Complete socketpair support

[RalfJung]

We pretend to implement socketpair, but that's a complete lie. Completing this however doesn't need any involvement with the rest of the epoll plans. We "just" need to allocate somewhere two buffers for the data to be sent between these two endpoints, and then wire up the read/write of the two sockets with the right buffers. This can be tested by directly using socketpair/read/write.

There's also #3231.

We can provide more detailed mentoring instructions if needed.

[RalfJung]

I think this should follow the proposed "project" process: #3443.

[oli-obk]

I wonder if there are libc compliance test suites that are licensed well enough for us to extract tests from

[tiif]

Hi, I am planning to work on this first since it is related to #3448. I will send the link to the proposal here once I finished localtime_r.

[tiif]

@rustbot claim

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[tiif]

I wonder if there are libc compliance test suites that are licensed well enough for us to extract tests from

Hmm I couldn't find any test for libc for now, if anyone knows where can I find one please let me know, thanks. Currently I imagine minimally the test should look like this (Note: they wrapped libc around their API, so it looks slightly different)

#[test]
pub fn test_socketpair() {
    use nix::sys::socket::{socketpair, AddressFamily, SockFlag, SockType};
    use nix::unistd::{read, write};

    let (fd1, fd2) = socketpair(
        AddressFamily::Unix,
        SockType::Stream,
        None,
        SockFlag::empty(),
    )
    .unwrap();
    write(&fd1, b"hello").unwrap();
    let mut buf = [0; 5];
    read(fd2.as_raw_fd(), &mut buf).unwrap();

    assert_eq!(&buf[..], b"hello");
}
//source: https://github.com/nix-rust/nix/blob/590ab4d5d0b6228f0529b036e4f8a42265ba6d31/test/sys/test_socket.rs#L311. 

source: nix-rust/nix

I might add similar test in the proposal that I am going to send soon.