Fixes recvm(m)sg blindly assuming correct initialization of received address

[Jan561]

recvm(m)sg now calling SockaddrLike::from_raw instead of assume_init on the received address

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[Jan561]

Always fucking apple

[asomers]

Are you aware that this creates a data copy? The original code was carefully designed to avoid such a thing.

[Jan561]

Yes, it creates a copy, but so does recvfrom (currently, on the master branch):

nix/src/sys/socket/mod.rs

Line 2218 in cb56e89

Ok((ret, T::from_raw(addr.assume_init().as_ptr(), Some(len))))

I feel like in a future PR we should make the address field of RecvMsg private and instead add a getter that lazily performs the checks and the copy, like this:

pub fn address(&self) -> Option<S> {
    S::from_raw(self.address.as_ptr(), len)
}

[asomers]

Why does it matter that some other function also performs a data copy? Are you pointing that out to suggest that we improve it? I'm requesting that you not add an additional data copy to recvmsg.

[Jan561]

Ah okay yeah, that makes sense, I think I misunderstood you before. I'll change the signature of read_mhdr back to expecting a pointer, and perform the conversion there.

[asomers]

Why does it matter that some other function also performs a data copy? Are you pointing that out to suggest that we improve it? I'm requesting that you not add an additional data copy to recvmsg.