Add read timeout to socket.

[meqif]

Currently, all reads from the underlying UDP are blocking. This raises several problems, such as:

• no timeout detection on ACKs for sent packets (which need to be resent),
• no timeout detection on receive (which may signal a dead connection),
• no timeout detection on close (meaning a socket may hang in the SocketState::FinSent state indefinitely).

This branch pull request creates a sub-crate with a new trait, WithReadTimeout, implemented for UdpSocket, adding the method recv_timeout(&mut [u8], i64) to it. This method is only properly implemented for POSIX targets, with the Windows implementation being a dummy which ignores the timeout.

[meqif]

I added a Windows implementation which compiles fine but seems to not be timing out at all.

[meqif]

In any case, it seems like we'll have proper methods soon-ish: rust-lang/rust#25818.

[dirvine]

Ah nice :-)

[meqif]

I re-read the manual page for select and noticed I should have done a more extensive check on the return value. Previously, I considered only -1 as indication of an error. However, 0 is also an error:

The select function returns the total number of socket handles that are ready and contained in the fd_set structures, zero if the time limit expired, or SOCKET_ERROR if an error occurred. If the return value is SOCKET_ERROR, WSAGetLastError can be used to retrieve a specific error code.

Now that is fixed, but select always returns 0.

[meqif]

It seems to be working correctly now. :)