UdpSocket split support

[blckngm]

The UDP counterpart of #1217.

A specialized split is implemented for UdpSocket, making it possible to receive and send on the socket concurrently, even from two different tasks.

There are no AsyncRead or AsyncWrite like traits for UdpSocket. So the halves have receive or send methods same with the socket. The methods return the same future types with their UdpSocket equivalents.

[LucioFranco]

This seems reasonable though I'd like to have @jonhoo or @carllerche look at it.

[carllerche]

LGTM 👍 Thanks! I tracked polish questions on #1250.

[ghost]

Why the API of UdpSocket::split is inconsistent with TcpStream::split? The former takes self while the latter borrows &mut self.

UdpSocket::split:

fn split(self) -> (UdpSocketRecvHalf, UdpSocketSendHalf)

TcpStream::split:

fn split(&mut self) -> (ReadHalf, WriteHalf)

What's more, UdpSocketSendHalf::send_to accepts only SocketAddr while UdpSocket::send_to accepts impl ToSocketAddrs.
UdpSocketSendHalf::send_to:

fn send_to(
    &'_ mut self,
    buf: &'_ [u8],
    target: &'_ SocketAddr
) -> impl Future<Output = Result<usize, Error>>

UdpSocket::send_to:

fn send_to<A>(
    &'_ mut self,
    buf: &'_ [u8],
    target: A
) -> impl Future<Output = Result<usize, Error>> where
    A: ToSocketAddrs

I have written a patch to make these API feel consistent, but I'm not sure whether it is considered a bug.

[blckngm]

Hi,

Why the API of UdpSocket::split is inconsistent with TcpStream::split? The former takes self while the latter borrows &mut self.

UdpSocket::split:

fn split(self) -> (UdpSocketRecvHalf, UdpSocketSendHalf)

TcpStream::split:

fn split(&mut self) -> (ReadHalf, WriteHalf)

Originally TcpStream::split also takes ownership (#1217). The split_mut method that takes &mut self was added in #1289. Later split was replaced by split_mut in #1521.

But I think it would problematic if UdpSocket loses split(self). The borrowed halves produced by split(&mut self) would be very difficult to use across multiple tasks.
TcpStream and UnixStream can at least use io::split, but there is no equivalent for UDP.

If we change UdpSocket::split to take &mut self for API consistency, I recommend that split(self) is kept and renamed, to e.g. split_owned.

What's more, UdpSocketSendHalf::send_to accepts only SocketAddr while UdpSocket::send_to accepts impl ToSocketAddrs.
UdpSocketSendHalf::send_to:

fn send_to(
    &'_ mut self,
    buf: &'_ [u8],
    target: &'_ SocketAddr
) -> impl Future<Output = Result<usize, Error>>

UdpSocket::send_to:

fn send_to<A>(
    &'_ mut self,
    buf: &'_ [u8],
    target: A
) -> impl Future<Output = Result<usize, Error>> where
    A: ToSocketAddrs

I have written a patch to make these API feel consistent, but I'm not sure whether it is considered a bug.

The ToSocketAddrs mechanism was added later. I agree that it should be used in the send half now.