close wrappers on exit to fix ResourceWarning

Author: adityuhkapoor

src/mcp/server/stdio.py

@@ -35,12 +35,15 @@ async def stdio_server(stdin: anyio.AsyncFile[str] | None = None, stdout: anyio.
     from the current process' stdin and writing to stdout.
     """
     # Re-wrap the `fd` with `closefd=False` to force UTF-8 (Windows encoding is
-    # platform-dependant) without taking ownership of process stdio.
+    # platform-dependent) without taking ownership of process stdio.
+    stdin_opened = stdout_opened = False
     if not stdin:
         stdin = anyio.wrap_file(TextIOWrapper(open(sys.stdin.fileno(), "rb", closefd=False), encoding="utf-8"))
+        stdin_opened = True
     if not stdout:
         stdout = anyio.wrap_file(TextIOWrapper(open(sys.stdout.fileno(), "wb", closefd=False), encoding="utf-8"))
-        
+        stdout_opened = True
+
     read_stream: MemoryObjectReceiveStream[SessionMessage | Exception]
     read_stream_writer: MemoryObjectSendStream[SessionMessage | Exception]
 
@@ -75,7 +78,13 @@ async def stdout_writer():
         except anyio.ClosedResourceError:  # pragma: no cover
             await anyio.lowlevel.checkpoint()
 
-    async with anyio.create_task_group() as tg:
-        tg.start_soon(stdin_reader)
-        tg.start_soon(stdout_writer)
-        yield read_stream, write_stream
+    try:
+        async with anyio.create_task_group() as tg:
+            tg.start_soon(stdin_reader)
+            tg.start_soon(stdout_writer)
+            yield read_stream, write_stream
+    finally:
+        if stdin_opened:
+            await stdin.aclose()
+        if stdout_opened:
+            await stdout.aclose()

tests/issues/test_1933_stdio_close.py

@@ -1,6 +1,5 @@
 """Test for issue #1933: stdio_server closes real process stdio handles."""
 
-import gc
 import io
 import os
 import sys
@@ -12,12 +11,7 @@
 
 @pytest.mark.anyio
 async def test_stdio_server_preserves_process_handles():
-    """After stdio_server() exits, the underlying stdin/stdout fds should still be open.
-
-    Before the fix, TextIOWrapper took ownership of sys.stdin.buffer and
-    sys.stdout.buffer. When the wrapper was garbage-collected, it closed the
-    underlying buffer, permanently killing process stdio.
-    """
+    """After stdio_server() exits, the underlying stdin/stdout fds should still be open."""
     # Create real pipes to stand in for process stdin/stdout.
     # Real fds are required because the bug involves TextIOWrapper closing
     # the underlying fd — StringIO doesn't have file descriptors.
@@ -39,14 +33,15 @@ async def test_stdio_server_preserves_process_handles():
             await write_stream.aclose()
 
         await read_stream.aclose()
-        gc.collect()
 
         # os.fstat raises OSError if the fd was closed
         os.fstat(stdin_r_fd)
         os.fstat(stdout_w_fd)
     finally:
         sys.stdin = saved_stdin
         sys.stdout = saved_stdout
+        fake_stdin.close()
+        fake_stdout.close()
         for fd in [stdin_r_fd, stdout_r_fd, stdout_w_fd]:
             try:
                 os.close(fd)