io: use intrusive wait list for I/O driver (#2828)

This refactors I/O registration in a few ways:

- Cleans up the cached readiness in `PollEvented`. This cache used to
  be helpful when readiness was a linked list of `*mut Node`s in
  `Registration`. Previous refactors have turned `Registration` into just
  an `AtomicUsize` holding the current readiness, so the cache is just
  extra work and complexity. Gone.
- Polling the `Registration` for readiness now gives a `ReadyEvent`,
  which includes the driver tick. This event must be passed back into
  `clear_readiness`, so that the readiness is only cleared from `Registration`
  if the tick hasn't changed. Previously, it was possible to clear the
  readiness even though another thread had *just* polled the driver and
  found the socket ready again.
- Registration now also contains an `async fn readiness`, which stores
  wakers in an instrusive linked list. This allows an unbounded number
  of tasks to register for readiness (previously, only 1 per direction (read
  and write)). By using the intrusive linked list, there is no concern of
  leaking the storage of the wakers, since they are stored inside the `async fn`
  and released when the future is dropped.
- Registration retains a `poll_readiness(Direction)` method, to support
  `AsyncRead` and `AsyncWrite`. They aren't able to use `async fn`s, and
  so there are 2 reserved slots for those methods.
- IO types where it makes sense to have multiple tasks waiting on them
  now take advantage of this new `async fn readiness`, such as `UdpSocket`
  and `UnixDatagram`.

Additionally, this makes the `io-driver` "feature" internal-only (no longer
documented, not part of public API), and adds a second internal-only
feature, `io-readiness`, to group together linked list part of registration
that is only used by some of the IO types.

After a bit of discussion, changing stream-based transports (like
`TcpStream`) to have `async fn read(&self)` is punted, since that
is likely too easy of a footgun to activate.

Refs: #2779, #2728 

4 files changed, 17 insertions, 59 deletions

diff --git a/tokio/tests/async_send_sync.rs b/tokio/tests/async_send_sync.rs
index e7011e3b..b3492b5e 100644
--- a/tokio/tests/async_send_sync.rs
+++ b/tokio/tests/async_send_sync.rs
@@ -172,10 +172,6 @@ async_assert_fn!(tokio::net::UdpSocket::send(_, &[u8]): Send & Sync);
 async_assert_fn!(tokio::net::UdpSocket::recv(_, &mut [u8]): Send & Sync);
 async_assert_fn!(tokio::net::UdpSocket::send_to(_, &[u8], SocketAddr): Send & Sync);
 async_assert_fn!(tokio::net::UdpSocket::recv_from(_, &mut [u8]): Send & Sync);
-async_assert_fn!(tokio::net::udp::RecvHalf::recv(_, &mut [u8]): Send & Sync);
-async_assert_fn!(tokio::net::udp::RecvHalf::recv_from(_, &mut [u8]): Send & Sync);
-async_assert_fn!(tokio::net::udp::SendHalf::send(_, &[u8]): Send & Sync);
-async_assert_fn!(tokio::net::udp::SendHalf::send_to(_, &[u8], &SocketAddr): Send & Sync);
 
 #[cfg(unix)]
 mod unix_datagram {
diff --git a/tokio/tests/rt_common.rs b/tokio/tests/rt_common.rs
index a8968be1..7f0491c4 100644
--- a/tokio/tests/rt_common.rs
+++ b/tokio/tests/rt_common.rs
@@ -827,6 +827,7 @@ rt_test! {
     #[test]
     fn io_notify_while_shutting_down() {
         use std::net::Ipv6Addr;
+        use std::sync::Arc;
 
         for _ in 1..10 {
             let runtime = rt();
@@ -834,7 +835,8 @@ rt_test! {
             runtime.block_on(async {
                 let socket = UdpSocket::bind((Ipv6Addr::LOCALHOST, 0)).await.unwrap();
                 let addr = socket.local_addr().unwrap();
-                let (mut recv_half, mut send_half) = socket.split();
+                let send_half = Arc::new(socket);
+                let recv_half = send_half.clone();
 
                 tokio::spawn(async move {
                     let mut buf = [0];
diff --git a/tokio/tests/udp.rs b/tokio/tests/udp.rs
index 62a2234f..9da67267 100644
--- a/tokio/tests/udp.rs
+++ b/tokio/tests/udp.rs
@@ -1,6 +1,7 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "full")]
 
+use std::sync::Arc;
 use tokio::net::UdpSocket;
 
 const MSG: &[u8] = b"hello";
@@ -8,8 +9,8 @@ const MSG_LEN: usize = MSG.len();
 
 #[tokio::test]
 async fn send_recv() -> std::io::Result<()> {
-    let mut sender = UdpSocket::bind("127.0.0.1:0").await?;
-    let mut receiver = UdpSocket::bind("127.0.0.1:0").await?;
+    let sender = UdpSocket::bind("127.0.0.1:0").await?;
+    let receiver = UdpSocket::bind("127.0.0.1:0").await?;
 
     sender.connect(receiver.local_addr()?).await?;
     receiver.connect(sender.local_addr()?).await?;
@@ -25,8 +26,8 @@ async fn send_recv() -> std::io::Result<()> {
 
 #[tokio::test]
 async fn send_to_recv_from() -> std::io::Result<()> {
-    let mut sender = UdpSocket::bind("127.0.0.1:0").await?;
-    let mut receiver = UdpSocket::bind("127.0.0.1:0").await?;
+    let sender = UdpSocket::bind("127.0.0.1:0").await?;
+    let receiver = UdpSocket::bind("127.0.0.1:0").await?;
 
     let receiver_addr = receiver.local_addr()?;
     sender.send_to(MSG, &receiver_addr).await?;
@@ -42,9 +43,10 @@ async fn send_to_recv_from() -> std::io::Result<()> {
 #[tokio::test]
 async fn split() -> std::io::Result<()> {
     let socket = UdpSocket::bind("127.0.0.1:0").await?;
-    let (mut r, mut s) = socket.split();
+    let s = Arc::new(socket);
+    let r = s.clone();
 
-    let addr = s.as_ref().local_addr()?;
+    let addr = s.local_addr()?;
     tokio::spawn(async move {
         s.send_to(MSG, &addr).await.unwrap();
     });
@@ -54,24 +56,6 @@ async fn split() -> std::io::Result<()> {
     Ok(())
 }
 
-#[tokio::test]
-async fn reunite() -> std::io::Result<()> {
-    let socket = UdpSocket::bind("127.0.0.1:0").await?;
-    let (s, r) = socket.split();
-    assert!(s.reunite(r).is_ok());
-    Ok(())
-}
-
-#[tokio::test]
-async fn reunite_error() -> std::io::Result<()> {
-    let socket = UdpSocket::bind("127.0.0.1:0").await?;
-    let socket1 = UdpSocket::bind("127.0.0.1:0").await?;
-    let (s, _) = socket.split();
-    let (_, r1) = socket1.split();
-    assert!(s.reunite(r1).is_err());
-    Ok(())
-}
-
 // # Note
 //
 // This test is purposely written such that each time `sender` sends data on
@@ -86,7 +70,7 @@ async fn try_send_spawn() {
     const MSG2_LEN: usize = MSG2.len();
 
     let sender = UdpSocket::bind("127.0.0.1:0").await.unwrap();
-    let mut receiver = UdpSocket::bind("127.0.0.1:0").await.unwrap();
+    let receiver = UdpSocket::bind("127.0.0.1:0").await.unwrap();
 
     receiver
         .connect(sender.local_addr().unwrap())
diff --git a/tokio/tests/uds_datagram.rs b/tokio/tests/uds_datagram.rs
index d3c3535e..bd47f19e 100644
--- a/tokio/tests/uds_datagram.rs
+++ b/tokio/tests/uds_datagram.rs
@@ -6,8 +6,9 @@ use tokio::net::UnixDatagram;
 use tokio::try_join;
 
 use std::io;
+use std::sync::Arc;
 
-async fn echo_server(mut socket: UnixDatagram) -> io::Result<()> {
+async fn echo_server(socket: UnixDatagram) -> io::Result<()> {
     let mut recv_buf = vec![0u8; 1024];
     loop {
         let (len, peer_addr) = socket.recv_from(&mut recv_buf[..]).await?;
@@ -32,7 +33,7 @@ async fn echo() -> io::Result<()> {
     });
 
     {
-        let mut socket = UnixDatagram::bind(&client_path).unwrap();
+        let socket = UnixDatagram::bind(&client_path).unwrap();
         socket.connect(server_path)?;
         socket.send(b"ECHO").await?;
         let mut recv_buf = [0u8; 16];
@@ -87,8 +88,8 @@ async fn try_send_recv_never_block() -> io::Result<()> {
 async fn split() -> std::io::Result<()> {
     let dir = tempfile::tempdir().unwrap();
     let path = dir.path().join("split.sock");
-    let socket = UnixDatagram::bind(path.clone())?;
-    let (mut r, mut s) = socket.into_split();
+    let s = Arc::new(UnixDatagram::bind(path.clone())?);
+    let r = s.clone();
 
     let msg = b"hello";
     let ((), ()) = try_join! {
@@ -106,28 +107,3 @@ async fn split() -> std::io::Result<()> {
 
     Ok(())
 }
-
-#[tokio::test]
-async fn reunite() -> std::io::Result<()> {
-    let dir = tempfile::tempdir().unwrap();
-    let path = dir.path().join("reunite.sock");
-    let socket = UnixDatagram::bind(path)?;
-    let (s, r) = socket.into_split();
-    assert!(s.reunite(r).is_ok());
-    Ok(())
-}
-
-#[tokio::test]
-async fn reunite_error() -> std::io::Result<()> {
-    let dir = tempfile::tempdir().unwrap();
-    let path = dir.path().join("reunit.sock");
-    let dir = tempfile::tempdir().unwrap();
-    let path1 = dir.path().join("reunit.sock");
-    let socket = UnixDatagram::bind(path)?;
-    let socket1 = UnixDatagram::bind(path1)?;
-
-    let (s, _) = socket.into_split();
-    let (_, r1) = socket1.into_split();
-    assert!(s.reunite(r1).is_err());
-    Ok(())
-}