Re-enable writev support in TcpStreams (#1956)

tokio/Cargo.toml

@@ -85,7 +85,7 @@ signal = [
 stream = ["futures-core"]
 sync = ["fnv"]
 test-util = []
-tcp = ["io-driver"]
+tcp = ["io-driver", "iovec"]
 time = ["slab"]
 udp = ["io-driver"]
 uds = ["io-driver", "mio-uds", "libc"]
@@ -103,6 +103,7 @@ futures-core = { version = "0.3.0", optional = true }
 lazy_static = { version = "1.0.2", optional = true }
 memchr = { version = "2.2", optional = true }
 mio = { version = "0.6.20", optional = true }
+iovec = { version = "0.1.4", optional = true }
 num_cpus = { version = "1.8.0", optional = true }
 # Backs `DelayQueue`
 slab = { version = "0.4.1", optional = true }

tokio/src/net/tcp/split.rs

@@ -11,6 +11,7 @@
 use crate::io::{AsyncRead, AsyncWrite};
 use crate::net::TcpStream;
 
+use bytes::Buf;
 use std::io;
 use std::mem::MaybeUninit;
 use std::net::Shutdown;
@@ -55,6 +56,14 @@ impl AsyncWrite for WriteHalf<'_> {
         self.0.poll_write_priv(cx, buf)
     }
 
+    fn poll_write_buf<B: Buf>(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut B,
+    ) -> Poll<io::Result<usize>> {
+        self.0.poll_write_buf_priv(cx, buf)
+    }
+
     #[inline]
     fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
         // tcp flush is a no-op

tokio/src/net/tcp/stream.rs

@@ -3,6 +3,8 @@ use crate::io::{AsyncRead, AsyncWrite, PollEvented};
 use crate::net::tcp::split::{split, ReadHalf, WriteHalf};
 use crate::net::ToSocketAddrs;
 
+use bytes::Buf;
+use iovec::IoVec;
 use std::convert::TryFrom;
 use std::fmt;
 use std::io::{self, Read, Write};
@@ -639,7 +641,7 @@ impl TcpStream {
         }
     }
 
-    pub(crate) fn poll_write_priv(
+    pub(super) fn poll_write_priv(
         &self,
         cx: &mut Context<'_>,
         buf: &[u8],
@@ -654,6 +656,61 @@ impl TcpStream {
             x => Poll::Ready(x),
         }
     }
+
+    pub(super) fn poll_write_buf_priv<B: Buf>(
+        &self,
+        cx: &mut Context<'_>,
+        buf: &mut B,
+    ) -> Poll<io::Result<usize>> {
+        use std::io::IoSlice;
+
+        ready!(self.io.poll_write_ready(cx))?;
+
+        // The `IoVec` (v0.1.x) type can't have a zero-length size, so create
+        // a dummy version from a 1-length slice which we'll overwrite with
+        // the `bytes_vectored` method.
+        static S: &[u8] = &[0];
+        const MAX_BUFS: usize = 64;
+
+        // IoSlice isn't Copy, so we must expand this manually ;_;
+        let mut slices: [IoSlice<'_>; MAX_BUFS] = [
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+            IoSlice::new(S), IoSlice::new(S), IoSlice::new(S), IoSlice::new(S),
+        ];
+        let cnt = buf.bytes_vectored(&mut slices);
+
+        let iovec = <&IoVec>::from(S);
+        let mut vecs = [iovec; MAX_BUFS];
+        for i in 0..cnt {
+            vecs[i] = (*slices[i]).into();
+        }
+
+        match self.io.get_ref().write_bufs(&vecs[..cnt]) {
+            Ok(n) => {
+                buf.advance(n);
+                Poll::Ready(Ok(n))
+            },
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_write_ready(cx)?;
+                Poll::Pending
+            },
+            Err(e) => Poll::Ready(Err(e)),
+        }
+    }
 }
 
 impl TryFrom<TcpStream> for mio::net::TcpStream {
@@ -707,6 +764,14 @@ impl AsyncWrite for TcpStream {
         self.poll_write_priv(cx, buf)
     }
 
+    fn poll_write_buf<B: Buf>(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut B,
+    ) -> Poll<io::Result<usize>> {
+        self.poll_write_buf_priv(cx, buf)
+    }
+
     #[inline]
     fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
         // tcp flush is a no-op