Document unconnected TcpStream returned by TcpStream::connect

Author: Thomasdezeeuw

src/net/tcp/stream.rs

@@ -49,6 +49,30 @@ pub struct TcpStream {
 impl TcpStream {
     /// Create a new TCP stream and issue a non-blocking connect to the
     /// specified address.
+    ///
+    /// # Notes
+    ///
+    /// The returned `TcpStream` may not be connected (and thus usable), unlike
+    /// the API found in `std::net::TcpStream`. Because Mio issues a
+    /// *non-blocking* connect it will not block the thread and instead return
+    /// an unconnected `TcpStream`.
+    ///
+    /// Ensuring the returned stream is connected is surprisingly complex when
+    /// considering cross-platform support. Doing this properly should follow
+    /// the steps below, an example implementation can be found
+    /// [here](https://github.com/Thomasdezeeuw/heph/blob/0c4f1ab3eaf08bea1d65776528bfd6114c9f8374/src/net/tcp/stream.rs#L560-L622).
+    ///
+    ///  1. Call `TcpStream::connect`
+    ///  2. Register the returned stream with at least [read interest].
+    ///  3. Wait for a (readable) event.
+    ///  4. Check `TcpStream::peer_addr`. If it returns `libc::EINPROGRESS` or
+    ///     `ErrorKind::NotConnected` it means the stream is not yet connected,
+    ///     go back to step 3. If it returns an address it means the stream is
+    ///     connected, go to step 5. If another error is returned something
+    ///     whent wrong.
+    ///  5. Now the stream can be used.
+    ///
+    /// [read interest]: Interest::READABLE
     pub fn connect(addr: SocketAddr) -> io::Result<TcpStream> {
         let socket = new_for_addr(addr)?;
         #[cfg(unix)]