Fix ResourceWarning by properly cleaning up subprocess objects

After killing processes with taskkill on Windows, we need to call
wait() on the Popen object to let it know the process is dead and
clean up its resources. This prevents the ResourceWarning.

This is the proper fix instead of suppressing the warning.

Author: felixweinberger

tests/client/test_stdio.py

@@ -365,7 +365,6 @@ def sigterm_handler(signum, frame):
 
 
 @pytest.mark.anyio
-@pytest.mark.filterwarnings("ignore::ResourceWarning")
 async def test_stdio_client_child_process_cleanup():
     """
     Test that child processes are properly terminated when the parent is killed.
@@ -444,6 +443,11 @@ async def test_stdio_client_child_process_cleanup():
         from mcp.client.stdio import _terminate_process_with_children
 
         await _terminate_process_with_children(proc)
+        
+        # Ensure the process object is properly cleaned up
+        if sys.platform == "win32":
+            # Wait for the process to be reaped to avoid ResourceWarning
+            proc._popen.wait()
 
         # Verify processes stopped
         await anyio.sleep(0.5)
@@ -469,7 +473,6 @@ async def test_stdio_client_child_process_cleanup():
 
 
 @pytest.mark.anyio
-@pytest.mark.filterwarnings("ignore::ResourceWarning")
 async def test_stdio_client_nested_process_tree():
     """
     Test that a nested process tree (parent → child → grandchild) is properly cleaned up.
@@ -554,6 +557,11 @@ async def test_stdio_client_nested_process_tree():
         from mcp.client.stdio import _terminate_process_with_children
 
         await _terminate_process_with_children(proc)
+        
+        # Ensure the process object is properly cleaned up
+        if sys.platform == "win32":
+            # Wait for the process to be reaped to avoid ResourceWarning
+            proc._popen.wait()
 
         # Verify all stopped
         await anyio.sleep(0.5)
@@ -576,7 +584,6 @@ async def test_stdio_client_nested_process_tree():
 
 
 @pytest.mark.anyio
-@pytest.mark.filterwarnings("ignore::ResourceWarning")
 async def test_stdio_client_early_parent_exit():
     """
     Test that child processes are cleaned up when parent exits during cleanup.
@@ -644,6 +651,11 @@ def handle_term(sig, frame):
         from mcp.client.stdio import _terminate_process_with_children
 
         await _terminate_process_with_children(proc)
+        
+        # Ensure the process object is properly cleaned up
+        if sys.platform == "win32":
+            # Wait for the process to be reaped to avoid ResourceWarning
+            proc._popen.wait()
 
         # Verify child stopped
         await anyio.sleep(0.5)