`asyncio` does not set `TCP_NODELAY` on macOS

Bug report

Bug description:

On macOS, asyncio server silently skips setting TCP_NODELAY on accepted connections.

Reproduce

import asyncio
import socket

async def main():
    async def on_connect(reader, writer):
        sock = writer.transport.get_extra_info('socket')
        nodelay = sock.getsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY)
        print(f'server side: sock.proto={sock.proto} TCP_NODELAY={nodelay}')
        writer.close()
        await writer.wait_closed()
        srv.close()

    srv = await asyncio.start_server(on_connect, '127.0.0.1', 0)
    port = srv.sockets[0].getsockname()[1]

    reader, writer = await asyncio.open_connection('127.0.0.1', port)
    sock = writer.transport.get_extra_info('socket')
    nodelay = sock.getsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY)
    print(f'client side: sock.proto={sock.proto} TCP_NODELAY={nodelay}')

    await asyncio.sleep(0.1)
    writer.close()
    await writer.wait_closed()
    await srv.wait_closed()

asyncio.run(main())

Cause

In socket.__init__, CPython tries to detect proto with getsockopt + SO_PROTOCOL and falls back to 0 if SO_PROTOCOL is not supported, which is the case on macOS.

https://github.com/python/cpython/blob/82767780f8de2fc492567ceb6a590101ae3b19ad/Modules/socketmodule.c#L5749-L5764

While socket.accept does pass in the proto from listening socket, it is silently overridden to zero by the above logic.

https://github.com/python/cpython/blob/82767780f8de2fc492567ceb6a590101ae3b19ad/Lib/socket.py#L300

asyncio sets TCP_NODELAY only when sock.proto == socket.IPPROTO_TCP, but it is 0 on macOS for the above reason.

https://github.com/python/cpython/blob/82767780f8de2fc492567ceb6a590101ae3b19ad/Lib/asyncio/base_events.py#L193-L201

Client socket works fine because no fd is passed to socket.__init__ so the proto detection doesn't happen.

CPython versions tested on:

3.13

Operating systems tested on:

macOS, Linux

Linked PRs

• gh-148784

Xem xét phần triển khai được liên kết trong Modules/socketmodule.c, Lib/socket.py và Lib/asyncio/base_events.py, bắt đầu với socket.init và socket.accept. Kiểm tra PR gh-148784 trước khi bắt đầu; công việc được xem là hoàn tất khi các kết nối được chấp nhận trên macOS nhận được thông tin giao thức TCP như mong đợi và asyncio thiết lập TCP_NODELAY nhất quán với các kết nối máy khách.