`asyncio` does not set `TCP_NODELAY` on macOS

[SEIAROTg]

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())

Platform	Side	sock.proto	TCP_NODELAY
Linux	client	6 ✅	1 ✅
Linux	server	6 ✅	1 ✅
macOS	client	6 ✅	4 ✅
macOS	server	0 ❌	0 ❌

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.

cpython/Modules/socketmodule.c

Lines 5749 to 5764 in 8276778

	#ifdef SO_PROTOCOL
	if (proto == -1) {
	int tmp;
	socklen_t slen = sizeof(tmp);
	if (getsockopt(fd, SOL_SOCKET, SO_PROTOCOL,
	(void *)&tmp, &slen) == 0)
	{
	proto = tmp;
	} else {
	set_error();
	return -1;
	}
	}
	#else
	proto = 0;
	#endif

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

cpython/Lib/socket.py

Line 300 in 8276778

sock = socket(self.family, self.type, self.proto, fileno=fd)

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

cpython/Lib/asyncio/base_events.py

Lines 193 to 201 in 8276778

	if hasattr(socket, 'TCP_NODELAY'):
	def _set_nodelay(sock):
	if (sock.family in {socket.AF_INET, socket.AF_INET6} and
	sock.type == socket.SOCK_STREAM and
	sock.proto == socket.IPPROTO_TCP):
	sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)
	else:
	def _set_nodelay(sock):
	pass

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-148783: Respect caller-specified socket proto #148784