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

Beginne mit asyncio.start_unix_server und create_unix_server und konzentriere dich darauf, wie sie sich verhalten, wenn sock übergeben wird, und wie cleanup_socket behandelt wird. Prüfe die im Issue verlinkte asyncio-Stream-Dokumentation und die bestehende Kommentardiskussion. Die Arbeit ist abgeschlossen, wenn das Verhalten für vorhandene Sockets und die Socket-Übergabe durch systemd geklärt ist und die fehlende Dokumentation für cleanup_socket ergänzt wurde.