Merge pull request #93 from eholk/happy-eyeballs

Example: Happy eyeballs

Cargo.toml

@@ -33,3 +33,5 @@ pin-project = "1.0.8"
 futures = "0.3.25"
 futures-lite = "1.12.0"
 criterion = { version = "0.3", features = ["async", "async_futures", "html_reports"] }
+async-std = { version = "1.12.0", features = ["attributes"] }
+futures-time = "3.0.0"

examples/happy_eyeballs.rs

@@ -0,0 +1,48 @@
+use async_std::io::prelude::*;
+use futures::future::TryFutureExt;
+use futures_concurrency::prelude::*;
+use futures_time::prelude::*;
+
+use async_std::io;
+use async_std::net::TcpStream;
+use futures::channel::oneshot;
+use futures_concurrency::vec::AggregateError;
+use futures_time::time::Duration;
+use std::error;
+
+#[async_std::main]
+async fn main() -> Result<(), Box<dyn error::Error + Send + Sync + 'static>> {
+    // Connect to a socket
+    let mut socket = open_tcp_socket("rust-lang.org", 80, 3).await?;
+
+    // Make an HTTP GET request.
+    socket.write_all(b"GET / \r\n").await?;
+    io::copy(&mut socket, &mut io::stdout()).await?;
+
+    Ok(())
+}
+
+/// Happy eyeballs algorithm!
+async fn open_tcp_socket(
+    addr: &str,
+    port: u16,
+    attempts: u64,
+) -> Result<TcpStream, AggregateError<io::Error>> {
+    let (mut sender, mut receiver) = oneshot::channel();
+    let mut futures = Vec::with_capacity(attempts as usize);
+
+    for attempt in 0..attempts {
+        // Start a next attempt if the previous one finishes, or timeout expires.
+        let tcp = TcpStream::connect((addr, port));
+        let start_event = receiver.timeout(Duration::from_secs(attempt));
+        futures.push(tcp.delay(start_event).map_err(|err| {
+            // If the socket fails, start the next attempt
+            let _ = sender.send(());
+            err
+        }));
+        (sender, receiver) = oneshot::channel();
+    }
+
+    // Start connecting. If an attempt succeeds, cancel all others attempts.
+    Ok(futures.race_ok().await?)
+}