Add socket-mark-id support for marking sockets.

[devnexen]

Add a configuration option to attach an operating system-specific identifier to Redis sockets, supporting advanced network configurations using iptables (Linux) or ipfw (FreeBSD).

[yossigo]

@devnexen Generally this looks good to me, but I'd like to consider if this should be a FreeBSD-specific feature or something more generic that we implement also on Linux (as a connection mark maybe? not sure, need to look into that).

[devnexen]

that might be a tricky decision indeed, might be a risk to give a false sense of equivalence, so far I have not seen anything even remotely ressemblant on Linux, socket options in this system have very different contexts (but which can be used in redis though but as system specific like controlling thp support in some sense).

[devnexen]

finally taking back what I said, I found some equivalence :)

[yossigo]

@devnexen It sounded this ID is the equivalent of a conn mark but I didn't have the bandwidth to dig deep and validate, thank you!

[yossigo]

Calling @redis/core-team for approval, this is a small change but potentially high value in some environments - see top comments.

[oranagra]

I'm a bit uncomfortable adding an interface that's not supported on Linux.
but i guess it won't harm anyone, and for some it could be useful.
so @yossigo if you're ok with it, i'm too.

[oranagra]

are we sure we wanna return OK here?
alternatively, maybe we should add an isValid callback to the config, and fail the configuration when this is not supported?

[devnexen]

In fact Linux is supported (finally). Originally not planned but found a way.

[oranagra]

ohh, i missed it.

[sundb]

SO_MARK (since Linux 2.6.25)
Check with #ifdef SO_MARK?

[yossigo]

@devnexen I've slightly modified the PR to be more generalized, and avoid calling setsockopt() if not explicitly enabled. I assume that a zero value is anyway the default and that there's no need to make this explicit call, please let me know if these changes make sense.

[devnexen]

Yes I ve seen it, it s fine by me.