Implement vectored write functionality for files

tokio/src/fs/file.rs

@@ -725,6 +725,81 @@ impl AsyncWrite for File {
         }
     }
 
+    fn poll_write_vectored(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        bufs: &[io::IoSlice<'_>],
+    ) -> Poll<Result<usize, io::Error>> {
+        ready!(crate::trace::trace_leaf(cx));
+        let me = self.get_mut();
+        let inner = me.inner.get_mut();
+
+        if let Some(e) = inner.last_write_err.take() {
+            return Ready(Err(e.into()));
+        }
+
+        loop {
+            match inner.state {
+                Idle(ref mut buf_cell) => {
+                    let mut buf = buf_cell.take().unwrap();
+
+                    let seek = if !buf.is_empty() {
+                        Some(SeekFrom::Current(buf.discard_read()))
+                    } else {
+                        None
+                    };
+
+                    let n = buf.copy_from_bufs(bufs);
+                    let std = me.std.clone();
+
+                    let blocking_task_join_handle = spawn_mandatory_blocking(move || {
+                        let res = if let Some(seek) = seek {
+                            (&*std).seek(seek).and_then(|_| buf.write_to(&mut &*std))
+                        } else {
+                            buf.write_to(&mut &*std)
+                        };
+
+                        (Operation::Write(res), buf)
+                    })
+                    .ok_or_else(|| {
+                        io::Error::new(io::ErrorKind::Other, "background task failed")
+                    })?;
+
+                    inner.state = Busy(blocking_task_join_handle);
+
+                    return Ready(Ok(n));
+                }
+                Busy(ref mut rx) => {
+                    let (op, buf) = ready!(Pin::new(rx).poll(cx))?;
+                    inner.state = Idle(Some(buf));
+
+                    match op {
+                        Operation::Read(_) => {
+                            // We don't care about the result here. The fact
+                            // that the cursor has advanced will be reflected in
+                            // the next iteration of the loop
+                            continue;
+                        }
+                        Operation::Write(res) => {
+                            // If the previous write was successful, continue.
+                            // Otherwise, error.
+                            res?;
+                            continue;
+                        }
+                        Operation::Seek(_) => {
+                            // Ignore the seek
+                            continue;
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    fn is_write_vectored(&self) -> bool {
+        true
+    }
+
     fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
         ready!(crate::trace::trace_leaf(cx));
         let inner = self.inner.get_mut();

tokio/src/io/blocking.rs

@@ -276,5 +276,22 @@ cfg_fs! {
             self.buf.truncate(0);
             ret
         }
+
+        pub(crate) fn copy_from_bufs(&mut self, bufs: &[io::IoSlice<'_>]) -> usize {
+            assert!(self.is_empty());
+
+            let mut rem = MAX_BUF;
+            for buf in bufs {
+                if rem == 0 {
+                    break
+                }
+
+                let len = buf.len().min(rem);
+                self.buf.extend_from_slice(&buf[..len]);
+                rem -= len;
+            }
+
+            MAX_BUF - rem
+        }
     }
 }

tokio/tests/fs_file.rs

@@ -2,6 +2,7 @@
 #![cfg(all(feature = "full", not(target_os = "wasi")))] // WASI does not support all fs operations
 
 use std::io::prelude::*;
+use std::io::IoSlice;
 use tempfile::NamedTempFile;
 use tokio::fs::File;
 use tokio::io::{AsyncReadExt, AsyncSeekExt, AsyncWriteExt, SeekFrom};
@@ -49,6 +50,40 @@ async fn basic_write_and_shutdown() {
     assert_eq!(file, HELLO);
 }
 
+#[tokio::test]
+async fn write_vectored() {
+    let tempfile = tempfile();
+
+    let mut file = File::create(tempfile.path()).await.unwrap();
+
+    let ret = file
+        .write_vectored(&[IoSlice::new(HELLO), IoSlice::new(HELLO)])
+        .await
+        .unwrap();
+    assert_eq!(ret, HELLO.len() * 2);
+    file.flush().await.unwrap();
+
+    let file = std::fs::read(tempfile.path()).unwrap();
+    assert_eq!(file, [HELLO, HELLO].concat());
+}
+
+#[tokio::test]
+async fn write_vectored_and_shutdown() {
+    let tempfile = tempfile();
+
+    let mut file = File::create(tempfile.path()).await.unwrap();
+
+    let ret = file
+        .write_vectored(&[IoSlice::new(HELLO), IoSlice::new(HELLO)])
+        .await
+        .unwrap();
+    assert_eq!(ret, HELLO.len() * 2);
+    file.shutdown().await.unwrap();
+
+    let file = std::fs::read(tempfile.path()).unwrap();
+    assert_eq!(file, [HELLO, HELLO].concat());
+}
+
 #[tokio::test]
 async fn rewind_seek_position() {
     let tempfile = tempfile();