Merge #891

891: Async Pipe r=Dirbaio a=Dirbaio



Co-authored-by: Dario Nieuwenhuis <[email protected]>

4 files changed, 706 insertions, 2 deletions

diff --git a/embassy-util/Cargo.toml b/embassy-util/Cargo.toml
index 32b796c0a..ef5acc0f0 100644
--- a/embassy-util/Cargo.toml
+++ b/embassy-util/Cargo.toml
@@ -6,11 +6,14 @@ edition = "2021"
 [package.metadata.embassy_docs]
 src_base = "https://github.com/embassy-rs/embassy/blob/embassy-util-v$VERSION/embassy-util/src/"
 src_base_git = "https://github.com/embassy-rs/embassy/blob/$COMMIT/embassy-util/src/"
-features = ["nightly", "defmt", "unstable-traits", "time", "time-tick-1mhz"]
+features = ["nightly"]
 flavors = [
     { name = "default", target = "x86_64-unknown-linux-gnu" },
 ]
 
+[features]
+nightly = ["embedded-io/async"]
+
 [dependencies]
 defmt = { version = "0.3", optional = true }
 log = { version = "0.4.14", optional = true }
@@ -20,6 +23,7 @@ atomic-polyfill = "0.1.5"
 critical-section = "0.2.5"
 heapless = "0.7.5"
 cfg-if = "1.0.0"
+embedded-io = "0.3.0"
 
 [dev-dependencies]
 futures-executor = { version = "0.3.17", features = [ "thread-pool" ] }
diff --git a/embassy-util/src/lib.rs b/embassy-util/src/lib.rs
index 07b1633ea..110c72811 100644
--- a/embassy-util/src/lib.rs
+++ b/embassy-util/src/lib.rs
@@ -1,6 +1,5 @@
 #![cfg_attr(not(any(feature = "std", feature = "wasm")), no_std)]
 #![cfg_attr(feature = "nightly", feature(generic_associated_types, type_alias_impl_trait))]
-#![cfg_attr(all(feature = "nightly", target_arch = "xtensa"), feature(asm_experimental_arch))]
 #![allow(clippy::new_without_default)]
 #![doc = include_str!("../../README.md")]
 #![warn(missing_docs)]
@@ -8,9 +7,13 @@
 // This mod MUST go first, so that the others see its macros.
 pub(crate) mod fmt;
 
+// internal use
+mod ring_buffer;
+
 pub mod blocking_mutex;
 pub mod channel;
 pub mod mutex;
+pub mod pipe;
 pub mod waitqueue;
 
 mod forever;
diff --git a/embassy-util/src/pipe.rs b/embassy-util/src/pipe.rs
new file mode 100644
index 000000000..9c20aeeff
--- /dev/null
+++ b/embassy-util/src/pipe.rs
@@ -0,0 +1,551 @@
+//! Async byte stream pipe.
+
+use core::cell::RefCell;
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+use crate::blocking_mutex::raw::RawMutex;
+use crate::blocking_mutex::Mutex;
+use crate::ring_buffer::RingBuffer;
+use crate::waitqueue::WakerRegistration;
+
+/// Write-only access to a [`Pipe`].
+#[derive(Copy)]
+pub struct Writer<'p, M, const N: usize>
+where
+    M: RawMutex,
+{
+    pipe: &'p Pipe<M, N>,
+}
+
+impl<'p, M, const N: usize> Clone for Writer<'p, M, N>
+where
+    M: RawMutex,
+{
+    fn clone(&self) -> Self {
+        Writer { pipe: self.pipe }
+    }
+}
+
+impl<'p, M, const N: usize> Writer<'p, M, N>
+where
+    M: RawMutex,
+{
+    /// Writes a value.
+    ///
+    /// See [`Pipe::write()`]
+    pub fn write<'a>(&'a self, buf: &'a [u8]) -> WriteFuture<'a, M, N> {
+        self.pipe.write(buf)
+    }
+
+    /// Attempt to immediately write a message.
+    ///
+    /// See [`Pipe::write()`]
+    pub fn try_write(&self, buf: &[u8]) -> Result<usize, TryWriteError> {
+        self.pipe.try_write(buf)
+    }
+}
+
+/// Future returned by [`Pipe::write`] and  [`Writer::write`].
+pub struct WriteFuture<'p, M, const N: usize>
+where
+    M: RawMutex,
+{
+    pipe: &'p Pipe<M, N>,
+    buf: &'p [u8],
+}
+
+impl<'p, M, const N: usize> Future for WriteFuture<'p, M, N>
+where
+    M: RawMutex,
+{
+    type Output = usize;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match self.pipe.try_write_with_context(Some(cx), self.buf) {
+            Ok(n) => Poll::Ready(n),
+            Err(TryWriteError::Full) => Poll::Pending,
+        }
+    }
+}
+
+impl<'p, M, const N: usize> Unpin for WriteFuture<'p, M, N> where M: RawMutex {}
+
+/// Read-only access to a [`Pipe`].
+#[derive(Copy)]
+pub struct Reader<'p, M, const N: usize>
+where
+    M: RawMutex,
+{
+    pipe: &'p Pipe<M, N>,
+}
+
+impl<'p, M, const N: usize> Clone for Reader<'p, M, N>
+where
+    M: RawMutex,
+{
+    fn clone(&self) -> Self {
+        Reader { pipe: self.pipe }
+    }
+}
+
+impl<'p, M, const N: usize> Reader<'p, M, N>
+where
+    M: RawMutex,
+{
+    /// Reads a value.
+    ///
+    /// See [`Pipe::read()`]
+    pub fn read<'a>(&'a self, buf: &'a mut [u8]) -> ReadFuture<'a, M, N> {
+        self.pipe.read(buf)
+    }
+
+    /// Attempt to immediately read a message.
+    ///
+    /// See [`Pipe::read()`]
+    pub fn try_read(&self, buf: &mut [u8]) -> Result<usize, TryReadError> {
+        self.pipe.try_read(buf)
+    }
+}
+
+/// Future returned by [`Pipe::read`] and  [`Reader::read`].
+pub struct ReadFuture<'p, M, const N: usize>
+where
+    M: RawMutex,
+{
+    pipe: &'p Pipe<M, N>,
+    buf: &'p mut [u8],
+}
+
+impl<'p, M, const N: usize> Future for ReadFuture<'p, M, N>
+where
+    M: RawMutex,
+{
+    type Output = usize;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match self.pipe.try_read_with_context(Some(cx), self.buf) {
+            Ok(n) => Poll::Ready(n),
+            Err(TryReadError::Empty) => Poll::Pending,
+        }
+    }
+}
+
+impl<'p, M, const N: usize> Unpin for ReadFuture<'p, M, N> where M: RawMutex {}
+
+/// Error returned by [`try_read`](Pipe::try_read).
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub enum TryReadError {
+    /// No data could be read from the pipe because it is currently
+    /// empty, and reading would require blocking.
+    Empty,
+}
+
+/// Error returned by [`try_write`](Pipe::try_write).
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub enum TryWriteError {
+    /// No data could be written to the pipe because it is
+    /// currently full, and writing would require blocking.
+    Full,
+}
+
+struct PipeState<const N: usize> {
+    buffer: RingBuffer<N>,
+    read_waker: WakerRegistration,
+    write_waker: WakerRegistration,
+}
+
+impl<const N: usize> PipeState<N> {
+    const fn new() -> Self {
+        PipeState {
+            buffer: RingBuffer::new(),
+            read_waker: WakerRegistration::new(),
+            write_waker: WakerRegistration::new(),
+        }
+    }
+
+    fn clear(&mut self) {
+        self.buffer.clear();
+        self.write_waker.wake();
+    }
+
+    fn try_read(&mut self, buf: &mut [u8]) -> Result<usize, TryReadError> {
+        self.try_read_with_context(None, buf)
+    }
+
+    fn try_read_with_context(&mut self, cx: Option<&mut Context<'_>>, buf: &mut [u8]) -> Result<usize, TryReadError> {
+        if self.buffer.is_full() {
+            self.write_waker.wake();
+        }
+
+        let available = self.buffer.pop_buf();
+        if available.is_empty() {
+            if let Some(cx) = cx {
+                self.read_waker.register(cx.waker());
+            }
+            return Err(TryReadError::Empty);
+        }
+
+        let n = available.len().min(buf.len());
+        buf[..n].copy_from_slice(&available[..n]);
+        self.buffer.pop(n);
+        Ok(n)
+    }
+
+    fn try_write(&mut self, buf: &[u8]) -> Result<usize, TryWriteError> {
+        self.try_write_with_context(None, buf)
+    }
+
+    fn try_write_with_context(&mut self, cx: Option<&mut Context<'_>>, buf: &[u8]) -> Result<usize, TryWriteError> {
+        if self.buffer.is_empty() {
+            self.read_waker.wake();
+        }
+
+        let available = self.buffer.push_buf();
+        if available.is_empty() {
+            if let Some(cx) = cx {
+                self.write_waker.register(cx.waker());
+            }
+            return Err(TryWriteError::Full);
+        }
+
+        let n = available.len().min(buf.len());
+        available[..n].copy_from_slice(&buf[..n]);
+        self.buffer.push(n);
+        Ok(n)
+    }
+}
+
+/// A bounded pipe for communicating between asynchronous tasks
+/// with backpressure.
+///
+/// The pipe will buffer up to the provided number of messages.  Once the
+/// buffer is full, attempts to `write` new messages will wait until a message is
+/// read from the pipe.
+///
+/// All data written will become available in the same order as it was written.
+pub struct Pipe<M, const N: usize>
+where
+    M: RawMutex,
+{
+    inner: Mutex<M, RefCell<PipeState<N>>>,
+}
+
+impl<M, const N: usize> Pipe<M, N>
+where
+    M: RawMutex,
+{
+    /// Establish a new bounded pipe. For example, to create one with a NoopMutex:
+    ///
+    /// ```
+    /// use embassy_util::pipe::Pipe;
+    /// use embassy_util::blocking_mutex::raw::NoopRawMutex;
+    ///
+    /// // Declare a bounded pipe, with a buffer of 256 bytes.
+    /// let mut pipe = Pipe::<NoopRawMutex, 256>::new();
+    /// ```
+    pub const fn new() -> Self {
+        Self {
+            inner: Mutex::new(RefCell::new(PipeState::new())),
+        }
+    }
+
+    fn lock<R>(&self, f: impl FnOnce(&mut PipeState<N>) -> R) -> R {
+        self.inner.lock(|rc| f(&mut *rc.borrow_mut()))
+    }
+
+    fn try_read_with_context(&self, cx: Option<&mut Context<'_>>, buf: &mut [u8]) -> Result<usize, TryReadError> {
+        self.lock(|c| c.try_read_with_context(cx, buf))
+    }
+
+    fn try_write_with_context(&self, cx: Option<&mut Context<'_>>, buf: &[u8]) -> Result<usize, TryWriteError> {
+        self.lock(|c| c.try_write_with_context(cx, buf))
+    }
+
+    /// Get a writer for this pipe.
+    pub fn writer(&self) -> Writer<'_, M, N> {
+        Writer { pipe: self }
+    }
+
+    /// Get a reader for this pipe.
+    pub fn reader(&self) -> Reader<'_, M, N> {
+        Reader { pipe: self }
+    }
+
+    /// Write a value, waiting until there is capacity.
+    ///
+    /// Writeing completes when the value has been pushed to the pipe's queue.
+    /// This doesn't mean the value has been read yet.
+    pub fn write<'a>(&'a self, buf: &'a [u8]) -> WriteFuture<'a, M, N> {
+        WriteFuture { pipe: self, buf }
+    }
+
+    /// Attempt to immediately write a message.
+    ///
+    /// This method differs from [`write`](Pipe::write) by returning immediately if the pipe's
+    /// buffer is full, instead of waiting.
+    ///
+    /// # Errors
+    ///
+    /// If the pipe capacity has been reached, i.e., the pipe has `n`
+    /// buffered values where `n` is the argument passed to [`Pipe`], then an
+    /// error is returned.
+    pub fn try_write(&self, buf: &[u8]) -> Result<usize, TryWriteError> {
+        self.lock(|c| c.try_write(buf))
+    }
+
+    /// Receive the next value.
+    ///
+    /// If there are no messages in the pipe's buffer, this method will
+    /// wait until a message is written.
+    pub fn read<'a>(&'a self, buf: &'a mut [u8]) -> ReadFuture<'a, M, N> {
+        ReadFuture { pipe: self, buf }
+    }
+
+    /// Attempt to immediately read a message.
+    ///
+    /// This method will either read a message from the pipe immediately or return an error
+    /// if the pipe is empty.
+    pub fn try_read(&self, buf: &mut [u8]) -> Result<usize, TryReadError> {
+        self.lock(|c| c.try_read(buf))
+    }
+
+    /// Clear the data in the pipe's buffer.
+    pub fn clear(&self) {
+        self.lock(|c| c.clear())
+    }
+
+    /// Return whether the pipe is full (no free space in the buffer)
+    pub fn is_full(&self) -> bool {
+        self.len() == N
+    }
+
+    /// Return whether the pipe is empty (no data buffered)
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Total byte capacity.
+    ///
+    /// This is the same as the `N` generic param.
+    pub fn capacity(&self) -> usize {
+        N
+    }
+
+    /// Used byte capacity.
+    pub fn len(&self) -> usize {
+        self.lock(|c| c.buffer.len())
+    }
+
+    /// Free byte capacity.
+    ///
+    /// This is equivalent to `capacity() - len()`
+    pub fn free_capacity(&self) -> usize {
+        N - self.len()
+    }
+}
+
+#[cfg(feature = "nightly")]
+mod io_impls {
+    use core::convert::Infallible;
+
+    use futures_util::FutureExt;
+
+    use super::*;
+
+    impl<M: RawMutex, const N: usize> embedded_io::Io for Pipe<M, N> {
+        type Error = Infallible;
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::asynch::Read for Pipe<M, N> {
+        type ReadFuture<'a> = impl Future<Output = Result<usize, Self::Error>>
+        where
+            Self: 'a;
+
+        fn read<'a>(&'a mut self, buf: &'a mut [u8]) -> Self::ReadFuture<'a> {
+            Pipe::read(self, buf).map(Ok)
+        }
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::asynch::Write for Pipe<M, N> {
+        type WriteFuture<'a> = impl Future<Output = Result<usize, Self::Error>>
+        where
+            Self: 'a;
+
+        fn write<'a>(&'a mut self, buf: &'a [u8]) -> Self::WriteFuture<'a> {
+            Pipe::write(self, buf).map(Ok)
+        }
+
+        type FlushFuture<'a> = impl Future<Output = Result<(), Self::Error>>
+        where
+            Self: 'a;
+
+        fn flush<'a>(&'a mut self) -> Self::FlushFuture<'a> {
+            futures_util::future::ready(Ok(()))
+        }
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::Io for &Pipe<M, N> {
+        type Error = Infallible;
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::asynch::Read for &Pipe<M, N> {
+        type ReadFuture<'a> = impl Future<Output = Result<usize, Self::Error>>
+        where
+            Self: 'a;
+
+        fn read<'a>(&'a mut self, buf: &'a mut [u8]) -> Self::ReadFuture<'a> {
+            Pipe::read(self, buf).map(Ok)
+        }
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::asynch::Write for &Pipe<M, N> {
+        type WriteFuture<'a> = impl Future<Output = Result<usize, Self::Error>>
+        where
+            Self: 'a;
+
+        fn write<'a>(&'a mut self, buf: &'a [u8]) -> Self::WriteFuture<'a> {
+            Pipe::write(self, buf).map(Ok)
+        }
+
+        type FlushFuture<'a> = impl Future<Output = Result<(), Self::Error>>
+        where
+            Self: 'a;
+
+        fn flush<'a>(&'a mut self) -> Self::FlushFuture<'a> {
+            futures_util::future::ready(Ok(()))
+        }
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::Io for Reader<'_, M, N> {
+        type Error = Infallible;
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::asynch::Read for Reader<'_, M, N> {
+        type ReadFuture<'a> = impl Future<Output = Result<usize, Self::Error>>
+        where
+            Self: 'a;
+
+        fn read<'a>(&'a mut self, buf: &'a mut [u8]) -> Self::ReadFuture<'a> {
+            Reader::read(self, buf).map(Ok)
+        }
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::Io for Writer<'_, M, N> {
+        type Error = Infallible;
+    }
+
+    impl<M: RawMutex, const N: usize> embedded_io::asynch::Write for Writer<'_, M, N> {
+        type WriteFuture<'a> = impl Future<Output = Result<usize, Self::Error>>
+        where
+            Self: 'a;
+
+        fn write<'a>(&'a mut self, buf: &'a [u8]) -> Self::WriteFuture<'a> {
+            Writer::write(self, buf).map(Ok)
+        }
+
+        type FlushFuture<'a> = impl Future<Output = Result<(), Self::Error>>
+        where
+            Self: 'a;
+
+        fn flush<'a>(&'a mut self) -> Self::FlushFuture<'a> {
+            futures_util::future::ready(Ok(()))
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use futures_executor::ThreadPool;
+    use futures_util::task::SpawnExt;
+
+    use super::*;
+    use crate::blocking_mutex::raw::{CriticalSectionRawMutex, NoopRawMutex};
+    use crate::Forever;
+
+    fn capacity<const N: usize>(c: &PipeState<N>) -> usize {
+        N - c.buffer.len()
+    }
+
+    #[test]
+    fn writing_once() {
+        let mut c = PipeState::<3>::new();
+        assert!(c.try_write(&[1]).is_ok());
+        assert_eq!(capacity(&c), 2);
+    }
+
+    #[test]
+    fn writing_when_full() {
+        let mut c = PipeState::<3>::new();
+        assert_eq!(c.try_write(&[42]), Ok(1));
+        assert_eq!(c.try_write(&[43]), Ok(1));
+        assert_eq!(c.try_write(&[44]), Ok(1));
+        assert_eq!(c.try_write(&[45]), Err(TryWriteError::Full));
+        assert_eq!(capacity(&c), 0);
+    }
+
+    #[test]
+    fn receiving_once_with_one_send() {
+        let mut c = PipeState::<3>::new();
+        assert!(c.try_write(&[42]).is_ok());
+        let mut buf = [0; 16];
+        assert_eq!(c.try_read(&mut buf), Ok(1));
+        assert_eq!(buf[0], 42);
+        assert_eq!(capacity(&c), 3);
+    }
+
+    #[test]
+    fn receiving_when_empty() {
+        let mut c = PipeState::<3>::new();
+        let mut buf = [0; 16];
+        assert_eq!(c.try_read(&mut buf), Err(TryReadError::Empty));
+        assert_eq!(capacity(&c), 3);
+    }
+
+    #[test]
+    fn simple_send_and_receive() {
+        let c = Pipe::<NoopRawMutex, 3>::new();
+        assert!(c.try_write(&[42]).is_ok());
+        let mut buf = [0; 16];
+        assert_eq!(c.try_read(&mut buf), Ok(1));
+        assert_eq!(buf[0], 42);
+    }
+
+    #[test]
+    fn cloning() {
+        let c = Pipe::<NoopRawMutex, 3>::new();
+        let r1 = c.reader();
+        let w1 = c.writer();
+
+        let _ = r1.clone();
+        let _ = w1.clone();
+    }
+
+    #[futures_test::test]
+    async fn receiver_receives_given_try_write_async() {
+        let executor = ThreadPool::new().unwrap();
+
+        static CHANNEL: Forever<Pipe<CriticalSectionRawMutex, 3>> = Forever::new();
+        let c = &*CHANNEL.put(Pipe::new());
+        let c2 = c;
+        let f = async move {
+            assert_eq!(c2.try_write(&[42]), Ok(1));
+        };
+        executor.spawn(f).unwrap();
+        let mut buf = [0; 16];
+        assert_eq!(c.read(&mut buf).await, 1);
+        assert_eq!(buf[0], 42);
+    }
+
+    #[futures_test::test]
+    async fn sender_send_completes_if_capacity() {
+        let c = Pipe::<CriticalSectionRawMutex, 1>::new();
+        c.write(&[42]).await;
+        let mut buf = [0; 16];
+        assert_eq!(c.read(&mut buf).await, 1);
+        assert_eq!(buf[0], 42);
+    }
+}
diff --git a/embassy-util/src/ring_buffer.rs b/embassy-util/src/ring_buffer.rs
new file mode 100644
index 000000000..521084024
--- /dev/null
+++ b/embassy-util/src/ring_buffer.rs
@@ -0,0 +1,146 @@
+pub struct RingBuffer<const N: usize> {
+    buf: [u8; N],
+    start: usize,
+    end: usize,
+    empty: bool,
+}
+
+impl<const N: usize> RingBuffer<N> {
+    pub const fn new() -> Self {
+        Self {
+            buf: [0; N],
+            start: 0,
+            end: 0,
+            empty: true,
+        }
+    }
+
+    pub fn push_buf(&mut self) -> &mut [u8] {
+        if self.start == self.end && !self.empty {
+            trace!("  ringbuf: push_buf empty");
+            return &mut self.buf[..0];
+        }
+
+        let n = if self.start <= self.end {
+            self.buf.len() - self.end
+        } else {
+            self.start - self.end
+        };
+
+        trace!("  ringbuf: push_buf {:?}..{:?}", self.end, self.end + n);
+        &mut self.buf[self.end..self.end + n]
+    }
+
+    pub fn push(&mut self, n: usize) {
+        trace!("  ringbuf: push {:?}", n);
+        if n == 0 {
+            return;
+        }
+
+        self.end = self.wrap(self.end + n);
+        self.empty = false;
+    }
+
+    pub fn pop_buf(&mut self) -> &mut [u8] {
+        if self.empty {
+            trace!("  ringbuf: pop_buf empty");
+            return &mut self.buf[..0];
+        }
+
+        let n = if self.end <= self.start {
+            self.buf.len() - self.start
+        } else {
+            self.end - self.start
+        };
+
+        trace!("  ringbuf: pop_buf {:?}..{:?}", self.start, self.start + n);
+        &mut self.buf[self.start..self.start + n]
+    }
+
+    pub fn pop(&mut self, n: usize) {
+        trace!("  ringbuf: pop {:?}", n);
+        if n == 0 {
+            return;
+        }
+
+        self.start = self.wrap(self.start + n);
+        self.empty = self.start == self.end;
+    }
+
+    pub fn is_full(&self) -> bool {
+        self.start == self.end && !self.empty
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.empty
+    }
+
+    #[allow(unused)]
+    pub fn len(&self) -> usize {
+        if self.empty {
+            0
+        } else if self.start < self.end {
+            self.end - self.start
+        } else {
+            N + self.end - self.start
+        }
+    }
+
+    pub fn clear(&mut self) {
+        self.start = 0;
+        self.end = 0;
+        self.empty = true;
+    }
+
+    fn wrap(&self, n: usize) -> usize {
+        assert!(n <= self.buf.len());
+        if n == self.buf.len() {
+            0
+        } else {
+            n
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn push_pop() {
+        let mut rb: RingBuffer<4> = RingBuffer::new();
+        let buf = rb.push_buf();
+        assert_eq!(4, buf.len());
+        buf[0] = 1;
+        buf[1] = 2;
+        buf[2] = 3;
+        buf[3] = 4;
+        rb.push(4);
+
+        let buf = rb.pop_buf();
+        assert_eq!(4, buf.len());
+        assert_eq!(1, buf[0]);
+        rb.pop(1);
+
+        let buf = rb.pop_buf();
+        assert_eq!(3, buf.len());
+        assert_eq!(2, buf[0]);
+        rb.pop(1);
+
+        let buf = rb.pop_buf();
+        assert_eq!(2, buf.len());
+        assert_eq!(3, buf[0]);
+        rb.pop(1);
+
+        let buf = rb.pop_buf();
+        assert_eq!(1, buf.len());
+        assert_eq!(4, buf[0]);
+        rb.pop(1);
+
+        let buf = rb.pop_buf();
+        assert_eq!(0, buf.len());
+
+        let buf = rb.push_buf();
+        assert_eq!(4, buf.len());
+    }
+}