Create copy_buf_abortable, which enables to stop copying in the middle.

futures-util/src/abortable.rs

@@ -75,7 +75,7 @@ impl<T> Abortable<T> {
 /// in calls to `Abortable::new`.
 #[derive(Debug)]
 pub struct AbortRegistration {
-    inner: Arc<AbortInner>,
+    pub(crate) inner: Arc<AbortInner>,
 }
 
 /// A handle to an `Abortable` task.
@@ -100,9 +100,9 @@ impl AbortHandle {
 // Inner type storing the waker to awaken and a bool indicating that it
 // should be aborted.
 #[derive(Debug)]
-struct AbortInner {
-    waker: AtomicWaker,
-    aborted: AtomicBool,
+pub(crate) struct AbortInner {
+    pub(crate) waker: AtomicWaker,
+    pub(crate) aborted: AtomicBool,
 }
 
 /// Indicator that the `Abortable` task was aborted.

futures-util/src/io/copy_buf_abortable.rs

@@ -0,0 +1,124 @@
+use crate::abortable::{AbortHandle, AbortInner, Aborted};
+use futures_core::future::Future;
+use futures_core::task::{Context, Poll};
+use futures_io::{AsyncBufRead, AsyncWrite};
+use pin_project_lite::pin_project;
+use std::io;
+use std::pin::Pin;
+use std::sync::atomic::Ordering;
+use std::sync::Arc;
+
+/// Creates a future which copies all the bytes from one object to another, with its `AbortHandle`.
+///
+/// The returned future will copy all the bytes read from this `AsyncBufRead` into the
+/// `writer` specified. This future will only complete once abort has been requested or the `reader` has hit
+/// EOF and all bytes have been written to and flushed from the `writer`
+/// provided.
+///
+/// On success the number of bytes is returned. If aborted, `Aborted` is returned. Otherwise, the underlying error is returned.
+///
+/// # Examples
+///
+/// ```
+/// # futures::executor::block_on(async {
+/// use futures::io::{self, AsyncWriteExt, Cursor};
+/// use futures::future::Aborted;
+///
+/// let reader = Cursor::new([1, 2, 3, 4]);
+/// let mut writer = Cursor::new(vec![0u8; 5]);
+///
+/// let (fut, abort_handle) = io::copy_buf_abortable(reader, &mut writer);
+/// let bytes = fut.await;
+/// abort_handle.abort();
+/// writer.close().await.unwrap();
+/// match bytes {
+///     Ok(Ok(n)) => {
+///         assert_eq!(n, 4);
+///         assert_eq!(writer.into_inner(), [1, 2, 3, 4, 0]);
+///         Ok(n)
+///     },
+///     Ok(Err(a)) => {
+///         Err::<u64, Aborted>(a)
+///     }
+///     Err(e) => panic!("{}", e)
+/// }
+/// #  }).unwrap();
+/// ```
+pub fn copy_buf_abortable<R, W>(
+    reader: R,
+    writer: &mut W,
+) -> (CopyBufAbortable<'_, R, W>, AbortHandle)
+where
+    R: AsyncBufRead,
+    W: AsyncWrite + Unpin + ?Sized,
+{
+    let (handle, reg) = AbortHandle::new_pair();
+    (CopyBufAbortable { reader, writer, amt: 0, inner: reg.inner }, handle)
+}
+
+pin_project! {
+    /// Future for the [`copy_buf()`] function.
+    #[derive(Debug)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct CopyBufAbortable<'a, R, W: ?Sized> {
+        #[pin]
+        reader: R,
+        writer: &'a mut W,
+        amt: u64,
+        inner: Arc<AbortInner>
+    }
+}
+
+macro_rules! ready_or_break {
+    ($e:expr $(,)?) => {
+        match $e {
+            $crate::task::Poll::Ready(t) => t,
+            $crate::task::Poll::Pending => break,
+        }
+    };
+}
+
+impl<R, W> Future for CopyBufAbortable<'_, R, W>
+where
+    R: AsyncBufRead,
+    W: AsyncWrite + Unpin + Sized,
+{
+    type Output = Result<Result<u64, Aborted>, io::Error>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mut this = self.project();
+        loop {
+            // Check if the task has been aborted
+            if this.inner.aborted.load(Ordering::Relaxed) {
+                return Poll::Ready(Ok(Err(Aborted)));
+            }
+
+            // Read some bytes from the reader, and if we have reached EOF, return total bytes read
+            let buffer = ready_or_break!(this.reader.as_mut().poll_fill_buf(cx))?;
+            if buffer.is_empty() {
+                ready_or_break!(Pin::new(&mut this.writer).poll_flush(cx))?;
+                return Poll::Ready(Ok(Ok(*this.amt)));
+            }
+
+            // Pass the buffer to the writer, and update the amount written
+            let i = ready_or_break!(Pin::new(&mut this.writer).poll_write(cx, buffer))?;
+            if i == 0 {
+                return Poll::Ready(Err(io::ErrorKind::WriteZero.into()));
+            }
+            *this.amt += i as u64;
+            this.reader.as_mut().consume(i);
+        }
+        // Schedule the task to be woken up again.
+        // Never called unless Poll::Pending is returned from io objects.
+        this.inner.waker.register(cx.waker());
+
+        // Check to see if the task was aborted between the first check and
+        // registration.
+        // Checking with  `Relaxed` is sufficient because
+        // `register` introduces an `AcqRel` barrier.
+        if this.inner.aborted.load(Ordering::Relaxed) {
+            return Poll::Ready(Ok(Err(Aborted)));
+        }
+        Poll::Pending
+    }
+}

futures-util/src/io/mod.rs

@@ -76,6 +76,9 @@ pub use self::copy::{copy, Copy};
 mod copy_buf;
 pub use self::copy_buf::{copy_buf, CopyBuf};
 
+mod copy_buf_abortable;
+pub use self::copy_buf_abortable::{copy_buf_abortable, CopyBufAbortable};
+
 mod cursor;
 pub use self::cursor::Cursor;