This is proposed to support some tests in tokio.

Ideally we could support F_SETFL and F_GETFL for all kind of file descriptions, but getting them to work on socketpair should be sufficient to unblock some of the tests there (here is the relevant codepath).

F_GETFL is quite straight forward as we only support SOCK_NONBLOCK and SOCK_CLOEXEC flags for socketpair.

For F_SETFL, as it has the ability to change O_NONBLOCK flag, we need to decide what should happen if a previously blocking fd is marked as non-blocking, and there are waiting threads. Should they wake up immediately, later at some point or never? It might make this previously unreachable case, reachable.

Ideally support this on:

• socketpair
• pipe

@rustbot label +A-shims +C-enhancement +A-concurrency