io: use vectored io for write_all_buf when possible

tokio/src/io/util/write_all_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::{Context, Poll};
@@ -42,9 +42,17 @@ where
     type Output = io::Result<()>;
 
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        const MAX_VECTOR_ELEMENTS: usize = 64;
+
         let me = self.project();
         while me.buf.has_remaining() {
-            let n = ready!(Pin::new(&mut *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);
             if n == 0 {
                 return Poll::Ready(Err(io::ErrorKind::WriteZero.into()));

tokio/tests/io_write_all_buf.rs

@@ -94,3 +94,52 @@ async fn write_buf_err() {
         Bytes::from_static(b"oworld")
     );
 }
+
+#[tokio::test]
+async fn write_all_buf_vectored() {
+    struct Wr {
+        buf: BytesMut,
+    }
+    impl AsyncWrite for Wr {
+        fn poll_write(
+            self: Pin<&mut Self>,
+            _cx: &mut Context<'_>,
+            _buf: &[u8],
+        ) -> Poll<io::Result<usize>> {
+            // When executing `write_all_buf` with this writer,
+            // `poll_write` is not called.
+            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>> {
+            for buf in bufs {
+                self.buf.extend_from_slice(buf);
+            }
+            let n = self.buf.len();
+            Ok(n).into()
+        }
+        fn is_write_vectored(&self) -> bool {
+            // Enable vectored write.
+            true
+        }
+    }
+
+    let mut wr = Wr {
+        buf: BytesMut::with_capacity(64),
+    };
+    let mut buf = Bytes::from_static(b"hello")
+        .chain(Bytes::from_static(b" "))
+        .chain(Bytes::from_static(b"world"));
+
+    wr.write_all_buf(&mut buf).await.unwrap();
+    assert_eq!(&wr.buf[..], b"hello world");
+}