net: move into tokio crate (#1683)

A step towards collapsing Tokio sub crates into a single `tokio`
crate (#1318).

The `net` implementation is now provided by the main `tokio` crate.
Functionality can be opted out of by using the various net related
feature flags.

tokio/tests/net_driver.rs

@@ -0,0 +1,83 @@
+#![warn(rust_2018_idioms)]
+
+use tokio::net::driver::Reactor;
+use tokio::net::TcpListener;
+use tokio_test::{assert_ok, assert_pending};
+
+use futures_util::task::{waker_ref, ArcWake};
+use std::future::Future;
+use std::net::TcpStream;
+use std::pin::Pin;
+use std::sync::{mpsc, Arc, Mutex};
+use std::task::Context;
+
+struct Task<T> {
+    future: Mutex<Pin<Box<T>>>,
+}
+
+impl<T: Send> ArcWake for Task<T> {
+    fn wake_by_ref(_: &Arc<Self>) {
+        // Do nothing...
+    }
+}
+
+impl<T> Task<T> {
+    fn new(future: T) -> Task<T> {
+        Task {
+            future: Mutex::new(Box::pin(future)),
+        }
+    }
+}
+
+#[test]
+fn test_drop_on_notify() {
+    // When the reactor receives a kernel notification, it notifies the
+    // task that holds the associated socket. If this notification results in
+    // the task being dropped, the socket will also be dropped.
+    //
+    // Previously, there was a deadlock scenario where the reactor, while
+    // notifying, held a lock and the task being dropped attempted to acquire
+    // that same lock in order to clean up state.
+    //
+    // To simulate this case, we create a fake executor that does nothing when
+    // the task is notified. This simulates an executor in the process of
+    // shutting down. Then, when the task handle is dropped, the task itself is
+    // dropped.
+
+    let mut reactor = assert_ok!(Reactor::new());
+    let (addr_tx, addr_rx) = mpsc::channel();
+
+    // Define a task that just drains the listener
+    let task = Arc::new(Task::new(async move {
+        // Create a listener
+        let mut listener = assert_ok!(TcpListener::bind("127.0.0.1:0").await);
+
+        // Send the address
+        let addr = listener.local_addr().unwrap();
+        addr_tx.send(addr).unwrap();
+
+        loop {
+            let _ = listener.accept().await;
+        }
+    }));
+
+    let _enter = tokio_executor::enter().unwrap();
+
+    {
+        let handle = reactor.handle();
+        let _reactor = tokio::net::driver::set_default(&handle);
+        let waker = waker_ref(&task);
+        let mut cx = Context::from_waker(&waker);
+        assert_pending!(task.future.lock().unwrap().as_mut().poll(&mut cx));
+    }
+
+    // Get the address
+    let addr = addr_rx.recv().unwrap();
+
+    drop(task);
+
+    // Establish a connection to the acceptor
+    let _s = TcpStream::connect(&addr).unwrap();
+
+    reactor.turn(None).unwrap();
+}