Add peek to TcpStream.

src/net/tcp/split_owned.rs

@@ -36,6 +36,19 @@ impl OwnedReadHalf {
     pub fn reunite(self, other: OwnedWriteHalf) -> Result<TcpStream, ReuniteError> {
         reunite(self, other)
     }
+
+    /// Attempt to receive data on the socket, without removing that data from the queue, registering the current task for wakeup if data is not yet available.
+    pub fn poll_peek(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut ReadBuf,
+    ) -> Poll<io::Result<usize>> {
+        Pin::new(&mut self.inner).poll_peek(cx, buf)
+    }
+
+    pub async fn peek(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        self.inner.peek(buf).await
+    }
 }
 
 /// Owned write half of a `TcpStream`, created by `into_split`.

src/net/tcp/stream.rs

@@ -1,3 +1,5 @@
+use bytes::{Buf, Bytes};
+use std::future::poll_fn;
 use std::{
     fmt::Debug,
     io::{self, Error, Result},
@@ -6,8 +8,6 @@ use std::{
     sync::Arc,
     task::{ready, Context, Poll},
 };
-
-use bytes::{Buf, Bytes};
 use tokio::{
     io::{AsyncRead, AsyncWrite, ReadBuf},
     runtime::Handle,
@@ -165,6 +165,16 @@ impl TcpStream {
     pub fn set_nodelay(&self, _nodelay: bool) -> Result<()> {
         Ok(())
     }
+
+    /// Receives data on the socket from the remote address to which it is connected,
+    /// without removing that data from the queue. On success, returns the number of bytes peeked.
+    pub async fn peek(&mut self, buf: &mut [u8]) -> Result<usize> {
+        self.read_half.peek(buf).await
+    }
+
+    pub fn poll_peek(&mut self, cx: &mut Context<'_>, buf: &mut ReadBuf) -> Poll<Result<usize>> {
+        self.read_half.poll_peek(cx, buf)
+    }
 }
 
 pub(crate) struct ReadHalf {
@@ -234,6 +244,48 @@ impl ReadHalf {
             Some(avail)
         }
     }
+
+    pub(crate) fn poll_peek(
+        &mut self,
+        cx: &mut Context<'_>,
+        buf: &mut ReadBuf,
+    ) -> Poll<Result<usize>> {
+        if self.is_closed || buf.capacity() == 0 {
+            return Poll::Ready(Ok(0));
+        }
+
+        // If we have buffered data, peek from it
+        if let Some(bytes) = &self.rx.buffer {
+            let len = std::cmp::min(bytes.len(), buf.remaining());
+            buf.put_slice(&bytes[..len]);
+            return Poll::Ready(Ok(len));
+        }
+
+        match ready!(self.rx.recv.poll_recv(cx)) {
+            Some(seg) => match seg {
+                SequencedSegment::Data(bytes) => {
+                    let len = std::cmp::min(bytes.len(), buf.remaining());
+                    buf.put_slice(&bytes[..len]);
+                    self.rx.buffer = Some(bytes);
+
+                    Poll::Ready(Ok(len))
+                }
+                SequencedSegment::Fin => {
+                    self.is_closed = true;
+                    Poll::Ready(Ok(0))
+                }
+            },
+            None => Poll::Ready(Err(io::Error::new(
+                io::ErrorKind::ConnectionReset,
+                "Connection reset",
+            ))),
+        }
+    }
+
+    pub(crate) async fn peek(&mut self, buf: &mut [u8]) -> Result<usize> {
+        let mut buf = ReadBuf::new(buf);
+        poll_fn(|cx| self.poll_peek(cx, &mut buf)).await
+    }
 }
 
 impl Debug for ReadHalf {

tests/tcp.rs

@@ -749,6 +749,145 @@ fn split() -> Result {
     sim.run()
 }
 
+#[test]
+fn peek_empty_buffer() -> Result {
+    let mut sim = Builder::new().build();
+
+    sim.client("server", async move {
+        let listener = bind().await?;
+        let _ = listener.accept().await?;
+        Ok(())
+    });
+
+    sim.client("client", async move {
+        let mut s = TcpStream::connect(("server", PORT)).await?;
+
+        // no-op peek with empty buffer
+        let mut buf = [0; 0];
+        let n = s.peek(&mut buf).await?;
+        assert_eq!(0, n);
+
+        Ok(())
+    });
+
+    sim.run()
+}
+
+#[test]
+fn peek_then_read() -> Result {
+    let mut sim = Builder::new().build();
+
+    sim.client("server", async move {
+        let listener = bind().await?;
+        let (mut s, _) = listener.accept().await?;
+
+        s.write_u64(1234).await?;
+        Ok(())
+    });
+
+    sim.client("client", async move {
+        let mut s = TcpStream::connect(("server", PORT)).await?;
+
+        // peek full message
+        let mut peek_buf = [0; 8];
+        assert_eq!(8, s.peek(&mut peek_buf).await?);
+        assert_eq!(1234u64, u64::from_be_bytes(peek_buf));
+
+        // peek again should see same data
+        let mut peek_buf2 = [0; 8];
+        assert_eq!(8, s.peek(&mut peek_buf2).await?);
+        assert_eq!(1234u64, u64::from_be_bytes(peek_buf2));
+
+        // read should consume the data
+        assert_eq!(1234, s.read_u64().await?);
+        let mut buf = [0; 8];
+        assert!(matches!(s.read(&mut buf).await, Ok(0)));
+
+        Ok(())
+    });
+
+    sim.run()
+}
+
+#[test]
+fn peek_partial() -> Result {
+    let mut sim = Builder::new().build();
+
+    sim.client("server", async move {
+        let listener = bind().await?;
+        let (mut s, _) = listener.accept().await?;
+
+        s.write_all(&[0, 0, 1, 1]).await?;
+        Ok(())
+    });
+
+    sim.client("client", async move {
+        let mut s = TcpStream::connect(("server", PORT)).await?;
+
+        // peek with smaller buffer
+        let mut peek_buf = [0; 2];
+        assert_eq!(2, s.peek(&mut peek_buf).await?);
+        assert_eq!([0, 0], peek_buf);
+
+        // peek with larger buffer should still see all data
+        let mut peek_buf2 = [0; 4];
+        assert_eq!(4, s.peek(&mut peek_buf2).await?);
+        assert_eq!([0, 0, 1, 1], peek_buf2);
+
+        // read partial
+        let mut read_buf = [0; 2];
+        assert_eq!(2, s.read(&mut read_buf).await?);
+        assert_eq!([0, 0], read_buf);
+
+        // peek remaining
+        let mut peek_buf3 = [0; 2];
+        assert_eq!(2, s.peek(&mut peek_buf3).await?);
+        assert_eq!([1, 1], peek_buf3);
+
+        Ok(())
+    });
+
+    sim.run()
+}
+
+#[test]
+fn peek_multiple_messages() -> Result {
+    let mut sim = Builder::new().build();
+
+    sim.client("server", async move {
+        let listener = bind().await?;
+        let (mut s, _) = listener.accept().await?;
+
+        s.write_u64(1234).await?;
+        s.write_u64(5678).await?;
+        Ok(())
+    });
+
+    sim.client("client", async move {
+        let mut s = TcpStream::connect(("server", PORT)).await?;
+
+        // peek first message
+        let mut peek_buf = [0; 8];
+        assert_eq!(8, s.peek(&mut peek_buf).await?);
+        assert_eq!(1234u64, u64::from_be_bytes(peek_buf));
+
+        // read first message
+        assert_eq!(1234, s.read_u64().await?);
+
+        // peek second message
+        let mut peek_buf2 = [0; 8];
+        assert_eq!(8, s.peek(&mut peek_buf2).await?);
+        assert_eq!(5678u64, u64::from_be_bytes(peek_buf2));
+
+        // read second message
+        assert_eq!(5678, s.read_u64().await?);
+
+        Ok(())
+    });
+
+    sim.run()
+}
+
 // # IpVersion specific tests
 
 #[test]