asyncio.start_unix_server maybe shouldn’t default to cleanup_socket=True when sock parameter is passed

Bug report

Bug description:

When asyncio.start_unix_server is called and the sock parameter is passed (rather than path), a server is wrapped around an existing socket. In this case, the fact that cleanup_socket defaults to True is very weird and potentially broken: it means that asyncio will try to delete a socket that someone else created (either other code in the same process, or, in the case of e.g. systemd socket passing, a socket created by a different program).

This is problematic for a couple of reasons when using systemd socket passing:

• If the server succeeds at deleting the socket, it breaks socket activation. Socket activation means systemd creates the socket, and when the first connection arrives, it starts the server. The way it’s intended to work is that, if the server terminates, it leaves the socket in place, and systemd will start the server again if another connection comes in, allowing virtually seamless service restarts. If the server deletes the listening socket, this breaks.
• If systemd passes the listening socket to the server, the server may not even have filesystem permission to see the socket—after all, only clients should need that. Then the os.stat call in create_unix_server blows up.

So this is definitely a nontrivial backwards compatibility break from 3.12 (which didn’t have cleanup logic at all and didn’t have the parameter) to 3.13, despite not being mentioned in the release notes. It also seems like a bit of a footgun in general, not to mention the fact that in implementation start_unix_server takes arbitrary kwargs and forwards them to create_unix_server, but the documentation instead mentions each parameter explicitly and doesn’t mention cleanup_socket at all.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Bắt đầu với asyncio.start_unix_server và create_unix_server, tập trung vào hành vi khi sock được cung cấp và cách cleanup_socket được xử lý. Xem lại tài liệu về stream của asyncio được liên kết trong issue và phần thảo luận hiện có trong các bình luận. Công việc được xem là hoàn tất khi hành vi của socket hiện có và việc truyền socket của systemd được giải quyết, đồng thời phần tài liệu còn thiếu về cleanup_socket được bổ sung.