From 0e1adc58f48a65c6af1d2ededa8712426fb3ab6e Mon Sep 17 00:00:00 2001
From: Dario Nieuwenhuis <dirbaio@dirbaio.net>
Date: Mon, 19 Oct 2020 21:13:26 +0200
Subject: [PATCH] Add AsyncBufReadExt::drain

---
 embassy/src/io/util/drain.rs | 43 ++++++++++++++++++++++++++++++++++++
 embassy/src/io/util/mod.rs   | 10 +++++++++
 2 files changed, 53 insertions(+)
 create mode 100644 embassy/src/io/util/drain.rs

diff --git a/embassy/src/io/util/drain.rs b/embassy/src/io/util/drain.rs
new file mode 100644
index 000000000..d3f21d071
--- /dev/null
+++ b/embassy/src/io/util/drain.rs
@@ -0,0 +1,43 @@
+use core::iter::Iterator;
+use core::pin::Pin;
+use futures::future::Future;
+use futures::ready;
+use futures::task::{Context, Poll};
+
+use super::super::error::{Error, Result};
+use super::super::traits::AsyncBufRead;
+
+pub struct Drain<'a, R: ?Sized> {
+    reader: &'a mut R,
+}
+
+impl<R: ?Sized + Unpin> Unpin for Drain<'_, R> {}
+
+impl<'a, R: AsyncBufRead + ?Sized + Unpin> Drain<'a, R> {
+    pub(super) fn new(reader: &'a mut R) -> Self {
+        Self { reader }
+    }
+}
+
+impl<'a, R: AsyncBufRead + ?Sized + Unpin> Future for Drain<'a, R> {
+    type Output = Result<usize>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let Self { reader } = &mut *self;
+        let mut reader = Pin::new(reader);
+
+        let mut n = 0;
+
+        loop {
+            match reader.as_mut().poll_fill_buf(cx) {
+                Poll::Pending => return Poll::Ready(Ok(n)),
+                Poll::Ready(Err(e)) => return Poll::Ready(Err(e)),
+                Poll::Ready(Ok(buf)) => {
+                    let len = buf.len();
+                    n += len;
+                    reader.as_mut().consume(len);
+                }
+            }
+        }
+    }
+}
diff --git a/embassy/src/io/util/mod.rs b/embassy/src/io/util/mod.rs
index c95a23f0a..a0bda6e36 100644
--- a/embassy/src/io/util/mod.rs
+++ b/embassy/src/io/util/mod.rs
@@ -24,6 +24,9 @@ pub use self::read_to_end::ReadToEnd;
 mod skip_while;
 pub use self::skip_while::SkipWhile;
 
+mod drain;
+pub use self::drain::Drain;
+
 mod write;
 pub use self::write::Write;
 
@@ -79,6 +82,13 @@ pub trait AsyncBufReadExt: AsyncBufRead {
         SkipWhile::new(self, f)
     }
 
+    fn drain<'a>(&'a mut self) -> Drain<'a, Self>
+    where
+        Self: Unpin,
+    {
+        Drain::new(self)
+    }
+
     fn read<'a>(&'a mut self, buf: &'a mut [u8]) -> Read<'a, Self>
     where
         Self: Unpin,