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//! Virtual pipes.
//!
//! These types provide easy implementations of `WasiFile` that mimic much of the behavior of Unix
//! pipes. These are particularly helpful for redirecting WASI stdio handles to destinations other
//! than OS files.
//!
//! Some convenience constructors are included for common backing types like `Vec<u8>` and `String`,
//! but the virtual pipes can be instantiated with any `Read` or `Write` type.
//!
use crate::poll::Subscribe;
use crate::{HostInputStream, HostOutputStream, StreamError};
use anyhow::anyhow;
use bytes::Bytes;
use std::sync::{Arc, Mutex};
use tokio::sync::mpsc;
pub use crate::write_stream::AsyncWriteStream;
#[derive(Debug, Clone)]
pub struct MemoryInputPipe {
buffer: Arc<Mutex<Bytes>>,
}
impl MemoryInputPipe {
pub fn new(bytes: impl Into<Bytes>) -> Self {
Self {
buffer: Arc::new(Mutex::new(bytes.into())),
}
}
pub fn is_empty(&self) -> bool {
self.buffer.lock().unwrap().is_empty()
}
}
#[async_trait::async_trait]
impl HostInputStream for MemoryInputPipe {
fn read(&mut self, size: usize) -> Result<Bytes, StreamError> {
let mut buffer = self.buffer.lock().unwrap();
if buffer.is_empty() {
return Err(StreamError::Closed);
}
let size = size.min(buffer.len());
let read = buffer.split_to(size);
Ok(read)
}
}
#[async_trait::async_trait]
impl Subscribe for MemoryInputPipe {
async fn ready(&mut self) {}
}
#[derive(Debug, Clone)]
pub struct MemoryOutputPipe {
capacity: usize,
buffer: Arc<Mutex<bytes::BytesMut>>,
}
impl MemoryOutputPipe {
pub fn new(capacity: usize) -> Self {
MemoryOutputPipe {
capacity,
buffer: std::sync::Arc::new(std::sync::Mutex::new(bytes::BytesMut::new())),
}
}
pub fn contents(&self) -> bytes::Bytes {
self.buffer.lock().unwrap().clone().freeze()
}
pub fn try_into_inner(self) -> Option<bytes::BytesMut> {
std::sync::Arc::into_inner(self.buffer).map(|m| m.into_inner().unwrap())
}
}
impl HostOutputStream for MemoryOutputPipe {
fn write(&mut self, bytes: Bytes) -> Result<(), StreamError> {
let mut buf = self.buffer.lock().unwrap();
if bytes.len() > self.capacity - buf.len() {
return Err(StreamError::Trap(anyhow!(
"write beyond capacity of MemoryOutputPipe"
)));
}
buf.extend_from_slice(bytes.as_ref());
// Always ready for writing
Ok(())
}
fn flush(&mut self) -> Result<(), StreamError> {
// This stream is always flushed
Ok(())
}
fn check_write(&mut self) -> Result<usize, StreamError> {
let consumed = self.buffer.lock().unwrap().len();
if consumed < self.capacity {
Ok(self.capacity - consumed)
} else {
// Since the buffer is full, no more bytes will ever be written
Err(StreamError::Closed)
}
}
}
#[async_trait::async_trait]
impl Subscribe for MemoryOutputPipe {
async fn ready(&mut self) {}
}
/// Provides a [`HostInputStream`] impl from a [`tokio::io::AsyncRead`] impl
pub struct AsyncReadStream {
closed: bool,
buffer: Option<Result<Bytes, StreamError>>,
receiver: mpsc::Receiver<Result<Bytes, StreamError>>,
join_handle: Option<crate::runtime::AbortOnDropJoinHandle<()>>,
}
impl AsyncReadStream {
/// Create a [`AsyncReadStream`]. In order to use the [`HostInputStream`] impl
/// provided by this struct, the argument must impl [`tokio::io::AsyncRead`].
pub fn new<T: tokio::io::AsyncRead + Send + Unpin + 'static>(mut reader: T) -> Self {
let (sender, receiver) = mpsc::channel(1);
let join_handle = crate::runtime::spawn(async move {
loop {
use tokio::io::AsyncReadExt;
let mut buf = bytes::BytesMut::with_capacity(4096);
let sent = match reader.read_buf(&mut buf).await {
Ok(nbytes) if nbytes == 0 => sender.send(Err(StreamError::Closed)).await,
Ok(_) => sender.send(Ok(buf.freeze())).await,
Err(e) => {
sender
.send(Err(StreamError::LastOperationFailed(e.into())))
.await
}
};
if sent.is_err() {
// no more receiver - stop trying to read
break;
}
}
});
AsyncReadStream {
closed: false,
buffer: None,
receiver,
join_handle: Some(join_handle),
}
}
}
#[async_trait::async_trait]
impl HostInputStream for AsyncReadStream {
fn read(&mut self, size: usize) -> Result<Bytes, StreamError> {
use mpsc::error::TryRecvError;
match self.buffer.take() {
Some(Ok(mut bytes)) => {
// TODO: de-duplicate the buffer management with the case below
let len = bytes.len().min(size);
let rest = bytes.split_off(len);
if !rest.is_empty() {
self.buffer = Some(Ok(rest));
}
return Ok(bytes);
}
Some(Err(e)) => {
self.closed = true;
return Err(e);
}
None => {}
}
match self.receiver.try_recv() {
Ok(Ok(mut bytes)) => {
let len = bytes.len().min(size);
let rest = bytes.split_off(len);
if !rest.is_empty() {
self.buffer = Some(Ok(rest));
}
Ok(bytes)
}
Ok(Err(e)) => {
self.closed = true;
Err(e)
}
Err(TryRecvError::Empty) => Ok(Bytes::new()),
Err(TryRecvError::Disconnected) => Err(StreamError::Trap(anyhow!(
"AsyncReadStream sender died - should be impossible"
))),
}
}
async fn cancel(&mut self) {
match self.join_handle.take() {
Some(task) => _ = task.abort_wait().await,
None => {}
}
}
}
#[async_trait::async_trait]
impl Subscribe for AsyncReadStream {
async fn ready(&mut self) {
if self.buffer.is_some() || self.closed {
return;
}
match self.receiver.recv().await {
Some(res) => self.buffer = Some(res),
None => {
panic!("no more sender for an open AsyncReadStream - should be impossible")
}
}
}
}
/// An output stream that consumes all input written to it, and is always ready.
#[derive(Copy, Clone)]
pub struct SinkOutputStream;
impl HostOutputStream for SinkOutputStream {
fn write(&mut self, _buf: Bytes) -> Result<(), StreamError> {
Ok(())
}
fn flush(&mut self) -> Result<(), StreamError> {
// This stream is always flushed
Ok(())
}
fn check_write(&mut self) -> Result<usize, StreamError> {
// This stream is always ready for writing.
Ok(usize::MAX)
}
}
#[async_trait::async_trait]
impl Subscribe for SinkOutputStream {
async fn ready(&mut self) {}
}
/// A stream that is ready immediately, but will always report that it's closed.
#[derive(Copy, Clone)]
pub struct ClosedInputStream;
#[async_trait::async_trait]
impl HostInputStream for ClosedInputStream {
fn read(&mut self, _size: usize) -> Result<Bytes, StreamError> {
Err(StreamError::Closed)
}
}
#[async_trait::async_trait]
impl Subscribe for ClosedInputStream {
async fn ready(&mut self) {}
}
/// An output stream that is always closed.
#[derive(Copy, Clone)]
pub struct ClosedOutputStream;
impl HostOutputStream for ClosedOutputStream {
fn write(&mut self, _: Bytes) -> Result<(), StreamError> {
Err(StreamError::Closed)
}
fn flush(&mut self) -> Result<(), StreamError> {
Err(StreamError::Closed)
}
fn check_write(&mut self) -> Result<usize, StreamError> {
Err(StreamError::Closed)
}
}
#[async_trait::async_trait]
impl Subscribe for ClosedOutputStream {
async fn ready(&mut self) {}
}
#[cfg(test)]
mod test {
use super::*;
use std::time::Duration;
use tokio::io::{AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt};
// This is a gross way to handle CI running under qemu for non-x86 architectures.
#[cfg(not(target_arch = "x86_64"))]
const TEST_ITERATIONS: usize = 10;
#[cfg(target_arch = "x86_64")]
const TEST_ITERATIONS: usize = 100;
async fn resolves_immediately<F, O>(fut: F) -> O
where
F: futures::Future<Output = O>,
{
// The input `fut` should resolve immediately, but in case it
// accidentally doesn't don't hang the test indefinitely. Provide a
// generous timeout to account for CI sensitivity and various systems.
tokio::time::timeout(Duration::from_secs(2), fut)
.await
.expect("operation timed out")
}
async fn never_resolves<F: futures::Future>(fut: F) {
// The input `fut` should never resolve, so only give it a small window
// of budget before we time out. If `fut` is actually resolved this
// should show up as a flaky test.
tokio::time::timeout(Duration::from_millis(10), fut)
.await
.err()
.expect("operation should time out");
}
pub fn simplex(size: usize) -> (impl AsyncRead, impl AsyncWrite) {
let (a, b) = tokio::io::duplex(size);
let (_read_half, write_half) = tokio::io::split(a);
let (read_half, _write_half) = tokio::io::split(b);
(read_half, write_half)
}
#[test_log::test(tokio::test(flavor = "multi_thread"))]
async fn empty_read_stream() {
let mut reader = AsyncReadStream::new(tokio::io::empty());
// In a multi-threaded context, the value of state is not deterministic -- the spawned
// reader task may run on a different thread.
match reader.read(10) {
// The reader task ran before we tried to read, and noticed that the input was empty.
Err(StreamError::Closed) => {}
// The reader task hasn't run yet. Call `ready` to await and fill the buffer.
Ok(bs) => {
assert!(bs.is_empty());
resolves_immediately(reader.ready()).await;
assert!(matches!(reader.read(0), Err(StreamError::Closed)));
}
res => panic!("unexpected: {res:?}"),
}
}
#[test_log::test(tokio::test(flavor = "multi_thread"))]
async fn infinite_read_stream() {
let mut reader = AsyncReadStream::new(tokio::io::repeat(0));
let bs = reader.read(10).unwrap();
if bs.is_empty() {
// Reader task hasn't run yet. Call `ready` to await and fill the buffer.
resolves_immediately(reader.ready()).await;
// Now a read should succeed
let bs = reader.read(10).unwrap();
assert_eq!(bs.len(), 10);
} else {
assert_eq!(bs.len(), 10);
}
// Subsequent reads should succeed
let bs = reader.read(10).unwrap();
assert_eq!(bs.len(), 10);
// Even 0-length reads should succeed and show its open
let bs = reader.read(0).unwrap();
assert_eq!(bs.len(), 0);
}
async fn finite_async_reader(contents: &[u8]) -> impl AsyncRead + Send + 'static {
let (r, mut w) = simplex(contents.len());
w.write_all(contents).await.unwrap();
r
}
#[test_log::test(tokio::test(flavor = "multi_thread"))]
async fn finite_read_stream() {
let mut reader = AsyncReadStream::new(finite_async_reader(&[1; 123]).await);
let bs = reader.read(123).unwrap();
if bs.is_empty() {
// Reader task hasn't run yet. Call `ready` to await and fill the buffer.
resolves_immediately(reader.ready()).await;
// Now a read should succeed
let bs = reader.read(123).unwrap();
assert_eq!(bs.len(), 123);
} else {
assert_eq!(bs.len(), 123);
}
// The AsyncRead's should be empty now, but we have a race where the reader task hasn't
// yet send that to the AsyncReadStream.
match reader.read(0) {
Err(StreamError::Closed) => {} // Correct!
Ok(bs) => {
assert!(bs.is_empty());
// Need to await to give this side time to catch up
resolves_immediately(reader.ready()).await;
// Now a read should show closed
assert!(matches!(reader.read(0), Err(StreamError::Closed)));
}
res => panic!("unexpected: {res:?}"),
}
}
#[test_log::test(tokio::test(flavor = "multi_thread"))]
// Test that you can write items into the stream, and they get read out in the order they were
// written, with the proper indications of readiness for reading:
async fn multiple_chunks_read_stream() {
let (r, mut w) = simplex(1024);
let mut reader = AsyncReadStream::new(r);
w.write_all(&[123]).await.unwrap();
let bs = reader.read(1).unwrap();
if bs.is_empty() {
// Reader task hasn't run yet. Call `ready` to await and fill the buffer.
resolves_immediately(reader.ready()).await;
// Now a read should succeed
let bs = reader.read(1).unwrap();
assert_eq!(*bs, [123u8]);
} else {
assert_eq!(*bs, [123u8]);
}
// The stream should be empty and open now:
let bs = reader.read(1).unwrap();
assert!(bs.is_empty());
// We can wait on readiness and it will time out:
never_resolves(reader.ready()).await;
// Still open and empty:
let bs = reader.read(1).unwrap();
assert!(bs.is_empty());
// Put something else in the stream:
w.write_all(&[45]).await.unwrap();
// Wait readiness (yes we could possibly win the race and read it out faster, leaving that
// out of the test for simplicity)
resolves_immediately(reader.ready()).await;
// read the something else back out:
let bs = reader.read(1).unwrap();
assert_eq!(*bs, [45u8]);
// nothing else in there:
let bs = reader.read(1).unwrap();
assert!(bs.is_empty());
// We can wait on readiness and it will time out:
never_resolves(reader.ready()).await;
// nothing else in there:
let bs = reader.read(1).unwrap();
assert!(bs.is_empty());
// Now close the pipe:
drop(w);
// Wait readiness (yes we could possibly win the race and read it out faster, leaving that
// out of the test for simplicity)
resolves_immediately(reader.ready()).await;
// empty and now closed:
assert!(matches!(reader.read(1), Err(StreamError::Closed)));
}
#[test_log::test(tokio::test(flavor = "multi_thread"))]
// At the moment we are restricting AsyncReadStream from buffering more than 4k. This isn't a
// suitable design for all applications, and we will probably make a knob or change the
// behavior at some point, but this test shows the behavior as it is implemented:
async fn backpressure_read_stream() {
let (r, mut w) = simplex(16 * 1024); // Make sure this buffer isn't a bottleneck
let mut reader = AsyncReadStream::new(r);
let writer_task = tokio::task::spawn(async move {
// Write twice as much as we can buffer up in an AsyncReadStream:
w.write_all(&[123; 8192]).await.unwrap();
w
});
resolves_immediately(reader.ready()).await;
// Now we expect the reader task has sent 4k from the stream to the reader.
// Try to read out one bigger than the buffer available:
let bs = reader.read(4097).unwrap();
assert_eq!(bs.len(), 4096);
// Allow the crank to turn more:
resolves_immediately(reader.ready()).await;
// Again we expect the reader task has sent 4k from the stream to the reader.
// Try to read out one bigger than the buffer available:
let bs = reader.read(4097).unwrap();
assert_eq!(bs.len(), 4096);
// The writer task is now finished - join with it:
let w = resolves_immediately(writer_task).await;
// And close the pipe:
drop(w);
// Allow the crank to turn more:
resolves_immediately(reader.ready()).await;
// Now we expect the reader to be empty, and the stream closed:
assert!(matches!(reader.read(4097), Err(StreamError::Closed)));
}
#[test_log::test(test_log::test(tokio::test(flavor = "multi_thread")))]
async fn sink_write_stream() {
let mut writer = AsyncWriteStream::new(2048, tokio::io::sink());
let chunk = Bytes::from_static(&[0; 1024]);
let readiness = resolves_immediately(writer.write_ready())
.await
.expect("write_ready does not trap");
assert_eq!(readiness, 2048);
// I can write whatever:
writer.write(chunk.clone()).expect("write does not error");
// This may consume 1k of the buffer:
let readiness = resolves_immediately(writer.write_ready())
.await
.expect("write_ready does not trap");
assert!(
readiness == 1024 || readiness == 2048,
"readiness should be 1024 or 2048, got {readiness}"
);
if readiness == 1024 {
writer.write(chunk.clone()).expect("write does not error");
let readiness = resolves_immediately(writer.write_ready())
.await
.expect("write_ready does not trap");
assert!(
readiness == 1024 || readiness == 2048,
"readiness should be 1024 or 2048, got {readiness}"
);
}
}
#[test_log::test(tokio::test(flavor = "multi_thread"))]
async fn closed_write_stream() {
// Run many times because the test is nondeterministic:
for n in 0..TEST_ITERATIONS {
closed_write_stream_(n).await
}
}
#[tracing::instrument]
async fn closed_write_stream_(n: usize) {
let (reader, writer) = simplex(1);
let mut writer = AsyncWriteStream::new(1024, writer);
// Drop the reader to allow the worker to transition to the closed state eventually.
drop(reader);
// First the api is going to report the last operation failed, then subsequently
// it will be reported as closed. We set this flag once we see LastOperationFailed.
let mut should_be_closed = false;
// Write some data to the stream to ensure we have data that cannot be flushed.
let chunk = Bytes::from_static(&[0; 1]);
writer
.write(chunk.clone())
.expect("first write should succeed");
// The rest of this test should be valid whether or not we check write readiness:
let mut write_ready_res = None;
if n % 2 == 0 {
let r = resolves_immediately(writer.write_ready()).await;
// Check write readiness:
match r {
// worker hasn't processed write yet:
Ok(1023) => {}
// worker reports failure:
Err(StreamError::LastOperationFailed(_)) => {
tracing::debug!("discovered stream failure in first write_ready");
should_be_closed = true;
}
r => panic!("unexpected write_ready: {r:?}"),
}
write_ready_res = Some(r);
}
// When we drop the simplex reader, that causes the simplex writer to return BrokenPipe on
// its write. Now that the buffering crank has turned, our next write will give BrokenPipe.
let flush_res = writer.flush();
match flush_res {
// worker reports failure:
Err(StreamError::LastOperationFailed(_)) => {
tracing::debug!("discovered stream failure trying to flush");
assert!(!should_be_closed);
should_be_closed = true;
}
// Already reported failure, now closed
Err(StreamError::Closed) => {
assert!(
should_be_closed,
"expected a LastOperationFailed before we see Closed. {write_ready_res:?}"
);
}
// Also possible the worker hasn't processed write yet:
Ok(()) => {}
Err(e) => panic!("unexpected flush error: {e:?} {write_ready_res:?}"),
}
// Waiting for the flush to complete should always indicate that the channel has been
// closed.
match resolves_immediately(writer.write_ready()).await {
// worker reports failure:
Err(StreamError::LastOperationFailed(_)) => {
tracing::debug!("discovered stream failure trying to flush");
assert!(!should_be_closed);
}
// Already reported failure, now closed
Err(StreamError::Closed) => {
assert!(should_be_closed);
}
r => {
panic!("stream should be reported closed by the end of write_ready after flush, got {r:?}. {write_ready_res:?} {flush_res:?}")
}
}
}
#[test_log::test(tokio::test(flavor = "multi_thread"))]
async fn multiple_chunks_write_stream() {
// Run many times because the test is nondeterministic:
for n in 0..TEST_ITERATIONS {
multiple_chunks_write_stream_aux(n).await
}
}
#[tracing::instrument]
async fn multiple_chunks_write_stream_aux(_: usize) {
use std::ops::Deref;
let (mut reader, writer) = simplex(1024);
let mut writer = AsyncWriteStream::new(1024, writer);
// Write a chunk:
let chunk = Bytes::from_static(&[123; 1]);
let permit = resolves_immediately(writer.write_ready())
.await
.expect("write should be ready");
assert_eq!(permit, 1024);
writer.write(chunk.clone()).expect("write does not trap");
// At this point the message will either be waiting for the worker to process the write, or
// it will be buffered in the simplex channel.
let permit = resolves_immediately(writer.write_ready())
.await
.expect("write should be ready");
assert!(matches!(permit, 1023 | 1024));
let mut read_buf = vec![0; chunk.len()];
let read_len = reader.read_exact(&mut read_buf).await.unwrap();
assert_eq!(read_len, chunk.len());
assert_eq!(read_buf.as_slice(), chunk.deref());
// Write a second, different chunk:
let chunk2 = Bytes::from_static(&[45; 1]);
// We're only guaranteed to see a consistent write budget if we flush.
writer.flush().expect("channel is still alive");
let permit = resolves_immediately(writer.write_ready())
.await
.expect("write should be ready");
assert_eq!(permit, 1024);
writer.write(chunk2.clone()).expect("write does not trap");
// At this point the message will either be waiting for the worker to process the write, or
// it will be buffered in the simplex channel.
let permit = resolves_immediately(writer.write_ready())
.await
.expect("write should be ready");
assert!(matches!(permit, 1023 | 1024));
let mut read2_buf = vec![0; chunk2.len()];
let read2_len = reader.read_exact(&mut read2_buf).await.unwrap();
assert_eq!(read2_len, chunk2.len());
assert_eq!(read2_buf.as_slice(), chunk2.deref());
// We're only guaranteed to see a consistent write budget if we flush.
writer.flush().expect("channel is still alive");
let permit = resolves_immediately(writer.write_ready())
.await
.expect("write should be ready");
assert_eq!(permit, 1024);
}
#[test_log::test(tokio::test(flavor = "multi_thread"))]
async fn backpressure_write_stream() {
// Run many times because the test is nondeterministic:
for n in 0..TEST_ITERATIONS {
backpressure_write_stream_aux(n).await
}
}
#[tracing::instrument]
async fn backpressure_write_stream_aux(_: usize) {
use futures::future::poll_immediate;
// The channel can buffer up to 1k, plus another 1k in the stream, before not
// accepting more input:
let (mut reader, writer) = simplex(1024);
let mut writer = AsyncWriteStream::new(1024, writer);
let chunk = Bytes::from_static(&[0; 1024]);
let permit = resolves_immediately(writer.write_ready())
.await
.expect("write should be ready");
assert_eq!(permit, 1024);
writer.write(chunk.clone()).expect("write succeeds");
// We might still be waiting for the worker to process the message, or the worker may have
// processed it and released all the budget back to us.
let permit = poll_immediate(writer.write_ready()).await;
assert!(matches!(permit, None | Some(Ok(1024))));
// Given a little time, the worker will process the message and release all the budget
// back.
let permit = resolves_immediately(writer.write_ready())
.await
.expect("write should be ready");
assert_eq!(permit, 1024);
// Now fill the buffer between here and the writer task. This should always indicate
// back-pressure because now both buffers (simplex and worker) are full.
writer.write(chunk.clone()).expect("write does not trap");
// Try shoving even more down there, and it shouldn't accept more input:
writer
.write(chunk.clone())
.err()
.expect("unpermitted write does trap");
// No amount of waiting will resolve the situation, as nothing is emptying the simplex
// buffer.
never_resolves(writer.write_ready()).await;
// There is 2k buffered between the simplex and worker buffers. I should be able to read
// all of it out:
let mut buf = [0; 2048];
reader.read_exact(&mut buf).await.unwrap();
// and no more:
never_resolves(reader.read(&mut buf)).await;
// Now the backpressure should be cleared, and an additional write should be accepted.
let permit = resolves_immediately(writer.write_ready())
.await
.expect("ready is ok");
assert_eq!(permit, 1024);
// and the write succeeds:
writer.write(chunk.clone()).expect("write does not trap");
}
#[test_log::test(tokio::test(flavor = "multi_thread"))]
async fn backpressure_write_stream_with_flush() {
for n in 0..TEST_ITERATIONS {
backpressure_write_stream_with_flush_aux(n).await;
}
}
async fn backpressure_write_stream_with_flush_aux(_: usize) {
// The channel can buffer up to 1k, plus another 1k in the stream, before not
// accepting more input:
let (mut reader, writer) = simplex(1024);
let mut writer = AsyncWriteStream::new(1024, writer);
let chunk = Bytes::from_static(&[0; 1024]);
let permit = resolves_immediately(writer.write_ready())
.await
.expect("write should be ready");
assert_eq!(permit, 1024);
writer.write(chunk.clone()).expect("write succeeds");
writer.flush().expect("flush succeeds");
// Waiting for write_ready to resolve after a flush should always show that we have the
// full budget available, as the message will have flushed to the simplex channel.
let permit = resolves_immediately(writer.write_ready())
.await
.expect("write_ready succeeds");
assert_eq!(permit, 1024);
// Write enough to fill the simplex buffer:
writer.write(chunk.clone()).expect("write does not trap");
// Writes should be refused until this flush succeeds.
writer.flush().expect("flush succeeds");
// Try shoving even more down there, and it shouldn't accept more input:
writer
.write(chunk.clone())
.err()
.expect("unpermitted write does trap");
// No amount of waiting will resolve the situation, as nothing is emptying the simplex
// buffer.
never_resolves(writer.write_ready()).await;
// There is 2k buffered between the simplex and worker buffers. I should be able to read
// all of it out:
let mut buf = [0; 2048];
reader.read_exact(&mut buf).await.unwrap();
// and no more:
never_resolves(reader.read(&mut buf)).await;
// Now the backpressure should be cleared, and an additional write should be accepted.
let permit = resolves_immediately(writer.write_ready())
.await
.expect("ready is ok");
assert_eq!(permit, 1024);
// and the write succeeds:
writer.write(chunk.clone()).expect("write does not trap");
writer.flush().expect("flush succeeds");
let permit = resolves_immediately(writer.write_ready())
.await
.expect("ready is ok");
assert_eq!(permit, 1024);
}
}