Fix stdio cleanup hanging on slow-terminating processes

The stdio cleanup was hanging indefinitely when processes ignored
termination signals or took too long to exit. This caused the MCP
client to freeze during shutdown, especially with servers that don't
handle SIGTERM properly.

The fix introduces a 2-second timeout for process termination. If a
process doesn't exit gracefully within this window, it's forcefully
killed. This ensures the client always completes cleanup in bounded
time while still giving well-behaved servers a chance to exit cleanly.

This resolves hanging issues reported when MCP servers ignore standard
termination signals or perform lengthy cleanup operations.

Also adds regression tests for #559.

resolves #555

Author: felixweinberger

src/mcp/client/stdio/__init__.py

@@ -16,7 +16,6 @@
 from .win32 import (
     create_windows_process,
     get_windows_executable_command,
-    terminate_windows_process,
 )
 
 # Environment variables to inherit by default
@@ -180,10 +179,12 @@ async def stdin_writer():
         finally:
             # Clean up process to prevent any dangling orphaned processes
             try:
-                if sys.platform == "win32":
-                    await terminate_windows_process(process)
-                else:
-                    process.terminate()
+                process.terminate()
+                with anyio.fail_after(2.0):
+                    await process.wait()
+            except TimeoutError:
+                # If process doesn't terminate in time, force kill it
+                process.kill()
             except ProcessLookupError:
                 # Process already exited, which is fine
                 pass

src/mcp/client/stdio/win32.py

@@ -8,9 +8,7 @@
 from pathlib import Path
 from typing import BinaryIO, TextIO, cast
 
-import anyio
 from anyio import to_thread
-from anyio.abc import Process
 from anyio.streams.file import FileReadStream, FileWriteStream
 
 
@@ -159,24 +157,3 @@ async def create_windows_process(
             bufsize=0,
         )
         return FallbackProcess(popen_obj)
-
-
-async def terminate_windows_process(process: Process | FallbackProcess):
-    """
-    Terminate a Windows process.
-
-    Note: On Windows, terminating a process with process.terminate() doesn't
-    always guarantee immediate process termination.
-    So we give it 2s to exit, or we call process.kill()
-    which sends a SIGKILL equivalent signal.
-
-    Args:
-        process: The process to terminate
-    """
-    try:
-        process.terminate()
-        with anyio.fail_after(2.0):
-            await process.wait()
-    except TimeoutError:
-        # Force kill if it doesn't terminate
-        process.kill()

tests/client/test_stdio.py

@@ -1,5 +1,9 @@
 import shutil
+import sys
+import textwrap
+import time
 
+import anyio
 import pytest
 
 from mcp.client.session import ClientSession
@@ -90,3 +94,121 @@ async def test_stdio_client_nonexistent_command():
         or "not found" in error_message.lower()
         or "cannot find the file" in error_message.lower()  # Windows error message
     )
+
+
+@pytest.mark.anyio
+async def test_stdio_client_universal_cleanup():
+    """
+    Test that stdio_client completes cleanup within reasonable time
+    even when connected to processes that exit slowly.
+    """
+
+    # Use a Python script that simulates a long-running process
+    # This ensures consistent behavior across platforms
+    long_running_script = textwrap.dedent(
+        """
+        import time
+        import sys
+        
+        # Simulate a long-running process
+        for i in range(100):
+            time.sleep(0.1)
+            # Flush to ensure output is visible
+            sys.stdout.flush()
+            sys.stderr.flush()
+        """
+    )
+
+    server_params = StdioServerParameters(
+        command=sys.executable,
+        args=["-c", long_running_script],
+    )
+
+    start_time = time.time()
+
+    with anyio.move_on_after(8.0) as cancel_scope:
+        async with stdio_client(server_params) as (read_stream, write_stream):
+            # Immediately exit - this triggers cleanup while process is still running
+            pass
+
+        end_time = time.time()
+        elapsed = end_time - start_time
+
+        # On Windows: 2s (stdin wait) + 2s (terminate wait) + overhead = ~5s expected
+        assert elapsed < 6.0, (
+            f"stdio_client cleanup took {elapsed:.1f} seconds, expected < 6.0 seconds. "
+            f"This suggests the timeout mechanism may not be working properly."
+        )
+
+    # Check if we timed out
+    if cancel_scope.cancelled_caught:
+        pytest.fail(
+            "stdio_client cleanup timed out after 8.0 seconds. "
+            "This indicates the cleanup mechanism is hanging and needs fixing."
+        )
+
+
+@pytest.mark.anyio
+@pytest.mark.skipif(sys.platform == "win32", reason="Windows signal handling is different")
+async def test_stdio_client_sigint_only_process():
+    """
+    Test cleanup with a process that ignores SIGTERM but responds to SIGINT.
+    """
+    # Create a Python script that ignores SIGTERM but handles SIGINT
+    script_content = textwrap.dedent(
+        """
+        import signal
+        import sys
+        import time
+
+        # Ignore SIGTERM (what process.terminate() sends)
+        signal.signal(signal.SIGTERM, signal.SIG_IGN)
+
+        # Handle SIGINT (Ctrl+C signal) by exiting cleanly
+        def sigint_handler(signum, frame):
+            sys.exit(0)
+
+        signal.signal(signal.SIGINT, sigint_handler)
+
+        # Keep running until SIGINT received
+        while True:
+            time.sleep(0.1)
+        """
+    )
+
+    server_params = StdioServerParameters(
+        command=sys.executable,
+        args=["-c", script_content],
+    )
+
+    start_time = time.time()
+
+    try:
+        # Use anyio timeout to prevent test from hanging forever
+        with anyio.move_on_after(5.0) as cancel_scope:
+            async with stdio_client(server_params) as (read_stream, write_stream):
+                # Let the process start and begin ignoring SIGTERM
+                await anyio.sleep(0.5)
+                # Exit context triggers cleanup - this should not hang
+                pass
+
+        if cancel_scope.cancelled_caught:
+            raise TimeoutError("Test timed out")
+
+        end_time = time.time()
+        elapsed = end_time - start_time
+
+        # Should complete quickly even with SIGTERM-ignoring process
+        # This will fail if cleanup only uses process.terminate() without fallback
+        assert elapsed < 5.0, (
+            f"stdio_client cleanup took {elapsed:.1f} seconds with SIGTERM-ignoring process. "
+            f"Expected < 5.0 seconds. This suggests the cleanup needs SIGINT/SIGKILL fallback."
+        )
+    except (TimeoutError, Exception) as e:
+        if isinstance(e, TimeoutError) or "timed out" in str(e):
+            pytest.fail(
+                "stdio_client cleanup timed out after 5.0 seconds with SIGTERM-ignoring process. "
+                "This confirms the cleanup needs SIGINT/SIGKILL fallback for processes that ignore SIGTERM."
+            )
+        else:
+            raise