1 files changed, 145 insertions, 0 deletions
diff --git a/tokio/src/io/util/chain.rs b/tokio/src/io/util/chain.rs
new file mode 100644
index 00000000..76144069
--- /dev/null
+++ b/tokio/src/io/util/chain.rs
@@ -0,0 +1,145 @@
+use crate::io::{AsyncBufRead, AsyncRead};
+
+use pin_project::{pin_project, project};
+use std::fmt;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Stream for the [`chain`](super::AsyncReadExt::chain) method.
+#[pin_project]
+#[must_use = "streams do nothing unless polled"]
+pub struct Chain<T, U> {
+    #[pin]
+    first: T,
+    #[pin]
+    second: U,
+    done_first: bool,
+}
+
+pub(super) fn chain<T, U>(first: T, second: U) -> Chain<T, U>
+where
+    T: AsyncRead,
+    U: AsyncRead,
+{
+    Chain {
+        first,
+        second,
+        done_first: false,
+    }
+}
+
+impl<T, U> Chain<T, U>
+where
+    T: AsyncRead,
+    U: AsyncRead,
+{
+    /// Gets references to the underlying readers in this `Chain`.
+    pub fn get_ref(&self) -> (&T, &U) {
+        (&self.first, &self.second)
+    }
+
+    /// Gets mutable references to the underlying readers in this `Chain`.
+    ///
+    /// Care should be taken to avoid modifying the internal I/O state of the
+    /// underlying readers as doing so may corrupt the internal state of this
+    /// `Chain`.
+    pub fn get_mut(&mut self) -> (&mut T, &mut U) {
+        (&mut self.first, &mut self.second)
+    }
+
+    /// Gets pinned mutable references to the underlying readers in this `Chain`.
+    ///
+    /// Care should be taken to avoid modifying the internal I/O state of the
+    /// underlying readers as doing so may corrupt the internal state of this
+    /// `Chain`.
+    pub fn get_pin_mut(self: Pin<&mut Self>) -> (Pin<&mut T>, Pin<&mut U>) {
+        let me = self.project();
+        (me.first, me.second)
+    }
+
+    /// Consumes the `Chain`, returning the wrapped readers.
+    pub fn into_inner(self) -> (T, U) {
+        (self.first, self.second)
+    }
+}
+
+impl<T, U> fmt::Debug for Chain<T, U>
+where
+    T: fmt::Debug,
+    U: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Chain")
+            .field("t", &self.first)
+            .field("u", &self.second)
+            .finish()
+    }
+}
+
+impl<T, U> AsyncRead for Chain<T, U>
+where
+    T: AsyncRead,
+    U: AsyncRead,
+{
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        let me = self.project();
+
+        if !*me.done_first {
+            match ready!(me.first.poll_read(cx, buf)?) {
+                0 if !buf.is_empty() => *me.done_first = true,
+                n => return Poll::Ready(Ok(n)),
+            }
+        }
+        me.second.poll_read(cx, buf)
+    }
+}
+
+impl<T, U> AsyncBufRead for Chain<T, U>
+where
+    T: AsyncBufRead,
+    U: AsyncBufRead,
+{
+    #[project]
+    fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
+        #[project]
+        let Chain {
+            first,
+            second,
+            done_first,
+        } = self.project();
+
+        if !*done_first {
+            match ready!(first.poll_fill_buf(cx)?) {
+                buf if buf.is_empty() => {
+                    *done_first = true;
+                }
+                buf => return Poll::Ready(Ok(buf)),
+            }
+        }
+        second.poll_fill_buf(cx)
+    }
+
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        let me = self.project();
+        if !*me.done_first {
+            me.first.consume(amt)
+        } else {
+            me.second.consume(amt)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        crate::is_unpin::<Chain<(), ()>>();
+    }
+}