test_vsock fails on Linux under QEMU

[asomers]

Sometime in between 6-March-2021 and 16-March-2021, sys::test_socket::test_vsock has begin to fail for the QEMU architectures (but it still passes on x86_64 Linux).

For example, from https://github.com/nix-rust/nix/pull/1401/checks?check_run_id=2124256635 ,

---- sys::test_socket::test_vsock stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `None`,
 right: `Some(Sys(EADDRNOTAVAIL))`', test/sys/test_socket.rs:1517:5

@stefano-garzarella @qwandor-google as the only two known users of Vsock with Nix, can one of you please debug this?

[asomers]

Possibly the same root cause as #1310

[stefano-garzarella]

@asomers sure! I can take a look!

[stefano-garzarella]

The test is failing here:

    let sockaddr = SockAddr::new_vsock(libc::VMADDR_CID_LOCAL, port);
    assert_eq!(bind(s1, &sockaddr).err(),
               Some(Error::Sys(Errno::EADDRNOTAVAIL)));

Starting from Linux 5.6, VMADDR_CID_LOCAL is supported to do local communication (loopback device).
Before this it was reserved, but probably the test runner now has a newer kernel and VMADDR_CID_LOCAL is supported.

I'll send a patch right away to remove that check and re-enable the test.

[asomers]

Fixed by #1407 .