io: implement vectored writes for write_buf (#7871)

tokio/src/io/util/write_buf.rs

@@ -3,7 +3,7 @@ use crate::io::AsyncWrite;
 use bytes::Buf;
 use pin_project_lite::pin_project;
 use std::future::Future;
-use std::io;
+use std::io::{self, IoSlice};
 use std::marker::PhantomPinned;
 use std::pin::Pin;
 use std::task::{ready, Context, Poll};
@@ -42,13 +42,22 @@ where
     type Output = io::Result<usize>;
 
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<usize>> {
+        const MAX_VECTOR_ELEMENTS: usize = 64;
+
         let me = self.project();
 
         if !me.buf.has_remaining() {
             return Poll::Ready(Ok(0));
         }
 
-        let n = ready!(Pin::new(me.writer).poll_write(cx, me.buf.chunk()))?;
+        let n = if me.writer.is_write_vectored() {
+            let mut slices = [IoSlice::new(&[]); MAX_VECTOR_ELEMENTS];
+            let cnt = me.buf.chunks_vectored(&mut slices);
+            ready!(Pin::new(&mut *me.writer).poll_write_vectored(cx, &slices[..cnt]))?
+        } else {
+            ready!(Pin::new(&mut *me.writer).poll_write(cx, me.buf.chunk()))?
+        };
+
         me.buf.advance(n);
         Poll::Ready(Ok(n))
     }

tokio/tests/io_write_buf.rs

@@ -54,3 +54,59 @@ async fn write_all() {
     assert_eq!(wr.cnt, 1);
     assert_eq!(buf.position(), 4);
 }
+
+#[tokio::test]
+async fn write_buf_vectored() {
+    struct Wr {
+        buf: BytesMut,
+        cnt: usize,
+    }
+
+    impl AsyncWrite for Wr {
+        fn poll_write(
+            self: Pin<&mut Self>,
+            _cx: &mut Context<'_>,
+            _buf: &[u8],
+        ) -> Poll<io::Result<usize>> {
+            panic!("shouldn't be called")
+        }
+
+        fn poll_flush(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+            Ok(()).into()
+        }
+
+        fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+            Ok(()).into()
+        }
+
+        fn poll_write_vectored(
+            mut self: Pin<&mut Self>,
+            _cx: &mut Context<'_>,
+            bufs: &[io::IoSlice<'_>],
+        ) -> Poll<Result<usize, io::Error>> {
+            let mut n = 0;
+            for buf in bufs {
+                self.buf.extend_from_slice(buf);
+                n += buf.len();
+            }
+            self.cnt += 1;
+            Ok(n).into()
+        }
+
+        fn is_write_vectored(&self) -> bool {
+            true
+        }
+    }
+
+    let mut wr = Wr {
+        buf: BytesMut::with_capacity(64),
+        cnt: 0,
+    };
+
+    let mut buf = Cursor::new(&b"hello world"[..]);
+
+    assert_ok!(wr.write_buf(&mut buf).await);
+    assert_eq!(wr.buf, b"hello world"[..]);
+    assert_eq!(wr.cnt, 1);
+    assert_eq!(buf.position(), 11);
+}