feat: add --socket-fd CLI option

[runleveldev]

Fixes #7939

[code-asher]

Very cool thank you!

[code-asher]

Actually, had one thought. Would it make sense to combine the flags? They seem so similar apart from the unlink behavior.

What do you think about reusing --socket, but if it is a number (parseInt() returns non-NaN) then we assume it is an fd, and we skip the unlink and mode change? Or does that overcomplicate the flag without any real benefit?

Do you know of any precedent from other existing applications?

[runleveldev]

Actually, had one thought. Would it make sense to combine the flags? They seem so similar apart from the unlink behavior.

What do you think about reusing --socket, but if it is a number (parseInt() returns non-NaN) then we assume it is an fd, and we skip the unlink and mode change? Or does that overcomplicate the flag without any real benefit?

Do you know of any precedent from other existing applications?

I was originally referencing dockerd but it uses a single "host" flag with schemes i.e. tcp://127.0.0.1:1234, unix:///path/to/sock, or fd://3. Obviously that would have been a more-jarring rewrite here. I did toy with combining this with the existing socket option, but I prefer to keep them explicit to avoid shadowing unlikely, but technically valid, paths (like 3 which currently would make a socket file in $PWD/3).