fs: move into `tokio` (#1672)

A step towards collapsing Tokio sub crates into a single `tokio`
crate (#1318).

The `fs` implementation is now provided by the main `tokio` crate. The
`fs` functionality may still be excluded from the build by skipping the
`fs` feature flag.

25 files changed, 1659 insertions, 0 deletions

diff --git a/tokio/src/fs/blocking.rs b/tokio/src/fs/blocking.rs
new file mode 100644
index 00000000..71b627c1
--- /dev/null
+++ b/tokio/src/fs/blocking.rs
@@ -0,0 +1,275 @@
+use crate::fs::sys;
+
+use tokio_io::{AsyncRead, AsyncWrite};
+
+use futures_core::ready;
+use std::cmp;
+use std::future::Future;
+use std::io;
+use std::io::prelude::*;
+use std::pin::Pin;
+use std::task::Poll::*;
+use std::task::{Context, Poll};
+
+use self::State::*;
+
+/// `T` should not implement _both_ Read and Write.
+#[derive(Debug)]
+pub(crate) struct Blocking<T> {
+    inner: Option<T>,
+    state: State<T>,
+    /// true if the lower IO layer needs flushing
+    need_flush: bool,
+}
+
+#[derive(Debug)]
+pub(crate) struct Buf {
+    buf: Vec<u8>,
+    pos: usize,
+}
+
+pub(crate) const MAX_BUF: usize = 16 * 1024;
+
+#[derive(Debug)]
+enum State<T> {
+    Idle(Option<Buf>),
+    Busy(sys::Blocking<(io::Result<usize>, Buf, T)>),
+}
+
+impl<T> Blocking<T> {
+    pub(crate) fn new(inner: T) -> Blocking<T> {
+        Blocking {
+            inner: Some(inner),
+            state: State::Idle(Some(Buf::with_capacity(0))),
+            need_flush: false,
+        }
+    }
+}
+
+impl<T> AsyncRead for Blocking<T>
+where
+    T: Read + Unpin + Send + 'static,
+{
+    fn poll_read(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        dst: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        loop {
+            match self.state {
+                Idle(ref mut buf_cell) => {
+                    let mut buf = buf_cell.take().unwrap();
+
+                    if !buf.is_empty() {
+                        let n = buf.copy_to(dst);
+                        *buf_cell = Some(buf);
+                        return Ready(Ok(n));
+                    }
+
+                    buf.ensure_capacity_for(dst);
+                    let mut inner = self.inner.take().unwrap();
+
+                    self.state = Busy(sys::run(move || {
+                        let res = buf.read_from(&mut inner);
+                        (res, buf, inner)
+                    }));
+                }
+                Busy(ref mut rx) => {
+                    let (res, mut buf, inner) = ready!(Pin::new(rx).poll(cx));
+                    self.inner = Some(inner);
+
+                    match res {
+                        Ok(_) => {
+                            let n = buf.copy_to(dst);
+                            self.state = Idle(Some(buf));
+                            return Ready(Ok(n));
+                        }
+                        Err(e) => {
+                            assert!(buf.is_empty());
+
+                            self.state = Idle(Some(buf));
+                            return Ready(Err(e));
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+impl<T> AsyncWrite for Blocking<T>
+where
+    T: Write + Unpin + Send + 'static,
+{
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        src: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        loop {
+            match self.state {
+                Idle(ref mut buf_cell) => {
+                    let mut buf = buf_cell.take().unwrap();
+
+                    assert!(buf.is_empty());
+
+                    let n = buf.copy_from(src);
+                    let mut inner = self.inner.take().unwrap();
+
+                    self.state = Busy(sys::run(move || {
+                        let n = buf.len();
+                        let res = buf.write_to(&mut inner).map(|_| n);
+
+                        (res, buf, inner)
+                    }));
+                    self.need_flush = true;
+
+                    return Ready(Ok(n));
+                }
+                Busy(ref mut rx) => {
+                    let (res, buf, inner) = ready!(Pin::new(rx).poll(cx));
+                    self.state = Idle(Some(buf));
+                    self.inner = Some(inner);
+
+                    // If error, return
+                    res?;
+                }
+            }
+        }
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        loop {
+            let need_flush = self.need_flush;
+            match self.state {
+                // The buffer is not used here
+                Idle(ref mut buf_cell) => {
+                    if need_flush {
+                        let buf = buf_cell.take().unwrap();
+                        let mut inner = self.inner.take().unwrap();
+
+                        self.state = Busy(sys::run(move || {
+                            let res = inner.flush().map(|_| 0);
+                            (res, buf, inner)
+                        }));
+
+                        self.need_flush = false;
+                    } else {
+                        return Ready(Ok(()));
+                    }
+                }
+                Busy(ref mut rx) => {
+                    let (res, buf, inner) = ready!(Pin::new(rx).poll(cx));
+                    self.state = Idle(Some(buf));
+                    self.inner = Some(inner);
+
+                    // If error, return
+                    res?;
+                }
+            }
+        }
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        Poll::Ready(Ok(()))
+    }
+}
+
+/// Repeates operations that are interrupted
+macro_rules! uninterruptibly {
+    ($e:expr) => {{
+        loop {
+            match $e {
+                Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {}
+                res => break res,
+            }
+        }
+    }};
+}
+
+impl Buf {
+    pub(crate) fn with_capacity(n: usize) -> Buf {
+        Buf {
+            buf: Vec::with_capacity(n),
+            pos: 0,
+        }
+    }
+
+    pub(crate) fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    pub(crate) fn len(&self) -> usize {
+        self.buf.len() - self.pos
+    }
+
+    pub(crate) fn copy_to(&mut self, dst: &mut [u8]) -> usize {
+        let n = cmp::min(self.len(), dst.len());
+        dst[..n].copy_from_slice(&self.bytes()[..n]);
+        self.pos += n;
+
+        if self.pos == self.buf.len() {
+            self.buf.truncate(0);
+            self.pos = 0;
+        }
+
+        n
+    }
+
+    pub(crate) fn copy_from(&mut self, src: &[u8]) -> usize {
+        assert!(self.is_empty());
+
+        let n = cmp::min(src.len(), MAX_BUF);
+
+        self.buf.extend_from_slice(&src[..n]);
+        n
+    }
+
+    pub(crate) fn bytes(&self) -> &[u8] {
+        &self.buf[self.pos..]
+    }
+
+    pub(crate) fn ensure_capacity_for(&mut self, bytes: &[u8]) {
+        assert!(self.is_empty());
+
+        let len = cmp::min(bytes.len(), MAX_BUF);
+
+        if self.buf.len() < len {
+            self.buf.reserve(len - self.buf.len());
+        }
+
+        unsafe {
+            self.buf.set_len(len);
+        }
+    }
+
+    pub(crate) fn read_from<T: Read>(&mut self, rd: &mut T) -> io::Result<usize> {
+        let res = uninterruptibly!(rd.read(&mut self.buf));
+
+        if let Ok(n) = res {
+            self.buf.truncate(n);
+        } else {
+            self.buf.clear();
+        }
+
+        assert_eq!(self.pos, 0);
+
+        res
+    }
+
+    pub(crate) fn write_to<T: Write>(&mut self, wr: &mut T) -> io::Result<()> {
+        assert_eq!(self.pos, 0);
+
+        // `write_all` already ignores interrupts
+        let res = wr.write_all(&self.buf);
+        self.buf.clear();
+        res
+    }
+
+    pub(crate) fn discard_read(&mut self) -> i64 {
+        let ret = -(self.bytes().len() as i64);
+        self.pos = 0;
+        self.buf.truncate(0);
+        ret
+    }
+}
diff --git a/tokio/src/fs/create_dir.rs b/tokio/src/fs/create_dir.rs
new file mode 100644
index 00000000..a74ca71d
--- /dev/null
+++ b/tokio/src/fs/create_dir.rs
@@ -0,0 +1,14 @@
+use crate::fs::asyncify;
+
+use std::io;
+use std::path::Path;
+
+/// Creates a new, empty directory at the provided path
+///
+/// This is an async version of [`std::fs::create_dir`][std]
+///
+/// [std]: https://doc.rust-lang.org/std/fs/fn.create_dir.html
+pub async fn create_dir<P: AsRef<Path>>(path: P) -> io::Result<()> {
+    let path = path.as_ref().to_owned();
+    asyncify(move || std::fs::create_dir(path)).await
+}
diff --git a/tokio/src/fs/create_dir_all.rs b/tokio/src/fs/create_dir_all.rs
new file mode 100644
index 00000000..2a7374a3
--- /dev/null
+++ b/tokio/src/fs/create_dir_all.rs
@@ -0,0 +1,15 @@
+use crate::fs::asyncify;
+
+use std::io;
+use std::path::Path;
+
+/// Recursively create a directory and all of its parent components if they
+/// are missing.
+///
+/// This is an async version of [`std::fs::create_dir_all`][std]
+///
+/// [std]: https://doc.rust-lang.org/std/fs/fn.create_dir_all.html
+pub async fn create_dir_all<P: AsRef<Path>>(path: P) -> io::Result<()> {
+    let path = path.as_ref().to_owned();
+    asyncify(move || std::fs::create_dir_all(path)).await
+}
diff --git a/tokio/src/fs/file.rs b/tokio/src/fs/file.rs
new file mode 100644
index 00000000..7c3a6948
--- /dev/null
+++ b/tokio/src/fs/file.rs
@@ -0,0 +1,610 @@
+//! Types for working with [`File`].
+//!
+//! [`File`]: file/struct.File.html
+
+use self::State::*;
+use crate::fs::blocking::Buf;
+use crate::fs::{asyncify, sys};
+
+use tokio_io::{AsyncRead, AsyncWrite};
+
+use futures_core::ready;
+use std::fmt;
+use std::fs::{Metadata, Permissions};
+use std::future::Future;
+use std::io::{self, Seek, SeekFrom};
+use std::path::Path;
+use std::pin::Pin;
+use std::sync::Arc;
+use std::task::Context;
+use std::task::Poll;
+use std::task::Poll::*;
+
+/// A reference to an open file on the filesystem.
+///
+/// This is a specialized version of [`std::fs::File`][std] for usage from the
+/// Tokio runtime.
+///
+/// An instance of a `File` can be read and/or written depending on what options
+/// it was opened with. Files also implement Seek to alter the logical cursor
+/// that the file contains internally.
+///
+/// Files are automatically closed when they go out of scope.
+///
+/// [std]: https://doc.rust-lang.org/std/fs/struct.File.html
+///
+/// # Examples
+///
+/// Create a new file and asynchronously write bytes to it:
+///
+/// ```no_run
+/// use tokio::fs::File;
+/// use tokio::prelude::*;
+///
+/// # async fn dox() -> std::io::Result<()> {
+/// let mut file = File::create("foo.txt").await?;
+/// file.write_all(b"hello, world!").await?;
+/// # Ok(())
+/// # }
+/// ```
+///
+/// Read the contents of a file into a buffer
+///
+/// ```no_run
+/// use tokio::fs::File;
+/// use tokio::prelude::*;
+///
+/// # async fn dox() -> std::io::Result<()> {
+/// let mut file = File::open("foo.txt").await?;
+///
+/// let mut contents = vec![];
+/// file.read_to_end(&mut contents).await?;
+///
+/// println!("len = {}", contents.len());
+/// # Ok(())
+/// # }
+/// ```
+pub struct File {
+    std: Arc<sys::File>,
+    state: State,
+
+    /// Errors from writes/flushes are returned in write/flush calls. If a write
+    /// error is observed while performing a read, it is saved until the next
+    /// write / flush call.
+    last_write_err: Option<io::ErrorKind>,
+}
+
+#[derive(Debug)]
+enum State {
+    Idle(Option<Buf>),
+    Busy(sys::Blocking<(Operation, Buf)>),
+}
+
+#[derive(Debug)]
+enum Operation {
+    Read(io::Result<usize>),
+    Write(io::Result<()>),
+    Seek(io::Result<u64>),
+}
+
+impl File {
+    /// Attempts to open a file in read-only mode.
+    ///
+    /// See [`OpenOptions`] for more details.
+    ///
+    /// [`OpenOptions`]: struct.OpenOptions.html
+    ///
+    /// # Errors
+    ///
+    /// This function will return an error if called from outside of the Tokio
+    /// runtime or if path does not already exist. Other errors may also be
+    /// returned according to OpenOptions::open.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::open("foo.txt").await?;
+    ///
+    /// let mut contents = vec![];
+    /// file.read_to_end(&mut contents).await?;
+    ///
+    /// println!("len = {}", contents.len());
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn open<P>(path: P) -> io::Result<File>
+    where
+        P: AsRef<Path>,
+    {
+        let path = path.as_ref().to_owned();
+        let std = asyncify(|| sys::File::open(path)).await?;
+
+        Ok(File::from_std(std))
+    }
+
+    /// Opens a file in write-only mode.
+    ///
+    /// This function will create a file if it does not exist, and will truncate
+    /// it if it does.
+    ///
+    /// See [`OpenOptions`] for more details.
+    ///
+    /// [`OpenOptions`]: struct.OpenOptions.html
+    ///
+    /// # Errors
+    ///
+    /// Results in an error if called from outside of the Tokio runtime or if
+    /// the underlying [`create`] call results in an error.
+    ///
+    /// [`create`]: https://doc.rust-lang.org/std/fs/struct.File.html#method.create
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::create("foo.txt").await?;
+    /// file.write_all(b"hello, world!").await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn create<P>(path: P) -> io::Result<File>
+    where
+        P: AsRef<Path>,
+    {
+        let path = path.as_ref().to_owned();
+        let std_file = asyncify(move || sys::File::create(path)).await?;
+        Ok(File::from_std(std_file))
+    }
+
+    /// Convert a [`std::fs::File`][std] to a [`tokio_fs::File`][file].
+    ///
+    /// [std]: https://doc.rust-lang.org/std/fs/struct.File.html
+    /// [file]: struct.File.html
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// // This line could block. It is not recommended to do this on the Tokio
+    /// // runtime.
+    /// let std_file = std::fs::File::open("foo.txt").unwrap();
+    /// let file = tokio::fs::File::from_std(std_file);
+    /// ```
+    pub fn from_std(std: sys::File) -> File {
+        File {
+            std: Arc::new(std),
+            state: State::Idle(Some(Buf::with_capacity(0))),
+            last_write_err: None,
+        }
+    }
+
+    /// Seek to an offset, in bytes, in a stream.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// use std::io::SeekFrom;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::open("foo.txt").await?;
+    /// file.seek(SeekFrom::Start(6)).await?;
+    ///
+    /// let mut contents = vec![0u8; 10];
+    /// file.read_exact(&mut contents).await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn seek(&mut self, mut pos: SeekFrom) -> io::Result<u64> {
+        self.complete_inflight().await;
+
+        let mut buf = match self.state {
+            Idle(ref mut buf_cell) => buf_cell.take().unwrap(),
+            _ => unreachable!(),
+        };
+
+        // Factor in any unread data from the buf
+        if !buf.is_empty() {
+            let n = buf.discard_read();
+
+            if let SeekFrom::Current(ref mut offset) = pos {
+                *offset += n;
+            }
+        }
+
+        let std = self.std.clone();
+
+        // Start the operation
+        self.state = Busy(sys::run(move || {
+            let res = (&*std).seek(pos);
+            (Operation::Seek(res), buf)
+        }));
+
+        let (op, buf) = match self.state {
+            Idle(_) => unreachable!(),
+            Busy(ref mut rx) => rx.await,
+        };
+
+        self.state = Idle(Some(buf));
+
+        match op {
+            Operation::Seek(res) => res,
+            _ => unreachable!(),
+        }
+    }
+
+    /// Attempts to sync all OS-internal metadata to disk.
+    ///
+    /// This function will attempt to ensure that all in-core data reaches the
+    /// filesystem before returning.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::create("foo.txt").await?;
+    /// file.write_all(b"hello, world!").await?;
+    /// file.sync_all().await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn sync_all(&mut self) -> io::Result<()> {
+        self.complete_inflight().await;
+
+        let std = self.std.clone();
+        asyncify(move || std.sync_all()).await
+    }
+
+    /// This function is similar to `sync_all`, except that it may not
+    /// synchronize file metadata to the filesystem.
+    ///
+    /// This is intended for use cases that must synchronize content, but don't
+    /// need the metadata on disk. The goal of this method is to reduce disk
+    /// operations.
+    ///
+    /// Note that some platforms may simply implement this in terms of `sync_all`.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::create("foo.txt").await?;
+    /// file.write_all(b"hello, world!").await?;
+    /// file.sync_data().await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn sync_data(&mut self) -> io::Result<()> {
+        self.complete_inflight().await;
+
+        let std = self.std.clone();
+        asyncify(move || std.sync_data()).await
+    }
+
+    /// Truncates or extends the underlying file, updating the size of this file to become size.
+    ///
+    /// If the size is less than the current file's size, then the file will be
+    /// shrunk. If it is greater than the current file's size, then the file
+    /// will be extended to size and have all of the intermediate data filled in
+    /// with 0s.
+    ///
+    /// # Errors
+    ///
+    /// This function will return an error if the file is not opened for
+    /// writing.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::create("foo.txt").await?;
+    /// file.write_all(b"hello, world!").await?;
+    /// file.set_len(10).await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn set_len(&mut self, size: u64) -> io::Result<()> {
+        self.complete_inflight().await;
+
+        let mut buf = match self.state {
+            Idle(ref mut buf_cell) => buf_cell.take().unwrap(),
+            _ => unreachable!(),
+        };
+
+        let seek = if !buf.is_empty() {
+            Some(SeekFrom::Current(buf.discard_read()))
+        } else {
+            None
+        };
+
+        let std = self.std.clone();
+
+        self.state = Busy(sys::run(move || {
+            let res = if let Some(seek) = seek {
+                (&*std).seek(seek).and_then(|_| std.set_len(size))
+            } else {
+                std.set_len(size)
+            }
+            .map(|_| 0); // the value is discarded later
+
+            // Return the result as a seek
+            (Operation::Seek(res), buf)
+        }));
+
+        let (op, buf) = match self.state {
+            Idle(_) => unreachable!(),
+            Busy(ref mut rx) => rx.await,
+        };
+
+        self.state = Idle(Some(buf));
+
+        match op {
+            Operation::Seek(res) => res.map(|_| ()),
+            _ => unreachable!(),
+        }
+    }
+
+    /// Queries metadata about the underlying file.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let file = File::open("foo.txt").await?;
+    /// let metadata = file.metadata().await?;
+    ///
+    /// println!("{:?}", metadata);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn metadata(&self) -> io::Result<Metadata> {
+        let std = self.std.clone();
+        asyncify(move || std.metadata()).await
+    }
+
+    /// Create a new `File` instance that shares the same underlying file handle
+    /// as the existing `File` instance. Reads, writes, and seeks will affect both
+    /// File instances simultaneously.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let file = File::open("foo.txt").await?;
+    /// let file_clone = file.try_clone().await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn try_clone(&self) -> io::Result<File> {
+        let std = self.std.clone();
+        let std_file = asyncify(move || std.try_clone()).await?;
+        Ok(File::from_std(std_file))
+    }
+
+    /// Changes the permissions on the underlying file.
+    ///
+    /// # Platform-specific behavior
+    ///
+    /// This function currently corresponds to the `fchmod` function on Unix and
+    /// the `SetFileInformationByHandle` function on Windows. Note that, this
+    /// [may change in the future][changes].
+    ///
+    /// [changes]: https://doc.rust-lang.org/std/io/index.html#platform-specific-behavior
+    ///
+    /// # Errors
+    ///
+    /// This function will return an error if the user lacks permission change
+    /// attributes on the underlying file. It may also return an error in other
+    /// os-specific unspecified cases.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let file = File::open("foo.txt").await?;
+    /// let mut perms = file.metadata().await?.permissions();
+    /// perms.set_readonly(true);
+    /// file.set_permissions(perms).await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn set_permissions(&self, perm: Permissions) -> io::Result<()> {
+        let std = self.std.clone();
+        asyncify(move || std.set_permissions(perm)).await
+    }
+
+    async fn complete_inflight(&mut self) {
+        use futures_util::future::poll_fn;
+
+        if let Err(e) = poll_fn(|cx| Pin::new(&mut *self).poll_flush(cx)).await {
+            self.last_write_err = Some(e.kind());
+        }
+    }
+}
+
+impl AsyncRead for File {
+    fn poll_read(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        dst: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        loop {
+            match self.state {
+                Idle(ref mut buf_cell) => {
+                    let mut buf = buf_cell.take().unwrap();
+
+                    if !buf.is_empty() {
+                        let n = buf.copy_to(dst);
+                        *buf_cell = Some(buf);
+                        return Ready(Ok(n));
+                    }
+
+                    buf.ensure_capacity_for(dst);
+                    let std = self.std.clone();
+
+                    self.state = Busy(sys::run(move || {
+                        let res = buf.read_from(&mut &*std);
+                        (Operation::Read(res), buf)
+                    }));
+                }
+                Busy(ref mut rx) => {
+                    let (op, mut buf) = ready!(Pin::new(rx).poll(cx));
+
+                    match op {
+                        Operation::Read(Ok(_)) => {
+                            let n = buf.copy_to(dst);
+                            self.state = Idle(Some(buf));
+                            return Ready(Ok(n));
+                        }
+                        Operation::Read(Err(e)) => {
+                            assert!(buf.is_empty());
+
+                            self.state = Idle(Some(buf));
+                            return Ready(Err(e));
+                        }
+                        Operation::Write(Ok(_)) => {
+                            assert!(buf.is_empty());
+                            self.state = Idle(Some(buf));
+                            continue;
+                        }
+                        Operation::Write(Err(e)) => {
+                            assert!(self.last_write_err.is_none());
+                            self.last_write_err = Some(e.kind());
+                            self.state = Idle(Some(buf));
+                        }
+                        Operation::Seek(_) => {
+                            assert!(buf.is_empty());
+                            self.state = Idle(Some(buf));
+                            continue;
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+impl AsyncWrite for File {
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        src: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        if let Some(e) = self.last_write_err.take() {
+            return Ready(Err(e.into()));
+        }
+
+        loop {
+            match self.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(src);
+                    let std = self.std.clone();
+
+                    self.state = Busy(sys::run(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)
+                    }));
+
+                    return Ready(Ok(n));
+                }
+                Busy(ref mut rx) => {
+                    let (op, buf) = ready!(Pin::new(rx).poll(cx));
+                    self.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 poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        if let Some(e) = self.last_write_err.take() {
+            return Ready(Err(e.into()));
+        }
+
+        let (op, buf) = match self.state {
+            Idle(_) => return Ready(Ok(())),
+            Busy(ref mut rx) => ready!(Pin::new(rx).poll(cx)),
+        };
+
+        // The buffer is not used here
+        self.state = Idle(Some(buf));
+
+        match op {
+            Operation::Read(_) => Ready(Ok(())),
+            Operation::Write(res) => Ready(res),
+            Operation::Seek(_) => Ready(Ok(())),
+        }
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>