fix: avoid stdio cleanup BrokenResourceError race

Author: lavish0000

src/mcp/client/stdio.py

@@ -205,10 +205,13 @@ async def stdin_writer():
             except ProcessLookupError:  # pragma: no cover
                 # Process already exited, which is fine
                 pass
-            await read_stream.aclose()
-            await write_stream.aclose()
-            await read_stream_writer.aclose()
-            await write_stream_reader.aclose()
+            # Stop background stream tasks before closing the memory streams they use.
+            tg.cancel_scope.cancel()
+
+    await read_stream.aclose()
+    await write_stream.aclose()
+    await read_stream_writer.aclose()
+    await write_stream_reader.aclose()
 
 
 def _get_executable_command(command: str) -> str:

tests/client/test_stdio.py

@@ -155,6 +155,35 @@ async def test_stdio_client_universal_cleanup():
         )
 
 
+@pytest.mark.anyio
+async def test_stdio_client_cleanup_cancels_backpressured_stdout_reader():
+    """Regression test for issue #1960.
+
+    Exiting the client without consuming the read stream leaves stdout_reader
+    blocked on a zero-buffer send. Cleanup must cancel the task before closing
+    its memory stream.
+    """
+    script_content = textwrap.dedent(
+        """
+        import sys
+        import time
+
+        sys.stdout.write('{"jsonrpc":"2.0","id":1,"result":{}}\\n')
+        sys.stdout.flush()
+        time.sleep(2.0)
+        """
+    )
+
+    server_params = StdioServerParameters(
+        command=sys.executable,
+        args=["-c", script_content],
+    )
+
+    with anyio.fail_after(5.0):
+        async with stdio_client(server_params) as (_, _):
+            await anyio.sleep(0.2)
+
+
 @pytest.mark.anyio
 @pytest.mark.skipif(sys.platform == "win32", reason="Windows signal handling is different")
 async def test_stdio_client_sigint_only_process():  # pragma: lax no cover