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//! A multi-producer, single-consumer, futures-aware, FIFO queue with back pressure. //! //! A channel can be used as a communication primitive between tasks running on //! `futures-rs` executors. Channel creation provides `Receiver` and `Sender` //! handles. `Receiver` implements `Stream` and allows a task to read values //! out of the channel. If there is no message to read from the channel, the //! current task will be notified when a new value is sent. `Sender` implements //! the `Sink` trait and allows a task to send messages into the channel. If //! the channel is at capacity, then send will be rejected and the task will be //! notified when additional capacity is available. //! //! # Disconnection //! //! When all `Sender` handles have been dropped, it is no longer possible to //! send values into the channel. This is considered the termination event of //! the stream. As such, `Sender::poll` will return `Ok(Ready(None))`. //! //! If the receiver handle is dropped, then messages can no longer be read out //! of the channel. In this case, a `send` will result in an error. //! //! # Clean Shutdown //! //! If the `Receiver` is simply dropped, then it is possible for there to be //! messages still in the channel that will not be processed. As such, it is //! usually desirable to perform a "clean" shutdown. To do this, the receiver //! will first call `close`, which will prevent any further messages to be sent //! into the channel. Then, the receiver consumes the channel to completion, at //! which point the receiver can be dropped. // At the core, the channel uses an atomic FIFO queue for message passing. This // queue is used as the primary coordination primitive. In order to enforce // capacity limits and handle back pressure, a secondary FIFO queue is used to // send parked task handles. // // The general idea is that the channel is created with a `buffer` size of `n`. // The channel capacity is `n + num-senders`. Each sender gets one "guaranteed" // slot to hold a message. This allows `Sender` to know for a fact that a send // will succeed *before* starting to do the actual work of sending the value. // Since most of this work is lock-free, once the work starts, it is impossible // to safely revert. // // If the sender is unable to process a send operation, then the current // task is parked and the handle is sent on the parked task queue. // // Note that the implementation guarantees that the channel capacity will never // exceed the configured limit, however there is no *strict* guarantee that the // receiver will wake up a parked task *immediately* when a slot becomes // available. However, it will almost always unpark a task when a slot becomes // available and it is *guaranteed* that a sender will be unparked when the // message that caused the sender to become parked is read out of the channel. // // The steps for sending a message are roughly: // // 1) Increment the channel message count // 2) If the channel is at capacity, push the task handle onto the wait queue // 3) Push the message onto the message queue. // // The steps for receiving a message are roughly: // // 1) Pop a message from the message queue // 2) Pop a task handle from the wait queue // 3) Decrement the channel message count. // // It's important for the order of operations on lock-free structures to happen // in reverse order between the sender and receiver. This makes the message // queue the primary coordination structure and establishes the necessary // happens-before semantics required for the acquire / release semantics used // by the queue structure. use std::fmt; use std::error::Error; use std::any::Any; use std::sync::atomic::AtomicUsize; use std::sync::atomic::Ordering::SeqCst; use std::sync::{Arc, Mutex}; use std::thread; use std::usize; use sync::mpsc::queue::{Queue, PopResult}; use sync::oneshot; use task::{self, Task}; use future::Executor; use sink::SendAll; use resultstream::{self, Results}; use {Async, AsyncSink, Future, Poll, StartSend, Sink, Stream}; mod queue; /// The transmission end of a channel which is used to send values. /// /// This is created by the `channel` method. #[derive(Debug)] pub struct Sender<T> { // Channel state shared between the sender and receiver. inner: Arc<Inner<T>>, // Handle to the task that is blocked on this sender. This handle is sent // to the receiver half in order to be notified when the sender becomes // unblocked. sender_task: Arc<Mutex<SenderTask>>, // True if the sender might be blocked. This is an optimization to avoid // having to lock the mutex most of the time. maybe_parked: bool, } /// The transmission end of a channel which is used to send values. /// /// This is created by the `unbounded` method. #[derive(Debug)] pub struct UnboundedSender<T>(Sender<T>); trait AssertKinds: Send + Sync + Clone {} impl AssertKinds for UnboundedSender<u32> {} /// The receiving end of a channel which implements the `Stream` trait. /// /// This is a concrete implementation of a stream which can be used to represent /// a stream of values being computed elsewhere. This is created by the /// `channel` method. #[derive(Debug)] pub struct Receiver<T> { inner: Arc<Inner<T>>, } /// The receiving end of a channel which implements the `Stream` trait. /// /// This is a concrete implementation of a stream which can be used to represent /// a stream of values being computed elsewhere. This is created by the /// `unbounded` method. #[derive(Debug)] pub struct UnboundedReceiver<T>(Receiver<T>); /// Error type for sending, used when the receiving end of a channel is /// dropped #[derive(Clone, PartialEq, Eq)] pub struct SendError<T>(T); /// Error type returned from `try_send` #[derive(Clone, PartialEq, Eq)] pub struct TrySendError<T> { kind: TrySendErrorKind<T>, } #[derive(Clone, PartialEq, Eq)] enum TrySendErrorKind<T> { Full(T), Disconnected(T), } impl<T> fmt::Debug for SendError<T> { fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result { fmt.debug_tuple("SendError") .field(&"...") .finish() } } impl<T> fmt::Display for SendError<T> { fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result { write!(fmt, "send failed because receiver is gone") } } impl<T: Any> Error for SendError<T> { fn description(&self) -> &str { "send failed because receiver is gone" } } impl<T> SendError<T> { /// Returns the message that was attempted to be sent but failed. pub fn into_inner(self) -> T { self.0 } } impl<T> fmt::Debug for TrySendError<T> { fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result { fmt.debug_tuple("TrySendError") .field(&"...") .finish() } } impl<T> fmt::Display for TrySendError<T> { fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result { if self.is_full() { write!(fmt, "send failed because channel is full") } else { write!(fmt, "send failed because receiver is gone") } } } impl<T: Any> Error for TrySendError<T> { fn description(&self) -> &str { if self.is_full() { "send failed because channel is full" } else { "send failed because receiver is gone" } } } impl<T> TrySendError<T> { /// Returns true if this error is a result of the channel being full pub fn is_full(&self) -> bool { use self::TrySendErrorKind::*; match self.kind { Full(_) => true, _ => false, } } /// Returns true if this error is a result of the receiver being dropped pub fn is_disconnected(&self) -> bool { use self::TrySendErrorKind::*; match self.kind { Disconnected(_) => true, _ => false, } } /// Returns the message that was attempted to be sent but failed. pub fn into_inner(self) -> T { use self::TrySendErrorKind::*; match self.kind { Full(v) | Disconnected(v) => v, } } } #[derive(Debug)] struct Inner<T> { // Max buffer size of the channel. If `None` then the channel is unbounded. buffer: Option<usize>, // Internal channel state. Consists of the number of messages stored in the // channel as well as a flag signalling that the channel is closed. state: AtomicUsize, // Atomic, FIFO queue used to send messages to the receiver message_queue: Queue<Option<T>>, // Atomic, FIFO queue used to send parked task handles to the receiver. parked_queue: Queue<Arc<Mutex<SenderTask>>>, // Number of senders in existence num_senders: AtomicUsize, // Handle to the receiver's task. recv_task: Mutex<ReceiverTask>, } // Struct representation of `Inner::state`. #[derive(Debug, Clone, Copy)] struct State { // `true` when the channel is open is_open: bool, // Number of messages in the channel num_messages: usize, } #[derive(Debug)] struct ReceiverTask { unparked: bool, task: Option<Task>, } // Returned from Receiver::try_park() enum TryPark { Parked, Closed, NotEmpty, } // The `is_open` flag is stored in the left-most bit of `Inner::state` const OPEN_MASK: usize = usize::MAX - (usize::MAX >> 1); // When a new channel is created, it is created in the open state with no // pending messages. const INIT_STATE: usize = OPEN_MASK; // The maximum number of messages that a channel can track is `usize::MAX >> 1` const MAX_CAPACITY: usize = !(OPEN_MASK); // The maximum requested buffer size must be less than the maximum capacity of // a channel. This is because each sender gets a guaranteed slot. const MAX_BUFFER: usize = MAX_CAPACITY >> 1; // Sent to the consumer to wake up blocked producers #[derive(Debug)] struct SenderTask { task: Option<Task>, is_parked: bool, } impl SenderTask { fn new() -> Self { SenderTask { task: None, is_parked: false, } } fn notify(&mut self) { self.is_parked = false; if let Some(task) = self.task.take() { task.notify(); } } } /// Creates an in-memory channel implementation of the `Stream` trait with /// bounded capacity. /// /// This method creates a concrete implementation of the `Stream` trait which /// can be used to send values across threads in a streaming fashion. This /// channel is unique in that it implements back pressure to ensure that the /// sender never outpaces the receiver. The channel capacity is equal to /// `buffer + num-senders`. In other words, each sender gets a guaranteed slot /// in the channel capacity, and on top of that there are `buffer` "first come, /// first serve" slots available to all senders. /// /// The `Receiver` returned implements the `Stream` trait and has access to any /// number of the associated combinators for transforming the result. pub fn channel<T>(buffer: usize) -> (Sender<T>, Receiver<T>) { // Check that the requested buffer size does not exceed the maximum buffer // size permitted by the system. assert!(buffer < MAX_BUFFER, "requested buffer size too large"); channel2(Some(buffer)) } /// Creates an in-memory channel implementation of the `Stream` trait with /// unbounded capacity. /// /// This method creates a concrete implementation of the `Stream` trait which /// can be used to send values across threads in a streaming fashion. A `send` /// on this channel will always succeed as long as the receive half has not /// been closed. If the receiver falls behind, messages will be buffered /// internally. /// /// **Note** that the amount of available system memory is an implicit bound to /// the channel. Using an `unbounded` channel has the ability of causing the /// process to run out of memory. In this case, the process will be aborted. pub fn unbounded<T>() -> (UnboundedSender<T>, UnboundedReceiver<T>) { let (tx, rx) = channel2(None); (UnboundedSender(tx), UnboundedReceiver(rx)) } fn channel2<T>(buffer: Option<usize>) -> (Sender<T>, Receiver<T>) { let inner = Arc::new(Inner { buffer: buffer, state: AtomicUsize::new(INIT_STATE), message_queue: Queue::new(), parked_queue: Queue::new(), num_senders: AtomicUsize::new(1), recv_task: Mutex::new(ReceiverTask { unparked: false, task: None, }), }); let tx = Sender { inner: inner.clone(), sender_task: Arc::new(Mutex::new(SenderTask::new())), maybe_parked: false, }; let rx = Receiver { inner: inner, }; (tx, rx) } /* * * ===== impl Sender ===== * */ impl<T> Sender<T> { /// Attempts to send a message on this `Sender<T>` without blocking. /// /// This function, unlike `start_send`, is safe to call whether it's being /// called on a task or not. Note that this function, however, will *not* /// attempt to block the current task if the message cannot be sent. /// /// It is not recommended to call this function from inside of a future, /// only from an external thread where you've otherwise arranged to be /// notified when the channel is no longer full. pub fn try_send(&mut self, msg: T) -> Result<(), TrySendError<T>> { // If the sender is currently blocked, reject the message if !self.poll_unparked(false).is_ready() { return Err(TrySendError { kind: TrySendErrorKind::Full(msg), }); } // The channel has capacity to accept the message, so send it self.do_send(Some(msg), false) .map_err(|SendError(v)| { TrySendError { kind: TrySendErrorKind::Disconnected(v), } }) } // Do the send without failing // None means close fn do_send(&mut self, msg: Option<T>, do_park: bool) -> Result<(), SendError<T>> { // First, increment the number of messages contained by the channel. // This operation will also atomically determine if the sender task // should be parked. // // None is returned in the case that the channel has been closed by the // receiver. This happens when `Receiver::close` is called or the // receiver is dropped. let park_self = match self.inc_num_messages(msg.is_none()) { Some(park_self) => park_self, None => { // The receiver has closed the channel. Only abort if actually // sending a message. It is important that the stream // termination (None) is always sent. This technically means // that it is possible for the queue to contain the following // number of messages: // // num-senders + buffer + 1 // if let Some(msg) = msg { return Err(SendError(msg)); } else { return Ok(()); } } }; // If the channel has reached capacity, then the sender task needs to // be parked. This will send the task handle on the parked task queue. // // However, when `do_send` is called while dropping the `Sender`, // `task::current()` can't be called safely. In this case, in order to // maintain internal consistency, a blank message is pushed onto the // parked task queue. if park_self { self.park(do_park); } self.queue_push_and_signal(msg); Ok(()) } // Do the send without parking current task. // // To be called from unbounded sender. fn do_send_nb(&self, msg: T) -> Result<(), SendError<T>> { match self.inc_num_messages(false) { Some(park_self) => assert!(!park_self), None => return Err(SendError(msg)), }; self.queue_push_and_signal(Some(msg)); Ok(()) } // Push message to the queue and signal to the receiver fn queue_push_and_signal(&self, msg: Option<T>) { // Push the message onto the message queue self.inner.message_queue.push(msg); // Signal to the receiver that a message has been enqueued. If the // receiver is parked, this will unpark the task. self.signal(); } // Increment the number of queued messages. Returns if the sender should // block. fn inc_num_messages(&self, close: bool) -> Option<bool> { let mut curr = self.inner.state.load(SeqCst); loop { let mut state = decode_state(curr); // The receiver end closed the channel. if !state.is_open { return None; } // This probably is never hit? Odds are the process will run out of // memory first. It may be worth to return something else in this // case? assert!(state.num_messages < MAX_CAPACITY, "buffer space exhausted; \ sending this messages would overflow the state"); state.num_messages += 1; // The channel is closed by all sender handles being dropped. if close { state.is_open = false; } let next = encode_state(&state); match self.inner.state.compare_exchange(curr, next, SeqCst, SeqCst) { Ok(_) => { // Block if the current number of pending messages has exceeded // the configured buffer size let park_self = match self.inner.buffer { Some(buffer) => state.num_messages > buffer, None => false, }; return Some(park_self) } Err(actual) => curr = actual, } } } // Signal to the receiver task that a message has been enqueued fn signal(&self) { // TODO // This logic can probably be improved by guarding the lock with an // atomic. // // Do this step first so that the lock is dropped when // `unpark` is called let task = { let mut recv_task = self.inner.recv_task.lock().unwrap(); // If the receiver has already been unparked, then there is nothing // more to do if recv_task.unparked { return; } // Setting this flag enables the receiving end to detect that // an unpark event happened in order to avoid unnecessarily // parking. recv_task.unparked = true; recv_task.task.take() }; if let Some(task) = task { task.notify(); } } fn park(&mut self, can_park: bool) { // TODO: clean up internal state if the task::current will fail let task = if can_park { Some(task::current()) } else { None }; { let mut sender = self.sender_task.lock().unwrap(); sender.task = task; sender.is_parked = true; } // Send handle over queue let t = self.sender_task.clone(); self.inner.parked_queue.push(t); // Check to make sure we weren't closed after we sent our task on the // queue let state = decode_state(self.inner.state.load(SeqCst)); self.maybe_parked = state.is_open; } /// Polls the channel to determine if there is guaranteed to be capacity to send at least one /// item without waiting. /// /// Returns `Ok(Async::Ready(_))` if there is sufficient capacity, or returns /// `Ok(Async::NotReady)` if the channel is not guaranteed to have capacity. Returns /// `Err(SendError(_))` if the receiver has been dropped. /// /// # Panics /// /// This method will panic if called from outside the context of a task or future. pub fn poll_ready(&mut self) -> Poll<(), SendError<()>> { let state = decode_state(self.inner.state.load(SeqCst)); if !state.is_open { return Err(SendError(())); } Ok(self.poll_unparked(true)) } /// Returns whether this channel is closed without needing a context. pub fn is_closed(&self) -> bool { !decode_state(self.inner.state.load(SeqCst)).is_open } fn poll_unparked(&mut self, do_park: bool) -> Async<()> { // First check the `maybe_parked` variable. This avoids acquiring the // lock in most cases if self.maybe_parked { // Get a lock on the task handle let mut task = self.sender_task.lock().unwrap(); if !task.is_parked { self.maybe_parked = false; return Async::Ready(()) } // At this point, an unpark request is pending, so there will be an // unpark sometime in the future. We just need to make sure that // the correct task will be notified. // // Update the task in case the `Sender` has been moved to another // task task.task = if do_park { Some(task::current()) } else { None }; Async::NotReady } else { Async::Ready(()) } } } impl<T> Sink for Sender<T> { type SinkItem = T; type SinkError = SendError<T>; fn start_send(&mut self, msg: T) -> StartSend<T, SendError<T>> { // If the sender is currently blocked, reject the message before doing // any work. if !self.poll_unparked(true).is_ready() { return Ok(AsyncSink::NotReady(msg)); } // The channel has capacity to accept the message, so send it. self.do_send(Some(msg), true)?; Ok(AsyncSink::Ready) } fn poll_complete(&mut self) -> Poll<(), SendError<T>> { self.poll_ready() // At this point, the value cannot be returned and `SendError` // cannot be created with a `T` without breaking backwards // comptibility. This means we cannot return an error. // // That said, there is also no guarantee that a `poll_complete` // returning `Ok` implies the receiver sees the message. .or_else(|_| Ok(().into())) } fn close(&mut self) -> Poll<(), SendError<T>> { Ok(Async::Ready(())) } } impl<T> UnboundedSender<T> { /// Returns whether this channel is closed without needing a context. pub fn is_closed(&self) -> bool { self.0.is_closed() } /// Sends the provided message along this channel. /// /// This is an unbounded sender, so this function differs from `Sink::send` /// by ensuring the return type reflects that the channel is always ready to /// receive messages. #[deprecated(note = "renamed to `unbounded_send`")] #[doc(hidden)] pub fn send(&self, msg: T) -> Result<(), SendError<T>> { self.unbounded_send(msg) } /// Sends the provided message along this channel. /// /// This is an unbounded sender, so this function differs from `Sink::send` /// by ensuring the return type reflects that the channel is always ready to /// receive messages. pub fn unbounded_send(&self, msg: T) -> Result<(), SendError<T>> { self.0.do_send_nb(msg) } } impl<T> Sink for UnboundedSender<T> { type SinkItem = T; type SinkError = SendError<T>; fn start_send(&mut self, msg: T) -> StartSend<T, SendError<T>> { self.0.start_send(msg) } fn poll_complete(&mut self) -> Poll<(), SendError<T>> { self.0.poll_complete() } fn close(&mut self) -> Poll<(), SendError<T>> { Ok(Async::Ready(())) } } impl<'a, T> Sink for &'a UnboundedSender<T> { type SinkItem = T; type SinkError = SendError<T>; fn start_send(&mut self, msg: T) -> StartSend<T, SendError<T>> { self.0.do_send_nb(msg)?; Ok(AsyncSink::Ready) } fn poll_complete(&mut self) -> Poll<(), SendError<T>> { Ok(Async::Ready(())) } fn close(&mut self) -> Poll<(), SendError<T>> { Ok(Async::Ready(())) } } impl<T> Clone for UnboundedSender<T> { fn clone(&self) -> UnboundedSender<T> { UnboundedSender(self.0.clone()) } } impl<T> Clone for Sender<T> { fn clone(&self) -> Sender<T> { // Since this atomic op isn't actually guarding any memory and we don't // care about any orderings besides the ordering on the single atomic // variable, a relaxed ordering is acceptable. let mut curr = self.inner.num_senders.load(SeqCst); loop { // If the maximum number of senders has been reached, then fail if curr == self.inner.max_senders() { panic!("cannot clone `Sender` -- too many outstanding senders"); } debug_assert!(curr < self.inner.max_senders()); let next = curr + 1; let actual = self.inner.num_senders.compare_and_swap(curr, next, SeqCst); // The ABA problem doesn't matter here. We only care that the // number of senders never exceeds the maximum. if actual == curr { return Sender { inner: self.inner.clone(), sender_task: Arc::new(Mutex::new(SenderTask::new())), maybe_parked: false, }; } curr = actual; } } } impl<T> Drop for Sender<T> { fn drop(&mut self) { // Ordering between variables don't matter here let prev = self.inner.num_senders.fetch_sub(1, SeqCst); if prev == 1 { let _ = self.do_send(None, false); } } } /* * * ===== impl Receiver ===== * */ impl<T> Receiver<T> { /// Closes the receiving half /// /// This prevents any further messages from being sent on the channel while /// still enabling the receiver to drain messages that are buffered. pub fn close(&mut self) { let mut curr = self.inner.state.load(SeqCst); loop { let mut state = decode_state(curr); if !state.is_open { break } state.is_open = false; let next = encode_state(&state); match self.inner.state.compare_exchange(curr, next, SeqCst, SeqCst) { Ok(_) => break, Err(actual) => curr = actual, } } // Wake up any threads waiting as they'll see that we've closed the // channel and will continue on their merry way. loop { match unsafe { self.inner.parked_queue.pop() } { PopResult::Data(task) => { task.lock().unwrap().notify(); } PopResult::Empty => break, PopResult::Inconsistent => thread::yield_now(), } } } fn next_message(&mut self) -> Async<Option<T>> { // Pop off a message loop { match unsafe { self.inner.message_queue.pop() } { PopResult::Data(msg) => { // If there are any parked task handles in the parked queue, // pop one and unpark it. self.unpark_one(); // Decrement number of messages self.dec_num_messages(); return Async::Ready(msg); } PopResult::Empty => { // The queue is empty, return NotReady return Async::NotReady; } PopResult::Inconsistent => { // Inconsistent means that there will be a message to pop // in a short time. This branch can only be reached if // values are being produced from another thread, so there // are a few ways that we can deal with this: // // 1) Spin // 2) thread::yield_now() // 3) task::current().unwrap() & return NotReady // // For now, thread::yield_now() is used, but it would // probably be better to spin a few times then yield. thread::yield_now(); } } } } // Unpark a single task handle if there is one pending in the parked queue fn unpark_one(&mut self) { loop { match unsafe { self.inner.parked_queue.pop() } { PopResult::Data(task) => { task.lock().unwrap().notify(); return; } PopResult::Empty => { // Queue empty, no task to wake up. return; } PopResult::Inconsistent => { // Same as above thread::yield_now(); } } } } // Try to park the receiver task fn try_park(&self) -> TryPark { let curr = self.inner.state.load(SeqCst); let state = decode_state(curr); // If the channel is closed, then there is no need to park. if state.is_closed() { return TryPark::Closed; } // First, track the task in the `recv_task` slot let mut recv_task = self.inner.recv_task.lock().unwrap(); if recv_task.unparked { // Consume the `unpark` signal without actually parking recv_task.unparked = false; return TryPark::NotEmpty; } recv_task.task = Some(task::current()); TryPark::Parked } fn dec_num_messages(&self) { let mut curr = self.inner.state.load(SeqCst); loop { let mut state = decode_state(curr); state.num_messages -= 1; let next = encode_state(&state); match self.inner.state.compare_exchange(curr, next, SeqCst, SeqCst) { Ok(_) => break, Err(actual) => curr = actual, } } } } impl<T> Stream for Receiver<T> { type Item = T; type Error = (); fn poll(&mut self) -> Poll<Option<T>, ()> { loop { // Try to read a message off of the message queue. match self.next_message() { Async::Ready(msg) => return Ok(Async::Ready(msg)), Async::NotReady => { // There are no messages to read, in this case, attempt to // park. The act of parking will verify that the channel is // still empty after the park operation has completed. match self.try_park() { TryPark::Parked => { // The task was parked, and the channel is still // empty, return NotReady. return Ok(Async::NotReady); } TryPark::Closed => { // The channel is closed, there will be no further // messages. return Ok(Async::Ready(None)); } TryPark::NotEmpty => { // A message has been sent while attempting to // park. Loop again, the next iteration is // guaranteed to get the message. continue; } } } } } } } impl<T> Drop for Receiver<T> { fn drop(&mut self) { // Drain the channel of all pending messages self.close(); loop { match self.next_message() { Async::Ready(_) => {} Async::NotReady => { let curr = self.inner.state.load(SeqCst); let state = decode_state(curr); // If the channel is closed, then there is no need to park. if state.is_closed() { return; } // TODO: Spinning isn't ideal, it might be worth // investigating using a condvar or some other strategy // here. That said, if this case is hit, then another thread // is about to push the value into the queue and this isn't // the only spinlock in the impl right now. thread::yield_now(); } } } } } impl<T> UnboundedReceiver<T> { /// Closes the receiving half /// /// This prevents any further messages from being sent on the channel while /// still enabling the receiver to drain messages that are buffered. pub fn close(&mut self) { self.0.close(); } } impl<T> Stream for UnboundedReceiver<T> { type Item = T; type Error = (); fn poll(&mut self) -> Poll<Option<T>, ()> { self.0.poll() } } /// Handle returned from the `spawn` function. /// /// This handle is a stream that proxies a stream on a separate `Executor`. /// Created through the `mpsc::spawn` function, this handle will produce /// the same values as the proxied stream, as they are produced in the executor, /// and uses a limited buffer to exert back-pressure on the remote stream. /// /// If this handle is dropped, then the stream will no longer be polled and is /// scheduled to be dropped. pub struct SpawnHandle<Item, Error> { rx: Receiver<Result<Item, Error>>, _cancel_tx: oneshot::Sender<()>, } /// Type of future which `Executor` instances must be able to execute for `spawn`. pub struct Execute<S: Stream> { inner: SendAll<Sender<Result<S::Item, S::Error>>, Results<S, SendError<Result<S::Item, S::Error>>>>, cancel_rx: oneshot::Receiver<()>, } /// Spawns a `stream` onto the instance of `Executor` provided, `executor`, /// returning a handle representing the remote stream. /// /// The `stream` will be canceled if the `SpawnHandle` is dropped. /// /// The `SpawnHandle` returned is a stream that is a proxy for `stream` itself. /// When `stream` has additional items available, then the `SpawnHandle` /// will have those same items available. /// /// At most `buffer + 1` elements will be buffered at a time. If the buffer /// is full, then `stream` will stop progressing until more space is available. /// This allows the `SpawnHandle` to exert backpressure on the `stream`. /// /// # Panics /// /// This function will panic if `executor` is unable spawn a `Future` containing /// the entirety of the `stream`. pub fn spawn<S, E>(stream: S, executor: &E, buffer: usize) -> SpawnHandle<S::Item, S::Error> where S: Stream, E: Executor<Execute<S>> { let (cancel_tx, cancel_rx) = oneshot::channel(); let (tx, rx) = channel(buffer); executor.execute(Execute { inner: tx.send_all(resultstream::new(stream)), cancel_rx: cancel_rx, }).expect("failed to spawn stream"); SpawnHandle { rx: rx, _cancel_tx: cancel_tx, } } /// Spawns a `stream` onto the instance of `Executor` provided, `executor`, /// returning a handle representing the remote stream, with unbounded buffering. /// /// The `stream` will be canceled if the `SpawnHandle` is dropped. /// /// The `SpawnHandle` returned is a stream that is a proxy for `stream` itself. /// When `stream` has additional items available, then the `SpawnHandle` /// will have those same items available. /// /// An unbounded buffer is used, which means that values will be buffered as /// fast as `stream` can produce them, without any backpressure. Therefore, if /// `stream` is an infinite stream, it can use an unbounded amount of memory, and /// potentially hog CPU resources. /// /// # Panics /// /// This function will panic if `executor` is unable spawn a `Future` containing /// the entirety of the `stream`. pub fn spawn_unbounded<S, E>(stream: S, executor: &E) -> SpawnHandle<S::Item, S::Error> where S: Stream, E: Executor<Execute<S>> { let (cancel_tx, cancel_rx) = oneshot::channel(); let (tx, rx) = channel2(None); executor.execute(Execute { inner: tx.send_all(resultstream::new(stream)), cancel_rx: cancel_rx, }).expect("failed to spawn stream"); SpawnHandle { rx: rx, _cancel_tx: cancel_tx, } } impl<I, E> Stream for SpawnHandle<I, E> { type Item = I; type Error = E; fn poll(&mut self) -> Poll<Option<I>, E> { match self.rx.poll() { Ok(Async::Ready(Some(Ok(t)))) => Ok(Async::Ready(Some(t.into()))), Ok(Async::Ready(Some(Err(e)))) => Err(e), Ok(Async::Ready(None)) => Ok(Async::Ready(None)), Ok(Async::NotReady) => Ok(Async::NotReady), Err(_) => unreachable!("mpsc::Receiver should never return Err"), } } } impl<I, E> fmt::Debug for SpawnHandle<I, E> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.debug_struct("SpawnHandle") .finish() } } impl<S: Stream> Future for Execute<S> { type Item = (); type Error = (); fn poll(&mut self) -> Poll<(), ()> { match self.cancel_rx.poll() { Ok(Async::NotReady) => (), _ => return Ok(Async::Ready(())), } match self.inner.poll() { Ok(Async::NotReady) => Ok(Async::NotReady), _ => Ok(Async::Ready(())) } } } impl<S: Stream> fmt::Debug for Execute<S> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.debug_struct("Execute") .finish() } } /* * * ===== impl Inner ===== * */ impl<T> Inner<T> { // The return value is such that the total number of messages that can be // enqueued into the channel will never exceed MAX_CAPACITY fn max_senders(&self) -> usize { match self.buffer { Some(buffer) => MAX_CAPACITY - buffer, None => MAX_BUFFER, } } } unsafe impl<T: Send> Send for Inner<T> {} unsafe impl<T: Send> Sync for Inner<T> {} impl State { fn is_closed(&self) -> bool { !self.is_open && self.num_messages == 0 } } /* * * ===== Helpers ===== * */ fn decode_state(num: usize) -> State { State { is_open: num & OPEN_MASK == OPEN_MASK, num_messages: num & MAX_CAPACITY, } } fn encode_state(state: &State) -> usize { let mut num = state.num_messages; if state.is_open { num |= OPEN_MASK; } num }