Enhance TCP transport: library should respect a port of 0

[ralexstokes]

A port of 0 when using the TCP transport for listening should defer to the operating system to provide a free port.

We should provide some way for a user of this library to do so and then expose the allocated port in a straightforward manner.

[AkMo3]

This issue can be closed safely now as we using trio to open tcp listening socket:

py-libp2p/libp2p/transport/tcp/tcp.py

Lines 55 to 64 in 5ca6f26

	async def serve_tcp(
	handler: Callable[[trio.SocketStream], Awaitable[None]],
	port: int,
	host: str,
	task_status: TaskStatus[Sequence[trio.SocketListener]] = None,
	) -> None:
	"""Just a proxy function to add logging here."""
	logger.debug("serve_tcp %s %s", host, port)
	await trio.serve_tcp(handler, port, host=host, task_status=task_status)

And as per trio documentation:

port – The port to listen on. Use 0 to let the kernel pick an open port. Passed to [open_tcp_listeners()](https://trio.readthedocs.io/en/stable/reference-io.html#trio.open_tcp_listeners).

We see this is practice in our chat example if we pass 0 as port and print the host multiaddrs, we find one of the random port being assigned automatically.

$ python3 main.py -p 0
Run this from the same folder in another console:

chat-demo -p 1 -d /ip4/0.0.0.0/tcp/45041/p2p/QmRXLGbyiCKHjwiZSE7tk6cLTdn2zUEVM4VD799AUmz7YG
Waiting for incoming connection...

Hence we can safely close this @seetadev

[seetadev]

@AkMo3 : Awesome work, Akash. Thank you so much for figuring this out! 🎉

Thanks for also sharing the detailed explanation and code examples - it's really helpful to see how trio's serve_tcp with port=0 handles the automatic port assignment. The chat demo output you shared makes it crystal clear how this works in practice.

Really appreciate you taking the time to track this down and document the solution. The trio documentation reference is perfect context too.

Going ahead and closing this issue - thanks again for the thorough investigation! 👍