when_get_event_loop_fails() -> None:
+    params = StdioServerParams(
+        command="npx",
+        args=[
+            "-y",
+            "@modelcontextprotocol/server-filesystem",
+            ".",
+        ],
+        read_timeout_seconds=60,
+    )
+    workbench = McpWorkbench(server_params=params)
+
+    await workbench.list_tools()
+    assert workbench._actor is not None  # type: ignore[reportPrivateUsage]
+    assert workbench._actor._active is True  # type: ignore[reportPrivateUsage]
+
+    actor = workbench._actor  # type: ignore[reportPrivateUsage]
+
+    def cleanup() -> None:
+        nonlocal workbench
+        del workbench
+
+    t = threading.Thread(target=cleanup)
+    t.start()
+    t.join()
+
+    await asyncio.sleep(0.1)
+    assert actor._active is False  # type: ignore[reportPrivateUsage]
+
+
+def test_del_raises_when_loop_closed() -> None:
+    loop = asyncio.new_event_loop()
+    asyncio.set_event_loop(loop)
+
+    params = StdioServerParams(command="echo", args=["ok"])
+    workbench = McpWorkbench(server_params=params)
+
+    workbench._actor_loop = loop  # type: ignore[reportPrivateUsage]
+    workbench._actor = cast(McpSessionActor, object())  # type: ignore[reportPrivateUsage]
+
+    loop.close()
+
+    with pytest.warns(RuntimeWarning, match="loop is closed or not running"):
+        del workbench