Provides AsyncWrite with flush

As per Tokio and others, this commit provides a `poll_flush` method on `AsyncWrite` so that a best-effort attempt at wakening once all bytes are flushed can be made.

embassy/src/io/traits.rs

@@ -89,6 +89,15 @@ pub trait AsyncWrite {
     /// `poll_write` must try to make progress by flushing the underlying object if
     /// that is the only way the underlying object can become writable again.
     fn poll_write(self: Pin<&mut Self>, cx: &mut Context<'_>, buf: &[u8]) -> Poll<Result<usize>>;
+
+    /// Attempt to flush the object, ensuring that any buffered data reach their destination.
+    ///
+    /// On success, returns Poll::Ready(Ok(())).
+    ///
+    /// If flushing cannot immediately complete, this method returns [Poll::Pending] and arranges for the
+    /// current task (via cx.waker()) to receive a notification when the object can make progress
+    /// towards flushing.
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<()>>;
 }
 
 macro_rules! defer_async_read {
@@ -135,6 +144,10 @@ macro_rules! deref_async_write {
         ) -> Poll<Result<usize>> {
             Pin::new(&mut **self).poll_write(cx, buf)
         }
+
+        fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<()>> {
+            Pin::new(&mut **self).poll_flush(cx)
+        }
     };
 }
 
@@ -155,4 +168,8 @@ where
     fn poll_write(self: Pin<&mut Self>, cx: &mut Context<'_>, buf: &[u8]) -> Poll<Result<usize>> {
         self.get_mut().as_mut().poll_write(cx, buf)
     }
+
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<()>> {
+        self.get_mut().as_mut().poll_flush(cx)
+    }
 }

embassy/src/io/util/flush.rs

@@ -0,0 +1,32 @@
+use core::pin::Pin;
+use futures::future::Future;
+use futures::ready;
+use futures::task::{Context, Poll};
+
+use super::super::error::Result;
+use super::super::traits::AsyncWrite;
+
+/// Future for the [`flush`](super::AsyncWriteExt::flush) method.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct Flush<'a, W: ?Sized> {
+    writer: &'a mut W,
+}
+
+impl<W: ?Sized + Unpin> Unpin for Flush<'_, W> {}
+
+impl<'a, W: AsyncWrite + ?Sized + Unpin> Flush<'a, W> {
+    pub(super) fn new(writer: &'a mut W) -> Self {
+        Flush { writer }
+    }
+}
+
+impl<W: AsyncWrite + ?Sized + Unpin> Future for Flush<'_, W> {
+    type Output = Result<()>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<()>> {
+        let this = &mut *self;
+        let _ = ready!(Pin::new(&mut this.writer).poll_flush(cx))?;
+        Poll::Ready(Ok(()))
+    }
+}

embassy/src/io/util/mod.rs

@@ -27,6 +27,9 @@ pub use self::skip_while::SkipWhile;
 mod drain;
 pub use self::drain::Drain;
 
+mod flush;
+pub use self::flush::Flush;
+
 mod write;
 pub use self::write::Write;
 
@@ -160,6 +163,15 @@ pub trait AsyncWriteExt: AsyncWrite {
     {
         Write::new(self, buf)
     }
+
+    /// Awaits until all bytes have actually been written, and
+    /// not just enqueued as per the other "write" methods.
+    fn flush<'a>(&mut self) -> Flush<Self>
+    where
+        Self: Unpin,
+    {
+        Flush::new(self)
+    }
 }
 
 impl<R: AsyncWrite + ?Sized> AsyncWriteExt for R {}