From dc975eca907fabb62fd48cee82606b39900b47b8 Mon Sep 17 00:00:00 2001
From: Wim Looman <wim@nemo157.com>
Date: Fri, 14 Jun 2019 20:08:10 +0200
Subject: [PATCH] Add AsyncBufReadExt::copy_buf_into

---
 futures-util/src/io/copy_buf_into.rs | 58 ++++++++++++++++++++++++++++
 futures-util/src/io/mod.rs           | 33 ++++++++++++++++
 2 files changed, 91 insertions(+)
 create mode 100644 futures-util/src/io/copy_buf_into.rs

diff --git a/futures-util/src/io/copy_buf_into.rs b/futures-util/src/io/copy_buf_into.rs
new file mode 100644
index 0000000000..0ceaec8e0e
--- /dev/null
+++ b/futures-util/src/io/copy_buf_into.rs
@@ -0,0 +1,58 @@
+use futures_core::future::Future;
+use futures_core::task::{Context, Poll};
+use futures_io::{AsyncBufRead, AsyncWrite};
+use std::io;
+use std::pin::Pin;
+
+/// Future for the [`copy_buf_into`](super::AsyncBufReadExt::copy_buf_into) method.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct CopyBufInto<R, W> {
+    reader: R,
+    writer: W,
+    amt: u64,
+}
+
+impl<R: Unpin, W: Unpin> Unpin for CopyBufInto<R, W> {}
+
+impl<R, W> CopyBufInto<R, W> {
+    pub(super) fn new(reader: R, writer: W) -> Self {
+        CopyBufInto {
+            reader,
+            writer,
+            amt: 0,
+        }
+    }
+
+    fn project<'a>(self: Pin<&'a mut Self>) -> (Pin<&'a mut R>, Pin<&'a mut W>, &'a mut u64) {
+        unsafe {
+            let this = self.get_unchecked_mut();
+            (Pin::new_unchecked(&mut this.reader), Pin::new_unchecked(&mut this.writer), &mut this.amt)
+        }
+    }
+}
+
+impl<R, W> Future for CopyBufInto<R, W>
+    where R: AsyncBufRead,
+          W: AsyncWrite,
+{
+    type Output = io::Result<u64>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let (mut reader, mut writer, amt) = self.project();
+        loop {
+            let buffer = ready!(reader.as_mut().poll_fill_buf(cx))?;
+            if buffer.is_empty() {
+                ready!(writer.as_mut().poll_flush(cx))?;
+                return Poll::Ready(Ok(*amt));
+            }
+
+            let i = ready!(writer.as_mut().poll_write(cx, buffer))?;
+            if i == 0 {
+                return Poll::Ready(Err(io::ErrorKind::WriteZero.into()))
+            }
+            *amt += i as u64;
+            reader.as_mut().consume(i);
+        }
+    }
+}
diff --git a/futures-util/src/io/mod.rs b/futures-util/src/io/mod.rs
index a08b74ea9d..43bb4f4da9 100644
--- a/futures-util/src/io/mod.rs
+++ b/futures-util/src/io/mod.rs
@@ -31,6 +31,9 @@ pub use self::buf_writer::BufWriter;
 mod copy_into;
 pub use self::copy_into::CopyInto;
 
+mod copy_buf_into;
+pub use self::copy_buf_into::CopyBufInto;
+
 mod flush;
 pub use self::flush::Flush;
 
@@ -404,6 +407,36 @@ impl<S: AsyncSeek + ?Sized> AsyncSeekExt for S {}
 
 /// An extension trait which adds utility methods to `AsyncBufRead` types.
 pub trait AsyncBufReadExt: AsyncBufRead {
+    /// Creates a future which copies all the bytes from one object to another.
+    ///
+    /// The returned future will copy all the bytes read from this `AsyncBufRead` into the
+    /// `writer` specified. This future will only complete once 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.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(async_await)]
+    /// # futures::executor::block_on(async {
+    /// use futures::io::AsyncBufReadExt;
+    /// use std::io::Cursor;
+    ///
+    /// let mut reader = Cursor::new([1, 2, 3, 4]);
+    /// let mut writer = Cursor::new([0u8; 5]);
+    ///
+    /// let bytes = reader.copy_buf_into(&mut writer).await?;
+    ///
+    /// assert_eq!(bytes, 4);
+    /// assert_eq!(writer.into_inner(), [1, 2, 3, 4, 0]);
+    /// # Ok::<(), Box<std::error::Error>>(()) }).unwrap();
+    /// ```
+    fn copy_buf_into<W: AsyncWrite>(self, writer: W) -> CopyBufInto<Self, W> where Self: Sized {
+        CopyBufInto::new(self, writer)
+    }
+
     /// Creates a future which will read all the bytes associated with this I/O
     /// object into `buf` until the delimiter `byte` or EOF is reached.
     /// This method is the async equivalent to [`BufRead::read_until`](std::io::BufRead::read_until).