Get ephemeral port number after binding to port 0

[rgov]

Listening on port 0 will ask the system to bind on an unused port in the ephemeral port range. For example, I can run MediaMTX with rtspAddress: :0 which logs

2025/07/17 22:48:57 INF [RTSP] listener opened on :0 (TCP)

In reality the operating system assigned it port 60780:

% sudo lsof -nP -iTCP -sTCP:LISTEN
COMMAND     PID   USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
mediamtx  53024    rgov   4u  IPv6   0x4c4d05e23c32f8      0t0  TCP *:60780 (LISTEN)

I think in server_tcp_listener:initialize() after calling Server.Listen(), you would extract the port number like this:

// I don't write Go...
port := sl.ln.Addr().(*net.TCPAddr).Port

There probably should be an interface for exposing the port back to functions like printAddresses() rather than have them rummage around in srv.serverTCPListener.ln which is marked private.

Why? I'd like to run an ephemeral RTSP server as part of a unit test of a video streaming application. It wouldn't do to use a static port; it could be in use (consider a buildbot that is running tests on multiple branches at once). If I pick a random port, there's a chance of collision / race condition. Allowing the system to choose by setting port = 0 and parsing the log message is the robust solution.

[aler9]

Hello, in #934 i've added Server.NetListener() that allows to access the underlying net listener.

[rgov]

Nice! But is this completed yet? There's an accessor function but nothing calls it.

[aler9]

@rgov the accessor function is all that is needed for users to access the underlying net.Listener and consequently the ephemeral port and any other socket-related thing, what else do you need?

[rgov]

Mm yeah, maybe I haven't had my coffee yet. bluenviron/mediamtx needs to use it, not this project.