Fix #1933: Prevent closing real stdio when server exits

When using transport="stdio", the server was closing sys.stdin.buffer
and sys.stdout.buffer when exiting, causing subsequent stdio operations
to fail with ValueError: I/O operation on closed file.

This fix uses os.dup() to create duplicate file descriptors, so the
original stdin/stdout remain open when the wrapper streams are closed.

Fixes #1933

Author: 717986230

src/mcp/server/stdio.py

@@ -1,22 +1,22 @@
 """Stdio Server Transport Module
 
 This module provides functionality for creating an stdio-based transport layer
-that can be used to communicate with an MCP client through standard input/output
-streams.
+that can be used to communicate with an MCP client through standard input/output streams.
 
 Example:
-    ```python
-    async def run_server():
-        async with stdio_server() as (read_stream, write_stream):
-            # read_stream contains incoming JSONRPCMessages from stdin
-            # write_stream allows sending JSONRPCMessages to stdout
-            server = await create_my_server()
-            await server.run(read_stream, write_stream, init_options)
+```python
+async def run_server():
+    async with stdio_server() as (read_stream, write_stream):
+        # read_stream contains incoming JSONRPCMessages from stdin
+        # write_stream allows sending JSONRPCMessages to stdout
+        server = await create_my_server()
+        await server.run(read_stream, write_stream, init_options)
 
-    anyio.run(run_server)
-    ```
+anyio.run(run_server)
+```
 """
 
+import os
 import sys
 from contextlib import asynccontextmanager
 from io import TextIOWrapper
@@ -38,10 +38,18 @@ async def stdio_server(stdin: anyio.AsyncFile[str] | None = None, stdout: anyio.
     # standard process handles. Encoding of stdin/stdout as text streams on
     # python is platform-dependent (Windows is particularly problematic), so we
     # re-wrap the underlying binary stream to ensure UTF-8.
+    #
+    # Fix #1933: Use os.dup() to avoid closing the original stdin/stdout
+    # when the wrapper is closed, preventing "I/O operation on closed file" errors.
     if not stdin:
-        stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8", errors="replace"))
+        stdin_fd = os.dup(sys.stdin.fileno())
+        stdin_bin = os.fdopen(stdin_fd, "rb", closefd=True)
+        stdin = anyio.wrap_file(TextIOWrapper(stdin_bin, encoding="utf-8", errors="replace"))
+    
     if not stdout:
-        stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))
+        stdout_fd = os.dup(sys.stdout.fileno())
+        stdout_bin = os.fdopen(stdout_fd, "wb", closefd=True)
+        stdout = anyio.wrap_file(TextIOWrapper(stdout_bin, encoding="utf-8"))
 
     read_stream_writer, read_stream = create_context_streams[SessionMessage | Exception](0)
     write_stream, write_stream_reader = create_context_streams[SessionMessage](0)