add IP_MULTICAST_IF for sockopt

[keepsimple1]

IP_MULTICAST_IF is not supported by sockopt in nix. See issue #1360 .

According to Linux man page, IP_MULTICAST_IF is used in this way:

quote:

IP_MULTICAST_IF (since Linux 1.2)
              Set the local device for a multicast socket.  The argument for
              setsockopt(2) is an ip_mreqn or (since Linux 3.5) ip_mreq
              structure similar to IP_ADD_MEMBERSHIP, or an in_addr struc‐
              ture.  (The kernel determines which structure is being passed
              based on the size passed in optlen.)  For getsockopt(2), the
              argument is an in_addr structure.

Please review this patch. Also I have a question: how do we add automated testing for such sockopt in nix? I've added a test case in my own code that uses nix and it works, but don't know about the practice of testing inside nix. Thanks.

Update: 12/24/2020

The original approach of defining an Enum did not work well because #[repr(transparent)] does not support Enum. Moreover, my use cases only care about in_addr value, which is the most common AFAIK. So I refactored the patch to address the major cases I actually understand, and added a test case.

Also, based on the Github Actions results, I excluded the patch from the (OS, env, arch) that failed with error ENOPROTOOPT.

[asomers]

Nix doesn't currently use anything fancy like mocking for tests. Just try to exercise the socket option. Don't write an exhaustive test suite for it; that's a job for the kernel maintainers. Nix's test suite should do just enough to ensure that the bindings are correct.

[keepsimple1]

Thanks @asomers for your answers. I've updated the diff and the PR Description, adding a test case. Please take a look when you have time.

[asomers]

My man page says The use of IP_MULTICAST_IF is not recommended, as multicast memberships are scoped to each individual interface. It is supported for legacy use only.... Given that, we probably should't add Nix support for it. Is there something you need it for that can't be done any other way?

[asomers]

cfg_if is helpful when you expect to have else or else if blocks. But in cases like this, it's simpler to do:

#[cfg(...)]
sockopt_impl!(...)

[asomers]

Does the option work with IPv6? If so you should use an enum type. Or maybe define different Nix sockopts, one for each supported address type.

[keepsimple1]

My man page says The use of IP_MULTICAST_IF is not recommended, as multicast memberships are scoped to each individual interface. It is supported for legacy use only.... Given that, we probably should't add Nix support for it. Is there something you need it for that can't be done any other way?

Could you please share where I can find this man page you referred to? Thanks.

[asomers]

https://www.freebsd.org/cgi/man.cgi?query=ip&apropos=0&sektion=4&manpath=FreeBSD+12.2-RELEASE+and+Ports&arch=default&format=html

[keepsimple1]

https://www.freebsd.org/cgi/man.cgi?query=ip&apropos=0&sektion=4&manpath=FreeBSD+12.2-RELEASE+and+Ports&arch=default&format=html

Thanks. Looks like FreeBSD deems it legacy (while Linux seems not).

And I found a workaround in my project at this point. I will close this PR for now until someone found its use again.