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//! An executor for isolating blocking I/O in async programs. //! //! Sometimes there's no way to avoid blocking I/O. Consider files or stdin, which have weak async //! support on modern operating systems. While [IOCP], [AIO], and [io_uring] are possible //! solutions, they're not always available or ideal. //! //! Since blocking is not allowed inside futures, we must move blocking I/O onto a special //! executor provided by this crate. On this executor, futures are allowed to "cheat" and block //! without any restrictions. The executor dynamically spawns and stops threads depending on the //! current number of running futures. //! //! Note that there is a limit on the number of active threads. Once that limit is hit, a running //! task has to complete or yield before other tasks get a chance to continue running. When a //! thread is idle, it waits for the next task or shuts down after a certain timeout. //! //! [IOCP]: https://en.wikipedia.org/wiki/Input/output_completion_port //! [AIO]: http://man7.org/linux/man-pages/man2/io_submit.2.html //! [io_uring]: https://lwn.net/Articles/776703/ //! //! # Examples //! //! Spawn a blocking future with [`Blocking::spawn()`]: //! //! ```no_run //! use blocking::Blocking; //! use std::fs; //! //! # futures::executor::block_on(async { //! let contents = Blocking::spawn(async { fs::read_to_string("file.txt") }).await?; //! # std::io::Result::Ok(()) }); //! ``` //! //! Or do the same with the [`blocking!`] macro: //! //! ```no_run //! use blocking::blocking; //! use std::fs; //! //! # futures::executor::block_on(async { //! let contents = blocking!(fs::read_to_string("file.txt"))?; //! # std::io::Result::Ok(()) }); //! ``` //! //! Read a file and pipe its contents to stdout: //! //! ```no_run //! use blocking::Blocking; //! use std::fs::File; //! use std::io::stdout; //! //! # futures::executor::block_on(async { //! let input = Blocking::new(File::open("file.txt")?); //! let mut output = Blocking::new(stdout()); //! //! futures::io::copy(input, &mut output).await?; //! # std::io::Result::Ok(()) }); //! ``` //! //! Iterate over the contents of a directory: //! //! ```no_run //! use blocking::Blocking; //! use futures::prelude::*; //! use std::fs; //! //! # futures::executor::block_on(async { //! let mut dir = Blocking::new(fs::read_dir(".")?); //! //! while let Some(item) = dir.next().await { //! println!("{}", item?.file_name().to_string_lossy()); //! } //! # std::io::Result::Ok(()) }); //! ``` use std::any::Any; use std::collections::VecDeque; use std::io::{self, Read, Write}; use std::mem; use std::panic; use std::pin::Pin; use std::slice; use std::sync::atomic::{self, AtomicBool, AtomicUsize, Ordering}; use std::sync::{Arc, Condvar, Mutex, MutexGuard}; use std::task::{Context, Poll}; use std::thread; use std::time::Duration; use futures::channel::mpsc; use futures::prelude::*; use futures::task::AtomicWaker; use once_cell::sync::Lazy; /// A runnable future, ready for execution. /// /// When a future is internally spawned using `async_task::spawn()` or `async_task::spawn_local()`, /// we get back two values: /// /// 1. an `async_task::Task<()>`, which we refer to as a `Runnable` /// 2. an `async_task::JoinHandle<T, ()>`, which is wrapped inside a `Task<T>` /// /// Once a `Runnable` is run, it "vanishes" and only reappears when its future is woken. When it's /// woken up, its schedule function is called, which means the `Runnable` gets pushed into the main /// task queue in the executor. type Runnable = async_task::Task<()>; struct Task<T>(Option<async_task::JoinHandle<T, ()>>); impl<T> Drop for Task<T> { fn drop(&mut self) { if let Some(handle) = &self.0 { handle.cancel(); } } } impl<T> Future for Task<T> { type Output = T; fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { match Pin::new(&mut self.0.as_mut().unwrap()).poll(cx) { Poll::Pending => Poll::Pending, Poll::Ready(output) => Poll::Ready(output.expect("task has failed")), } } } /// The blocking executor. struct Executor { /// Inner state of the executor. inner: Mutex<Inner>, /// Used to put idle threads to sleep and wake them up when new work comes in. cvar: Condvar, } /// Inner state of the blocking executor. struct Inner { /// Number of idle threads in the pool. /// /// Idle threads are sleeping, waiting to get a task to run. idle_count: usize, /// Total number of threads in the pool. /// /// This is the number of idle threads + the number of active threads. thread_count: usize, /// The queue of blocking tasks. queue: VecDeque<Runnable>, } impl Executor { /// Spawns a future onto this executor. /// /// Returns a [`Task`] handle for the spawned task. fn spawn<T: Send + 'static>(future: impl Future<Output = T> + Send + 'static) -> Task<T> { static EXECUTOR: Lazy<Executor> = Lazy::new(|| Executor { inner: Mutex::new(Inner { idle_count: 0, thread_count: 0, queue: VecDeque::new(), }), cvar: Condvar::new(), }); // Create a task, schedule it, and return its `Task` handle. let (runnable, handle) = async_task::spawn(future, |r| EXECUTOR.schedule(r), ()); runnable.schedule(); Task(Some(handle)) } /// Runs the main loop on the current thread. /// /// This function runs blocking tasks until it becomes idle and times out. fn main_loop(&'static self) { let mut inner = self.inner.lock().unwrap(); loop { // This thread is not idle anymore because it's going to run tasks. inner.idle_count -= 1; // Run tasks in the queue. while let Some(runnable) = inner.queue.pop_front() { // We have found a task - grow the pool if needed. self.grow_pool(inner); // Run the task. let _ = panic::catch_unwind(|| runnable.run()); // Re-lock the inner state and continue. inner = self.inner.lock().unwrap(); } // This thread is now becoming idle. inner.idle_count += 1; // Put the thread to sleep until another task is scheduled. let timeout = Duration::from_millis(500); let (lock, res) = self.cvar.wait_timeout(inner, timeout).unwrap(); inner = lock; // If there are no tasks after a while, stop this thread. if res.timed_out() && inner.queue.is_empty() { inner.idle_count -= 1; inner.thread_count -= 1; break; } } } /// Schedules a runnable task for execution. fn schedule(&'static self, runnable: Runnable) { let mut inner = self.inner.lock().unwrap(); inner.queue.push_back(runnable); // Notify a sleeping thread and spawn more threads if needed. self.cvar.notify_one(); self.grow_pool(inner); } /// Spawns more blocking threads if the pool is overloaded with work. fn grow_pool(&'static self, mut inner: MutexGuard<'static, Inner>) { // If runnable tasks greatly outnumber idle threads and there aren't too many threads // already, then be aggressive: wake all idle threads and spawn one more thread. while inner.queue.len() > inner.idle_count * 5 && inner.thread_count < 500 { // The new thread starts in idle state. inner.idle_count += 1; inner.thread_count += 1; // Notify all existing idle threads because we need to hurry up. self.cvar.notify_all(); // Spawn the new thread. thread::spawn(move || self.main_loop()); } } } /// Spawns blocking I/O onto a thread. /// /// Note that `blocking!(expr)` is just syntax sugar for /// `Blocking::spawn(async move { expr }).await`. /// /// # Examples /// /// Read a file into a string: /// /// ```no_run /// use blocking::blocking; /// use std::fs; /// /// # futures::executor::block_on(async { /// let contents = blocking!(fs::read_to_string("file.txt"))?; /// # std::io::Result::Ok(()) }); /// ``` /// /// Spawn a process: /// /// ```no_run /// use blocking::blocking; /// use std::process::Command; /// /// # futures::executor::block_on(async { /// let out = blocking!(Command::new("dir").output())?; /// # std::io::Result::Ok(()) }); /// ``` #[macro_export] macro_rules! blocking { ($($expr:tt)*) => { $crate::Blocking::spawn(async move { $($expr)* }).await }; } /// Async I/O that runs on a thread. /// /// This handle represents a future performing some blocking I/O on the special thread pool. The /// output of the future can be awaited because [`Blocking`] itself is a future. /// /// It's also possible to interact with [`Blocking`] through [`Stream`], [`AsyncRead`] and /// [`AsyncWrite`] traits if the inner type implements [`Iterator`], [`Read`], or [`Write`]. /// /// To spawn a future and start it immediately, use [`Blocking::spawn()`]. To create an I/O handle /// that will lazily spawn an I/O future on its own, use [`Blocking::new()`]. /// /// If the [`Blocking`] handle is dropped, the future performing I/O will be canceled if it hasn't /// completed yet. However, note that it's not possible to forcibly cancel blocking I/O, so if the /// future is currently running, it won't be canceled until it yields. /// /// If writing some data through the [`AsyncWrite`] trait, make sure to flush before dropping the /// [`Blocking`] handle or some written data might get lost. Alternatively, await the handle to /// complete the pending work and extract the inner blocking I/O handle. /// /// # Examples /// /// ``` /// use blocking::Blocking; /// use futures::prelude::*; /// use std::io::stdout; /// /// # futures::executor::block_on(async { /// let mut stdout = Blocking::new(stdout()); /// stdout.write_all(b"Hello world!").await?; /// /// let inner = stdout.await; /// # std::io::Result::Ok(()) }); /// ``` pub struct Blocking<T>(State<T>); impl<T> Blocking<T> { /// Wraps a blocking I/O handle into an async interface. /// /// # Examples /// /// ```no_run /// use blocking::Blocking; /// use std::io::stdin; /// /// # futures::executor::block_on(async { /// // Create an async handle to standard input. /// let stdin = Blocking::new(stdin()); /// # std::io::Result::Ok(()) }); /// ``` pub fn new(io: T) -> Blocking<T> { Blocking(State::Idle(Some(Box::new(io)))) } /// Gets a mutable reference to the blocking I/O handle. /// /// This is an async method because the I/O handle might be on a different thread and needs to /// be moved onto the current thread before we can get a reference to it. /// /// # Examples /// /// ```no_run /// use blocking::Blocking; /// use std::fs::File; /// /// # futures::executor::block_on(async { /// let mut file = Blocking::new(File::create("file.txt")?); /// let metadata = file.get_mut().await.metadata()?; /// # std::io::Result::Ok(()) }); /// ``` pub async fn get_mut(&mut self) -> &mut T { // Wait for the running task to stop and ignore I/O errors if there are any. let _ = future::poll_fn(|cx| self.poll_stop(cx)).await; // Assume idle state and get a reference to the inner value. match &mut self.0 { State::Idle(t) => t.as_mut().expect("inner value was taken out"), State::Streaming(..) | State::Reading(..) | State::Writing(..) | State::Task(..) => { unreachable!("when stopped, the state machine must be in idle state"); } } } /// Extracts the inner blocking I/O handle. /// /// This is an async method because the I/O handle might be on a different thread and needs to /// be moved onto the current thread before we can extract it. /// /// Note that awaiting this method is equivalent to awaiting the [`Blocking`] handle. /// /// # Examples /// /// ```no_run /// use blocking::Blocking; /// use futures::prelude::*; /// use std::fs::File; /// /// # futures::executor::block_on(async { /// let mut file = Blocking::new(File::create("file.txt")?); /// file.write_all(b"Hello world!").await?; /// /// let file = file.into_inner().await; /// # std::io::Result::Ok(()) }); /// ``` pub async fn into_inner(self) -> T { // There's a bug in rustdoc causing it to render `mut self` as `__arg0: Self`, so we just // bind `self` to a local mutable variable. let mut this = self; // Wait for the running task to stop and ignore I/O errors if there are any. let _ = future::poll_fn(|cx| this.poll_stop(cx)).await; // Assume idle state and extract the inner value. match &mut this.0 { State::Idle(t) => *t.take().expect("inner value was taken out"), State::Streaming(..) | State::Reading(..) | State::Writing(..) | State::Task(..) => { unreachable!("when stopped, the state machine must be in idle state"); } } } /// Waits for the running task to stop. /// /// On success, the state machine is moved into the idle state. fn poll_stop(&mut self, cx: &mut Context<'_>) -> Poll<io::Result<()>> { loop { match &mut self.0 { State::Idle(_) => return Poll::Ready(Ok(())), State::Streaming(any, task) => { // Drop the receiver to close the channel. This stops the `send()` operation in // the task, after which the task returns the iterator back. any.take(); // Poll the task to retrieve the iterator. let iter = futures::ready!(Pin::new(task).poll(cx)); self.0 = State::Idle(Some(iter)); } State::Reading(reader, task) => { // Drop the reader to close the pipe. This stops the `futures::io::copy` // operation in the task, after which the task returns the I/O handle back. reader.take(); // Poll the task to retrieve the I/O handle. let (res, io) = futures::ready!(Pin::new(task).poll(cx)); // Make sure to move into the idle state before reporting errors. self.0 = State::Idle(Some(io)); res?; } State::Writing(writer, task) => { // Drop the writer to close the pipe. This stops the `futures::io::copy` // operation in the task, after which the task flushes the I/O handle and // returns it back. writer.take(); // Poll the task to retrieve the I/O handle. let (res, io) = futures::ready!(Pin::new(task).poll(cx)); // Make sure to move into the idle state before reporting errors. self.0 = State::Idle(Some(io)); res?; } State::Task(task) => { // Poll the task to retrieve the inner value. let t = futures::ready!(Pin::new(task).poll(cx)); self.0 = State::Idle(Some(Box::new(t))); } } } } } impl<T: Send + 'static> Blocking<T> { /// Spawns a future that is allowed to do blocking I/O. /// /// If the [`Blocking`] handle is dropped, the future will be canceled if it hasn't completed /// yet. However, note that it's not possible to forcibly cancel blocking I/O, so if the future /// is currently running, it won't be canceled until it yields. /// /// # Examples /// /// ```no_run /// use blocking::Blocking; /// use std::fs; /// /// # futures::executor::block_on(async { /// let contents = Blocking::spawn(async { fs::read_to_string("file.txt") }).await?; /// # std::io::Result::Ok(()) }); /// ``` pub fn spawn(future: impl Future<Output = T> + Send + 'static) -> Blocking<T> { let task = Executor::spawn(future); Blocking(State::Task(task)) } } impl<T> Future for Blocking<T> { type Output = T; fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { // Wait for the running task to stop and ignore I/O errors if there are any. let _ = futures::ready!(self.poll_stop(cx)); // Assume idle state and extract the inner value. match &mut self.0 { State::Idle(t) => Poll::Ready(*t.take().expect("inner value was taken out")), State::Streaming(..) | State::Reading(..) | State::Writing(..) | State::Task(..) => { unreachable!("when stopped, the state machine must be in idle state"); } } } } /// Current state of a blocking task. enum State<T> { /// There is no blocking task. /// /// The inner value is readily available, unless it has already been extracted. The value is /// extracted out by [`Blocking::into_inner()`], [`AsyncWrite::poll_close()`], or by awaiting /// [`Blocking`]. Idle(Option<Box<T>>), /// A task was spawned by [`Blocking::spawn()`] and is still running. Task(Task<T>), /// The inner value is an [`Iterator`] currently iterating in a task. /// /// The `dyn Any` value here is a `mpsc::Receiver<<T as Iterator>::Item>`. Streaming(Option<Box<dyn Any>>, Task<Box<T>>), /// The inner value is a [`Read`] currently reading in a task. Reading(Option<Reader>, Task<(io::Result<()>, Box<T>)>), /// The inner value is a [`Write`] currently writing in a task. Writing(Option<Writer>, Task<(io::Result<()>, Box<T>)>), } impl<T: Iterator + Send + 'static> Stream for Blocking<T> where T::Item: Send + 'static, { type Item = T::Item; fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T::Item>> { loop { match &mut self.0 { // If not in idle or active streaming state, stop the running task. State::Task(..) | State::Streaming(None, _) | State::Reading(..) | State::Writing(..) => { // Wait for the running task to stop. let _ = futures::ready!(self.poll_stop(cx)); } // If idle, start a streaming task. State::Idle(iter) => { // If idle, take the iterator out to run it on a blocking task. let mut iter = iter.take().unwrap(); // This channel capacity seems to work well in practice. If it's too low, there // will be too much synchronization between tasks. If too high, memory // consumption increases. let (mut sender, receiver) = mpsc::channel(8 * 1024); // 8192 items // Spawn a blocking task that runs the iterator and returns it when done. let task = Executor::spawn(async move { for item in &mut iter { if sender.send(item).await.is_err() { break; } } iter }); // Move into the busy state and poll again. self.0 = State::Streaming(Some(Box::new(receiver)), task); } // If streaming, receive an item. State::Streaming(Some(any), task) => { let receiver = any.downcast_mut::<mpsc::Receiver<T::Item>>().unwrap(); // Poll the channel. let opt = futures::ready!(Pin::new(receiver).poll_next(cx)); // If the channel is closed, retrieve the iterator back from the blocking task. // This is not really a required step, but it's cleaner to drop the iterator on // the same thread that created it. if opt.is_none() { // Poll the task to retrieve the iterator. let iter = futures::ready!(Pin::new(task).poll(cx)); self.0 = State::Idle(Some(iter)); } return Poll::Ready(opt); } } } } } impl<T: Read + Send + 'static> AsyncRead for Blocking<T> { fn poll_read( mut self: Pin<&mut Self>, cx: &mut Context<'_>, buf: &mut [u8], ) -> Poll<io::Result<usize>> { loop { match &mut self.0 { // If not in idle or active reading state, stop the running task. State::Task(..) | State::Reading(None, _) | State::Streaming(..) | State::Writing(..) => { // Wait for the running task to stop. futures::ready!(self.poll_stop(cx))?; } // If idle, start a reading task. State::Idle(io) => { // If idle, take the I/O handle out to read it on a blocking task. let mut io = io.take().unwrap(); // This pipe capacity seems to work well in practice. If it's too low, there // will be too much synchronization between tasks. If too high, memory // consumption increases. let (reader, mut writer) = pipe(8 * 1024 * 1024); // 8 MB // Spawn a blocking task that reads and returns the I/O handle when done. let task = Executor::spawn(async move { // Copy bytes from the I/O handle into the pipe until the pipe is closed or // an error occurs. loop { match future::poll_fn(|cx| writer.poll_write(cx, &mut io)).await { Ok(0) => return (Ok(()), io), Ok(_) => {} Err(err) => return (Err(err), io), } } }); // Move into the busy state and poll again. self.0 = State::Reading(Some(reader), task); } // If reading, read bytes from the pipe. State::Reading(Some(reader), task) => { // Poll the pipe. let n = futures::ready!(Pin::new(reader).poll_read(cx, buf))?; // If the pipe is closed, retrieve the I/O handle back from the blocking task. // This is not really a required step, but it's cleaner to drop the handle on // the same thread that created it. if n == 0 { // Poll the task to retrieve the I/O handle. let (res, io) = futures::ready!(Pin::new(task).poll(cx)); // Make sure to move into the idle state before reporting errors. self.0 = State::Idle(Some(io)); res?; } return Poll::Ready(Ok(n)); } } } } } impl<T: Write + Send + 'static> AsyncWrite for Blocking<T> { fn poll_write( mut self: Pin<&mut Self>, cx: &mut Context<'_>, buf: &[u8], ) -> Poll<io::Result<usize>> { loop { match &mut self.0 { // If not in idle or active writing state, stop the running task. State::Task(..) | State::Writing(None, _) | State::Streaming(..) | State::Reading(..) => { // Wait for the running task to stop. futures::ready!(self.poll_stop(cx))?; } // If idle, start the writing task. State::Idle(io) => { // If idle, take the I/O handle out to write on a blocking task. let mut io = io.take().unwrap(); // This pipe capacity seems to work well in practice. If it's too low, there will // be too much synchronization between tasks. If too high, memory consumption // increases. let (mut reader, writer) = pipe(8 * 1024 * 1024); // 8 MB // Spawn a blocking task that writes and returns the I/O handle when done. let task = Executor::spawn(async move { // Copy bytes from the pipe into the I/O handle until the pipe is closed or an // error occurs. Flush the I/O handle at the end. loop { match future::poll_fn(|cx| reader.poll_read(cx, &mut io)).await { Ok(0) => return (io.flush(), io), Ok(_) => {} Err(err) => { let _ = io.flush(); return (Err(err), io); } } } }); // Move into the busy state. self.0 = State::Writing(Some(writer), task); } // If writing,write more bytes into the pipe. State::Writing(Some(writer), _) => return Pin::new(writer).poll_write(cx, buf), } } } fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> { loop { match &mut self.0 { // If not in idle state, stop the running task. State::Task(..) | State::Streaming(..) | State::Writing(..) | State::Reading(..) => { // Wait for the running task to stop. futures::ready!(self.poll_stop(cx))?; } // Idle implies flushed. State::Idle(_) => return Poll::Ready(Ok(())), } } } fn poll_close(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> { // First, make sure the I/O handle is flushed. futures::ready!(Pin::new(&mut *self).poll_flush(cx))?; // Then move into the idle state with no I/O handle, thus dropping it. self.0 = State::Idle(None); Poll::Ready(Ok(())) } } /// Creates a bounded single-producer single-consumer pipe. /// /// A pipe is a ring buffer of `cap` bytes that implements traits [`AsyncRead`] and [`AsyncWrite`]. /// /// When the sender is dropped, remaining bytes in the pipe can still be read. After that, attempts /// to read will result in `Ok(0)`, i.e. they will always 'successfully' read 0 bytes. /// /// When the receiver is dropped, the pipe is closed and no more bytes and be written into it. /// Further writes will result in `Ok(0)`, i.e. they will always 'successfully' write 0 bytes. fn pipe(cap: usize) -> (Reader, Writer) { assert!(cap > 0, "capacity must be positive"); assert!(cap.checked_mul(2).is_some(), "capacity is too large"); // Allocate the ring buffer. let mut v = Vec::with_capacity(cap); let buffer = v.as_mut_ptr(); mem::forget(v); let inner = Arc::new(Pipe { head: AtomicUsize::new(0), tail: AtomicUsize::new(0), reader: AtomicWaker::new(), writer: AtomicWaker::new(), closed: AtomicBool::new(false), buffer, cap, }); let r = Reader { inner: inner.clone(), head: 0, tail: 0, }; let w = Writer { inner, head: 0, tail: 0, zeroed_until: 0, }; (r, w) } /// The reading side of a pipe. #[derive(Debug)] struct Reader { /// The inner ring buffer. inner: Arc<Pipe>, /// The head index, moved by the reader, in the range `0..2*cap`. /// /// This index always matches `inner.head`. head: usize, /// The tail index, moved by the writer, in the range `0..2*cap`. /// /// This index is a snapshot of `index.tail` that might become stale at any point. tail: usize, } /// The writing side of a pipe. #[derive(Debug)] struct Writer { /// The inner ring buffer. inner: Arc<Pipe>, /// The head index, moved by the reader, in the range `0..2*cap`. /// /// This index is a snapshot of `index.head` that might become stale at any point. head: usize, /// The tail index, moved by the writer, in the range `0..2*cap`. /// /// This index always matches `inner.tail`. tail: usize, /// How many bytes at the beginning of the buffer have been zeroed. /// /// The pipe allocates an uninitialized buffer, and we must be careful about passing /// uninitialized data to user code. Zeroing the buffer right after allocation would be too /// expensive, so we zero it in smaller chunks as the writer makes progress. zeroed_until: usize, } unsafe impl Send for Reader {} unsafe impl Send for Writer {} /// The inner ring buffer. /// /// Head and tail indices are in the range `0..2*cap`, even though they really map onto the /// `0..cap` range. The distance between head and tail indices is never more than `cap`. /// /// The reason why indices are not in the range `0..cap` is because we need to distinguish between /// the pipe being empty and being full. If head and tail were in `0..cap`, then `head == tail` /// could mean the pipe is either empty or full, but we don't know which! #[derive(Debug)] struct Pipe { /// The head index, moved by the reader, in the range `0..2*cap`. head: AtomicUsize, /// The tail index, moved by the writer, in the range `0..2*cap`. tail: AtomicUsize, /// A waker representing the blocked reader. reader: AtomicWaker, /// A waker representing the blocked writer. writer: AtomicWaker, /// Set to `true` if the reader or writer was dropped. closed: AtomicBool, /// The byte buffer. buffer: *mut u8, /// The buffer capacity. cap: usize, } impl Drop for Pipe { fn drop(&mut self) { // Deallocate the byte buffer. unsafe { Vec::from_raw_parts(self.buffer, 0, self.cap); } } } impl Drop for Reader { fn drop(&mut self) { // Dropping closes the pipe and then wakes the writer. self.inner.closed.store(true, Ordering::SeqCst); self.inner.writer.wake(); } } impl Drop for Writer { fn drop(&mut self) { // Dropping closes the pipe and then wakes the reader. self.inner.closed.store(true, Ordering::SeqCst); self.inner.reader.wake(); } } impl Reader { fn poll_read(&mut self, cx: &mut Context<'_>, mut dest: impl Write) -> Poll<io::Result<usize>> { let cap = self.inner.cap; // Calculates the distance between two indices. let distance = |a: usize, b: usize| { if a <= b { b - a } else { 2 * cap - (a - b) } }; // If the pipe appears to be empty... if distance(self.head, self.tail) == 0 { // Reload the tail in case it's become stale. self.tail = self.inner.tail.load(Ordering::Acquire); // If the pipe is now really empty... if distance(self.head, self.tail) == 0 { // Register the waker. self.inner.reader.register(cx.waker()); atomic::fence(Ordering::SeqCst); // Reload the tail after registering the waker. self.tail = self.inner.tail.load(Ordering::Acquire); // If the pipe is still empty... if distance(self.head, self.tail) == 0 { // Check whether the pipe is closed or just empty. if self.inner.closed.load(Ordering::Relaxed) { return Poll::Ready(Ok(0)); } else { return Poll::Pending; } } } } // The pipe is not empty so remove the waker. self.inner.reader.take(); // Given an index in `0..2*cap`, returns the real index in `0..cap`. let real_index = |i: usize| { if i < cap { i } else { i - cap } }; // Number of bytes read so far. let mut count = 0; loop { // Calculate how many bytes to read in this iteration. let n = (128 * 1024) // Not too many bytes in one go - better to wake the writer soon! .min(distance(self.head, self.tail)) // No more than bytes in the pipe. .min(cap - real_index(self.head)); // Don't go past the buffer boundary. // Create a slice of data in the pipe buffer. let pipe_slice = unsafe { slice::from_raw_parts(self.inner.buffer.add(real_index(self.head)), n) }; // Copy bytes from the pipe buffer into `dest`. let n = dest .write(pipe_slice) .expect("shouldn't fail because `dest` is a slice"); count += n; // If pipe is empty or `dest` is full, return. if n == 0 { return Poll::Ready(Ok(count)); } // Move the head forward. if self.head + n < 2 * cap { self.head += n; } else { self.head = 0; } // Store the current head index. self.inner.head.store(self.head, Ordering::Release); // Wake the writer because the pipe is not full. self.inner.writer.wake(); } } } impl Writer { fn poll_write(&mut self, cx: &mut Context<'_>, mut src: impl Read) -> Poll<io::Result<usize>> { // Just a quick check if the pipe is closed, which is why a relaxed load is okay. if self.inner.closed.load(Ordering::Relaxed) { return Poll::Ready(Ok(0)); } // Calculates the distance between two indices. let cap = self.inner.cap; let distance = |a: usize, b: usize| { if a <= b { b - a } else { 2 * cap - (a - b) } }; // If the pipe appears to be full... if distance(self.head, self.tail) == cap { // Reload the head in case it's become stale. self.head = self.inner.head.load(Ordering::Acquire); // If the pipe is now really empty... if distance(self.head, self.tail) == cap { // Register the waker. self.inner.writer.register(cx.waker()); atomic::fence(Ordering::SeqCst); // Reload the head after registering the waker. self.head = self.inner.head.load(Ordering::Acquire); // If the pipe is still full... if distance(self.head, self.tail) == cap { // Check whether the pipe is closed or just full. if self.inner.closed.load(Ordering::Relaxed) { return Poll::Ready(Ok(0)); } else { return Poll::Pending; } } } } // The pipe is not full so remove the waker. self.inner.writer.take(); // Given an index in `0..2*cap`, returns the real index in `0..cap`. let real_index = |i: usize| { if i < cap { i } else { i - cap } }; // Number of bytes written so far. let mut count = 0; loop { // Calculate how many bytes to write in this iteration. let n = (128 * 1024) // Not too many bytes in one go - better to wake the reader soon! .min(self.zeroed_until * 2 + 4096) // Don't zero too many bytes when starting. .min(cap - distance(self.head, self.tail)) // No more than space in the pipe. .min(cap - real_index(self.tail)); // Don't go past the buffer boundary. // Create a slice of available space in the pipe buffer. let pipe_slice_mut = unsafe { let from = real_index(self.tail); let to = from + n; // Make sure all bytes in the slice are initialized. if self.zeroed_until < to { self.inner .buffer .add(self.zeroed_until) .write_bytes(0u8, to - self.zeroed_until); self.zeroed_until = to; } slice::from_raw_parts_mut(self.inner.buffer.add(from), n) }; // Copy bytes from `src` into the piper buffer. let n = src .read(pipe_slice_mut) .expect("shouldn't fail because `src` is a slice"); count += n; // If the pipe is full or `src` is empty, return. if n == 0 { return Poll::Ready(Ok(count)); } // Move the tail forward. if self.tail + n < 2 * cap { self.tail += n; } else { self.tail = 0; } // Store the current tail index. self.inner.tail.store(self.tail, Ordering::Release); // Wake the reader because the pipe is not empty. self.inner.reader.wake(); } } }