net: add UdpSocket::peek_sender()

closes #5491

tokio/src/net/udp.rs

@@ -1331,6 +1331,11 @@ impl UdpSocket {
     /// Make sure to always use a sufficiently large buffer to hold the
     /// maximum UDP packet size, which can be up to 65536 bytes in size.
     ///
+    /// MacOS will return an error if you pass a zero-sized buffer.
+    ///
+    /// If you're merely interested in learning the sender of the data at the head of the queue,
+    /// try [`peek_sender`].
+    ///
     /// # Examples
     ///
     /// ```no_run
@@ -1349,6 +1354,8 @@ impl UdpSocket {
     ///     Ok(())
     /// }
     /// ```
+    ///
+    /// [`peek_sender`]: method@Self::peek_sender
     pub async fn peek_from(&self, buf: &mut [u8]) -> io::Result<(usize, SocketAddr)> {
         self.io
             .registration()
@@ -1371,6 +1378,11 @@ impl UdpSocket {
     /// Make sure to always use a sufficiently large buffer to hold the
     /// maximum UDP packet size, which can be up to 65536 bytes in size.
     ///
+    /// MacOS will return an error if you pass a zero-sized buffer.
+    ///
+    /// If you're merely interested in learning the sender of the data at the head of the queue,
+    /// try [`poll_peek_sender`].
+    ///
     /// # Return value
     ///
     /// The function returns:
@@ -1382,6 +1394,8 @@ impl UdpSocket {
     /// # Errors
     ///
     /// This function may encounter any standard I/O error except `WouldBlock`.
+    ///
+    /// [`poll_peek_sender`]: method@Self::poll_peek_sender
     pub fn poll_peek_from(
         &self,
         cx: &mut Context<'_>,
@@ -1404,6 +1418,53 @@ impl UdpSocket {
         Poll::Ready(Ok(addr))
     }
 
+    /// Retrieve the sender of the data at the head of the input queue, waiting if empty.
+    ///
+    /// This is equivalent to calling [`peek_from`] with a zero-sized buffer,
+    /// but suppresses the `WSAEMSGSIZE` error on Windows and the "invalid argument" error on macOS.
+    ///
+    /// [`peek_from`]: method@Self::peek_from
+    pub async fn peek_sender(&self) -> io::Result<SocketAddr> {
+        self.io
+            .registration()
+            .async_io(Interest::READABLE, || {
+                self
+                    .as_socket()
+                    .peek_sender()?
+                    .as_socket()
+                    // Not clear what conditions could cause this,
+                    // but we probably ought not to panic.
+                    .ok_or_else(|| io::Error::new(io::ErrorKind::Other, "sender not available"))
+            })
+            .await
+    }
+
+    /// Retrieve the sender of the data at the head of the input queue.
+    ///
+    /// This is equivalent to calling [`poll_peek_from`] with a zero-sized buffer,
+    /// but suppresses the `WSAEMSGSIZE` error on Windows and the "invalid argument" error on macOS.
+    ///
+    /// # Notes
+    ///
+    /// Note that on multiple calls to a `poll_*` method in the recv direction, only the
+    /// `Waker` from the `Context` passed to the most recent call will be scheduled to
+    /// receive a wakeup.
+    ///
+    /// [`poll_peek_from`]: method@Self::poll_peek_from
+    pub fn poll_peek_sender(&self, cx: &mut Context<'_>) -> Poll<io::Result<SocketAddr>> {
+        self.io
+            .registration()
+            .poll_read_io(cx,  || {
+                self
+                    .as_socket()
+                    .peek_sender()?
+                    .as_socket()
+                    // Not clear what conditions could cause this,
+                    // but we probably ought not to panic.
+                    .ok_or_else(|| io::Error::new(io::ErrorKind::Other, "sender not available"))
+            })
+    }
+
     /// Gets the value of the `SO_BROADCAST` option for this socket.
     ///
     /// For more information about this option, see [`set_broadcast`].

tokio/tests/udp.rs

@@ -134,6 +134,57 @@ async fn send_to_peek_from_poll() -> std::io::Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn peek_sender() -> std::io::Result<()> {
+    let sender = UdpSocket::bind("127.0.0.1:0").await?;
+    let receiver = UdpSocket::bind("127.0.0.1:0").await?;
+
+    let sender_addr = sender.local_addr()?;
+    let receiver_addr = receiver.local_addr()?;
+
+    let msg = b"Hello, world!";
+    sender.send_to(msg, receiver_addr).await?;
+
+    let peeked_sender = receiver.peek_sender().await?;
+    assert_eq!(peeked_sender, sender_addr);
+
+    // Assert that `peek_sender()` returns the right sender but
+    // doesn't remove from the receive queue.
+    let mut recv_buf = [0u8; 32];
+    let (read, received_sender) = receiver.recv_from(&mut recv_buf).await?;
+
+    assert_eq!(&recv_buf[..read], msg);
+    assert_eq!(received_sender, peeked_sender);
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn poll_peek_sender() -> std::io::Result<()> {
+    let sender = UdpSocket::bind("127.0.0.1:0").await?;
+    let receiver = UdpSocket::bind("127.0.0.1:0").await?;
+
+    let sender_addr = sender.local_addr()?;
+    let receiver_addr = receiver.local_addr()?;
+
+    let msg = b"Hello, world!";
+    poll_fn(|cx| sender.poll_send_to(cx, msg, receiver_addr)).await?;
+
+    let peeked_sender = poll_fn(|cx| receiver.poll_peek_sender(cx)).await?;
+    assert_eq!(peeked_sender, sender_addr);
+
+    // Assert that `poll_peek_sender()` returns the right sender but
+    // doesn't remove from the receive queue.
+    let mut recv_buf = [0u8; 32];
+    let mut read = ReadBuf::new(&mut recv_buf);
+    let received_sender = poll_fn(|cx| receiver.poll_recv_from(cx, &mut read)).await?;
+
+    assert_eq!(read.filled(), msg);
+    assert_eq!(received_sender, peeked_sender);
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn split() -> std::io::Result<()> {
     let socket = UdpSocket::bind("127.0.0.1:0").await?;