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.

tests-integration/tests/process_stdio.rs

@@ -0,0 +1,127 @@
+#![warn(rust_2018_idioms)]
+
+use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
+use tokio::process::{Child, Command};
+use tokio_test::assert_ok;
+
+use futures::future::{self, FutureExt};
+use futures::stream::StreamExt;
+use std::env;
+use std::io;
+use std::process::{ExitStatus, Stdio};
+
+fn cat() -> Command {
+    let mut me = env::current_exe().unwrap();
+    me.pop();
+
+    if me.ends_with("deps") {
+        me.pop();
+    }
+
+    me.push("test-cat");
+
+    let mut cmd = Command::new(me);
+    cmd.stdin(Stdio::piped()).stdout(Stdio::piped());
+    cmd
+}
+
+async fn feed_cat(mut cat: Child, n: usize) -> io::Result<ExitStatus> {
+    let mut stdin = cat.stdin().take().unwrap();
+    let stdout = cat.stdout().take().unwrap();
+
+    // Produce n lines on the child's stdout.
+    let write = async {
+        for i in 0..n {
+            let bytes = format!("line {}\n", i).into_bytes();
+            stdin.write_all(&bytes).await.unwrap();
+        }
+
+        drop(stdin);
+    };
+
+    let read = async {
+        let mut reader = BufReader::new(stdout).lines();
+        let mut num_lines = 0;
+
+        // Try to read `n + 1` lines, ensuring the last one is empty
+        // (i.e. EOF is reached after `n` lines.
+        loop {
+            let data = reader
+                .next()
+                .await
+                .unwrap_or_else(|| Ok(String::new()))
+                .expect("failed to read line");
+
+            let num_read = data.len();
+            let done = num_lines >= n;
+
+            match (done, num_read) {
+                (false, 0) => panic!("broken pipe"),
+                (true, n) if n != 0 => panic!("extraneous data"),
+                _ => {
+                    let expected = format!("line {}", num_lines);
+                    assert_eq!(expected, data);
+                }
+            };
+
+            num_lines += 1;
+            if num_lines >= n {
+                break;
+            }
+        }
+    };
+
+    // Compose reading and writing concurrently.
+    future::join3(write, read, cat)
+        .map(|(_, _, status)| status)
+        .await
+}
+
+/// Check for the following properties when feeding stdin and
+/// consuming stdout of a cat-like process:
+///
+/// - A number of lines that amounts to a number of bytes exceeding a
+///   typical OS buffer size can be fed to the child without
+///   deadlock. This tests that we also consume the stdout
+///   concurrently; otherwise this would deadlock.
+///
+/// - We read the same lines from the child that we fed it.
+///
+/// - The child does produce EOF on stdout after the last line.
+#[tokio::test]
+async fn feed_a_lot() {
+    let child = cat().spawn().unwrap();
+    let status = feed_cat(child, 10000).await.unwrap();
+    assert_eq!(status.code(), Some(0));
+}
+
+#[tokio::test]
+async fn wait_with_output_captures() {
+    let mut child = cat().spawn().unwrap();
+    let mut stdin = child.stdin().take().unwrap();
+
+    let write_bytes = b"1234";
+
+    let future = async {
+        stdin.write_all(write_bytes).await?;
+        drop(stdin);
+        let out = child.wait_with_output();
+        out.await
+    };
+
+    let output = future.await.unwrap();
+
+    assert!(output.status.success());
+    assert_eq!(output.stdout, write_bytes);
+    assert_eq!(output.stderr.len(), 0);
+}
+
+#[tokio::test]
+async fn status_closes_any_pipes() {
+    // Cat will open a pipe between the parent and child.
+    // If `status_async` doesn't ensure the handles are closed,
+    // we would end up blocking forever (and time out).
+    let child = cat().status();
+
+    assert_ok!(child.await);
+}