Unify process termination on POSIX & Windows (+ tests) (#1044)

Co-authored-by: Cristian Pufu <cristian.pufu@uipath.com>

src/mcp/client/stdio/__init__.py

@@ -16,7 +16,6 @@ from mcp.shared.message import SessionMessage
 from .win32 import (
     create_windows_process,
     get_windows_executable_command,
-    terminate_windows_process,
 )
 
 # Environment variables to inherit by default
@@ -38,6 +37,9 @@ DEFAULT_INHERITED_ENV_VARS = (
     else ["HOME", "LOGNAME", "PATH", "SHELL", "TERM", "USER"]
 )
 
+# Timeout for process termination before falling back to force kill
+PROCESS_TERMINATION_TIMEOUT = 2.0
+
 
 def get_default_environment() -> dict[str, str]:
     """
@@ -180,10 +182,12 @@ async def stdio_client(server: StdioServerParameters, errlog: TextIO = sys.stder
         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(PROCESS_TERMINATION_TIMEOUT):
+                    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

@@ -12,6 +12,7 @@ import anyio
 from anyio import to_thread
 from anyio.abc import Process
 from anyio.streams.file import FileReadStream, FileWriteStream
+from typing_extensions import deprecated
 
 
 def get_windows_executable_command(command: str) -> str:
@@ -115,13 +116,14 @@ async def create_windows_process(
     env: dict[str, str] | None = None,
     errlog: TextIO | None = sys.stderr,
     cwd: Path | str | None = None,
-) -> FallbackProcess:
+) -> Process | FallbackProcess:
     """
     Creates a subprocess in a Windows-compatible way.
 
-    On Windows, asyncio.create_subprocess_exec has incomplete support
-    (NotImplementedError when trying to open subprocesses).
-    Therefore, we fallback to subprocess.Popen and wrap it for async usage.
+    Attempt to use anyio's open_process for async subprocess creation.
+    In some cases this will throw NotImplementedError on Windows, e.g.
+    when using the SelectorEventLoop which does not support async subprocesses.
+    In that case, we fall back to using subprocess.Popen.
 
     Args:
         command (str): The executable to run
@@ -133,6 +135,45 @@ async def create_windows_process(
     Returns:
         FallbackProcess: Async-compatible subprocess with stdin and stdout streams
     """
+    try:
+        # First try using anyio with Windows-specific flags to hide console window
+        process = await anyio.open_process(
+            [command, *args],
+            env=env,
+            # Ensure we don't create console windows for each process
+            creationflags=subprocess.CREATE_NO_WINDOW  # type: ignore
+            if hasattr(subprocess, "CREATE_NO_WINDOW")
+            else 0,
+            stderr=errlog,
+            cwd=cwd,
+        )
+        return process
+    except NotImplementedError:
+        # Windows often doesn't support async subprocess creation, use fallback
+        return await _create_windows_fallback_process(command, args, env, errlog, cwd)
+    except Exception:
+        # Try again without creation flags
+        process = await anyio.open_process(
+            [command, *args],
+            env=env,
+            stderr=errlog,
+            cwd=cwd,
+        )
+        return process
+
+
+async def _create_windows_fallback_process(
+    command: str,
+    args: list[str],
+    env: dict[str, str] | None = None,
+    errlog: TextIO | None = sys.stderr,
+    cwd: Path | str | None = None,
+) -> FallbackProcess:
+    """
+    Create a subprocess using subprocess.Popen as a fallback when anyio fails.
+
+    This function wraps the sync subprocess.Popen in an async-compatible interface.
+    """
     try:
         # Try launching with creationflags to avoid opening a new console window
         popen_obj = subprocess.Popen(
@@ -161,6 +202,10 @@ async def create_windows_process(
         return FallbackProcess(popen_obj)
 
 
+@deprecated(
+    "terminate_windows_process is deprecated and will be removed in a future version. "
+    "Process termination is now handled internally by the stdio_client context manager."
+)
 async def terminate_windows_process(process: Process | FallbackProcess):
     """
     Terminate a Windows process.