net: add TcpStream::ready and non-blocking ops (#3130)

Adds function to await for readiness on the TcpStream and non-blocking read/write functions.

`async fn TcpStream::ready(Interest)` waits for socket readiness satisfying **any** of the specified
interest. There are also two shorthand functions, `readable()` and `writable()`.

Once the stream is in a ready state, the caller may perform non-blocking operations on it using
`try_read()` and `try_write()`. These function return `WouldBlock` if the stream is not, in fact, ready.

The await readiness function are similar to `AsyncFd`, but do not require a guard. The guard in
`AsyncFd` protect against a potential race between receiving the readiness notification and clearing
it. The guard is needed as Tokio does not control the operations. With `TcpStream`, the `try_read()`
and `try_write()` function handle clearing stream readiness as needed.

This also exposes `Interest` and `Ready`, both defined in Tokio as wrappers for Mio types. These
types will also be useful for fixing #3072 .

Other I/O types, such as `TcpListener`, `UdpSocket`, `Unix*` should get similar functions, but this
is left for later PRs.

Refs: #3130

1 files changed, 2 insertions, 2 deletions

diff --git a/tokio/src/signal/unix/driver.rs b/tokio/src/signal/unix/driver.rs
index 323bb9df..315f3bd5 100644
--- a/tokio/src/signal/unix/driver.rs
+++ b/tokio/src/signal/unix/driver.rs
@@ -2,8 +2,8 @@
 
 //! Signal driver
 
-use crate::io::driver::Driver as IoDriver;
-use crate::io::{Interest, PollEvented};
+use crate::io::driver::{Driver as IoDriver, Interest};
+use crate::io::PollEvented;
 use crate::park::Park;
 use crate::signal::registry::globals;