process: expose AW::poll_write_vectored on unix

tests-integration/Cargo.toml

@@ -58,3 +58,4 @@ tokio = { path = "../tokio" }
 tokio-test = { path = "../tokio-test", optional = true }
 doc-comment = "0.3.1"
 futures = { version = "0.3.0", features = ["async-await"] }
+bytes = "1.0.0"

tests-integration/tests/process_stdio.rs

@@ -190,3 +190,54 @@ async fn pipe_from_one_command_to_another() {
     assert!(second_status.expect("second status").success());
     assert!(third_status.expect("third status").success());
 }
+
+#[tokio::test]
+async fn vectored_writes() {
+    use bytes::{Buf, Bytes};
+    use std::{io::IoSlice, pin::Pin};
+    use tokio::io::AsyncWrite;
+
+    let mut cat = cat().spawn().unwrap();
+    let mut stdin = cat.stdin.take().unwrap();
+    let are_writes_vectored = stdin.is_write_vectored();
+    let mut stdout = cat.stdout.take().unwrap();
+
+    let write = async {
+        let mut input = Bytes::from_static(b"hello\n").chain(Bytes::from_static(b"world!\n"));
+        let mut writes_completed = 0;
+
+        futures::future::poll_fn(|cx| loop {
+            let mut slices = [IoSlice::new(&[]); 2];
+            let vectored = input.chunks_vectored(&mut slices);
+            if vectored == 0 {
+                return std::task::Poll::Ready(std::io::Result::Ok(()));
+            }
+            let n = futures::ready!(Pin::new(&mut stdin).poll_write_vectored(cx, &slices))?;
+            writes_completed += 1;
+            input.advance(n);
+        })
+        .await?;
+
+        drop(stdin);
+
+        std::io::Result::Ok(writes_completed)
+    };
+
+    let read = async {
+        let mut buffer = Vec::with_capacity(6 + 7);
+        stdout.read_to_end(&mut buffer).await?;
+        std::io::Result::Ok(buffer)
+    };
+
+    let (write, read, status) = future::join3(write, read, cat.wait()).await;
+
+    assert!(status.unwrap().success());
+
+    let writes_completed = write.unwrap();
+    // on unix our small payload should always fit in whatever default sized pipe with a single
+    // syscall. if multiple are used, then the forwarding does not work, or we are on a platform
+    // for which the `std` does not support vectored writes.
+    assert_eq!(writes_completed == 1, are_writes_vectored);
+
+    assert_eq!(&read.unwrap(), b"hello\nworld!\n");
+}

tokio/src/io/poll_evented.rs

@@ -208,7 +208,7 @@ feature! {
             }
         }
 
-        #[cfg(feature = "net")]
+        #[cfg(any(feature = "net", feature = "process"))]
         pub(crate) fn poll_write_vectored<'a>(
             &'a self,
             cx: &mut Context<'_>,

tokio/src/process/mod.rs

@@ -1329,6 +1329,18 @@ impl AsyncWrite for ChildStdin {
     fn poll_shutdown(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
         Pin::new(&mut self.inner).poll_shutdown(cx)
     }
+
+    fn poll_write_vectored(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        bufs: &[io::IoSlice<'_>],
+    ) -> Poll<Result<usize, io::Error>> {
+        Pin::new(&mut self.inner).poll_write_vectored(cx, bufs)
+    }
+
+    fn is_write_vectored(&self) -> bool {
+        self.inner.is_write_vectored()
+    }
 }
 
 impl AsyncRead for ChildStdout {

tokio/src/process/unix/mod.rs

@@ -182,6 +182,10 @@ impl<'a> io::Write for &'a Pipe {
     fn flush(&mut self) -> io::Result<()> {
         (&self.fd).flush()
     }
+
+    fn write_vectored(&mut self, bufs: &[io::IoSlice<'_>]) -> io::Result<usize> {
+        (&self.fd).write_vectored(bufs)
+    }
 }
 
 impl AsRawFd for Pipe {
@@ -258,6 +262,18 @@ impl AsyncWrite for ChildStdio {
     fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
         Poll::Ready(Ok(()))
     }
+
+    fn poll_write_vectored(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        bufs: &[io::IoSlice<'_>],
+    ) -> Poll<Result<usize, io::Error>> {
+        self.inner.poll_write_vectored(cx, bufs)
+    }
+
+    fn is_write_vectored(&self) -> bool {
+        true
+    }
 }
 
 impl AsyncRead for ChildStdio {