io: bring back split utility

[carllerche]

Bring back split utility as a free fn instead of a method on
AsyncRead. This utility wraps the stream in an Arc and uses mutual
exclusion to ensure correct access.

Additionally, the specialized split_mut fn on TcpStream and UdsStream
is promoted to split.

[LucioFranco]

👍 LGTM

[DoumanAsh]

Just a quick check, is UnsafeCell necessary here?

[jonhoo]

I'm generally happy for this change, but the loss of an owned split for TcpStream that doesn't require locking is pretty sad. Any particular reason why that was removed?

[vojtechkral]

@jonhoo +1, also not sure why the owning version was removed/replaced. I can't use the referencing ReadHalf in my code... @carllerche thoughts? cc @sopium

[carllerche]

An owning version exists as a free fn: io::split(stream).

The idea is to place emphasis on the async/await friendly zero-cost options.

[vojtechkral]

Hm, ok, but the read and write halves being references reduces the usefulness of this split quite a lot. The TcpStream is really just a socket file descriptor and tcp is fully duplex, so unless I'm missing something, the halves could be owning and it would be just a matter of when to close the fd, which could be done with and Arc without hurting the zero-costness, no?

[carllerche]

You are correct, except that introducing an Arc means it is not zero-cost.

Could you describe more what you are trying to do?

[vojtechkral]

Our code is here (using tokio 0.1 with async/await preview, beware, there are some ugly hacks, especially my attemp at tcp_split()). Basically we need to have separate structures for the read and write halves which have a bunch of stuff on top of them and are used in separate tokio tasks.

I'm in the process of attempting to port that code to tokio 0.2.

So, would it be considered non-zero-cost even if the Arc were to be used only on split() and drop with the I/O going through the file descriptor without indirection? If so, then I don't see a way to do truly zero cost owning variant. But still, we could have an owning variant with much lower cost compared to locking...