net: restore TcpStream::{poll_read_ready, poll_write_ready}

[masnagam]

Motivation

In some situations, it's necessary to detect TCP disconnect without reading or writing anything to/from that socket.

This PR restores TcpStream::{poll_read_ready, poll_write_ready} which were removed at 6d8cc4e.

Solution

Restore TcpStream::{poll_read_ready, poll_write_ready}.

New test cases for those functions are added. See tcp_ready.rs.

This PR closes #2228.

[carllerche]

cc @seanmonstar

[seanmonstar]

Thanks for the PR!

As described in #2728, we want to remove the public dependency on mio from the API. That issue describes a proposal around how to expose readiness for socket types. The internals are changing in #2828. Once that lands, would you be interested in trying to implement what's proposed in #2728?

[masnagam]

@seanmonstar I created a new working branch for that.

I implemented TcpStream::is_readable and TcpStream::is_writable for testing purposes (I'll implement correct methods later). These methods simply call PollEvented::poll_xxx_ready in poll_fn like what TcpStream::connect_addr does.

Unfortunately, tests fail, which passed before the rebase.

Probably, there are some mistakes in my changes.
Could you point out mistakes if you find?

[masnagam]

#3130 was merged. Should we close this PR, or continue implementing poll_[read|write]_ready for poll_[read|write]?

[Darksonn]

We might just have an poll_ready that takes an Interest? What do you think @carllerche?

[carllerche]

I don't think we can have poll_ready. poll_read_ready and poll_write_ready makes it easier to explain the contract. There can only be one task waiting on each fn. The async fn version doesn't have this limitation because of how it is implemented.

[carllerche]

I think the various socket types can still provide poll_read_ready and poll_write_ready even though there are async fn variants. Not everyone can use those (i.e. future impls).

[masnagam]

PR has been rebased onto the current master branch.

TcpStream::poll_[read|write]_ready have been reimplemented using Registration::poll_[read|write]_ready.

[carllerche]

Thanks 👍