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Diffstat (limited to 'third_party/rust/tokio/src/sync/oneshot.rs')
| -rw-r--r-- | third_party/rust/tokio/src/sync/oneshot.rs | 1366 |
1 files changed, 1366 insertions, 0 deletions
diff --git a/third_party/rust/tokio/src/sync/oneshot.rs b/third_party/rust/tokio/src/sync/oneshot.rs new file mode 100644 index 0000000000..2240074e73 --- /dev/null +++ b/third_party/rust/tokio/src/sync/oneshot.rs @@ -0,0 +1,1366 @@ +#![cfg_attr(not(feature = "sync"), allow(dead_code, unreachable_pub))] + +//! A one-shot channel is used for sending a single message between +//! asynchronous tasks. The [`channel`] function is used to create a +//! [`Sender`] and [`Receiver`] handle pair that form the channel. +//! +//! The `Sender` handle is used by the producer to send the value. +//! The `Receiver` handle is used by the consumer to receive the value. +//! +//! Each handle can be used on separate tasks. +//! +//! Since the `send` method is not async, it can be used anywhere. This includes +//! sending between two runtimes, and using it from non-async code. +//! +//! # Examples +//! +//! ``` +//! use tokio::sync::oneshot; +//! +//! #[tokio::main] +//! async fn main() { +//! let (tx, rx) = oneshot::channel(); +//! +//! tokio::spawn(async move { +//! if let Err(_) = tx.send(3) { +//! println!("the receiver dropped"); +//! } +//! }); +//! +//! match rx.await { +//! Ok(v) => println!("got = {:?}", v), +//! Err(_) => println!("the sender dropped"), +//! } +//! } +//! ``` +//! +//! If the sender is dropped without sending, the receiver will fail with +//! [`error::RecvError`]: +//! +//! ``` +//! use tokio::sync::oneshot; +//! +//! #[tokio::main] +//! async fn main() { +//! let (tx, rx) = oneshot::channel::<u32>(); +//! +//! tokio::spawn(async move { +//! drop(tx); +//! }); +//! +//! match rx.await { +//! Ok(_) => panic!("This doesn't happen"), +//! Err(_) => println!("the sender dropped"), +//! } +//! } +//! ``` +//! +//! To use a oneshot channel in a `tokio::select!` loop, add `&mut` in front of +//! the channel. +//! +//! ``` +//! use tokio::sync::oneshot; +//! use tokio::time::{interval, sleep, Duration}; +//! +//! #[tokio::main] +//! # async fn _doc() {} +//! # #[tokio::main(flavor = "current_thread", start_paused = true)] +//! async fn main() { +//! let (send, mut recv) = oneshot::channel(); +//! let mut interval = interval(Duration::from_millis(100)); +//! +//! # let handle = +//! tokio::spawn(async move { +//! sleep(Duration::from_secs(1)).await; +//! send.send("shut down").unwrap(); +//! }); +//! +//! loop { +//! tokio::select! { +//! _ = interval.tick() => println!("Another 100ms"), +//! msg = &mut recv => { +//! println!("Got message: {}", msg.unwrap()); +//! break; +//! } +//! } +//! } +//! # handle.await.unwrap(); +//! } +//! ``` +//! +//! To use a `Sender` from a destructor, put it in an [`Option`] and call +//! [`Option::take`]. +//! +//! ``` +//! use tokio::sync::oneshot; +//! +//! struct SendOnDrop { +//! sender: Option<oneshot::Sender<&'static str>>, +//! } +//! impl Drop for SendOnDrop { +//! fn drop(&mut self) { +//! if let Some(sender) = self.sender.take() { +//! // Using `let _ =` to ignore send errors. +//! let _ = sender.send("I got dropped!"); +//! } +//! } +//! } +//! +//! #[tokio::main] +//! # async fn _doc() {} +//! # #[tokio::main(flavor = "current_thread")] +//! async fn main() { +//! let (send, recv) = oneshot::channel(); +//! +//! let send_on_drop = SendOnDrop { sender: Some(send) }; +//! drop(send_on_drop); +//! +//! assert_eq!(recv.await, Ok("I got dropped!")); +//! } +//! ``` + +use crate::loom::cell::UnsafeCell; +use crate::loom::sync::atomic::AtomicUsize; +use crate::loom::sync::Arc; +#[cfg(all(tokio_unstable, feature = "tracing"))] +use crate::util::trace; + +use std::fmt; +use std::future::Future; +use std::mem::MaybeUninit; +use std::pin::Pin; +use std::sync::atomic::Ordering::{self, AcqRel, Acquire}; +use std::task::Poll::{Pending, Ready}; +use std::task::{Context, Poll, Waker}; + +/// Sends a value to the associated [`Receiver`]. +/// +/// A pair of both a [`Sender`] and a [`Receiver`] are created by the +/// [`channel`](fn@channel) function. +/// +/// # Examples +/// +/// ``` +/// use tokio::sync::oneshot; +/// +/// #[tokio::main] +/// async fn main() { +/// let (tx, rx) = oneshot::channel(); +/// +/// tokio::spawn(async move { +/// if let Err(_) = tx.send(3) { +/// println!("the receiver dropped"); +/// } +/// }); +/// +/// match rx.await { +/// Ok(v) => println!("got = {:?}", v), +/// Err(_) => println!("the sender dropped"), +/// } +/// } +/// ``` +/// +/// If the sender is dropped without sending, the receiver will fail with +/// [`error::RecvError`]: +/// +/// ``` +/// use tokio::sync::oneshot; +/// +/// #[tokio::main] +/// async fn main() { +/// let (tx, rx) = oneshot::channel::<u32>(); +/// +/// tokio::spawn(async move { +/// drop(tx); +/// }); +/// +/// match rx.await { +/// Ok(_) => panic!("This doesn't happen"), +/// Err(_) => println!("the sender dropped"), +/// } +/// } +/// ``` +/// +/// To use a `Sender` from a destructor, put it in an [`Option`] and call +/// [`Option::take`]. +/// +/// ``` +/// use tokio::sync::oneshot; +/// +/// struct SendOnDrop { +/// sender: Option<oneshot::Sender<&'static str>>, +/// } +/// impl Drop for SendOnDrop { +/// fn drop(&mut self) { +/// if let Some(sender) = self.sender.take() { +/// // Using `let _ =` to ignore send errors. +/// let _ = sender.send("I got dropped!"); +/// } +/// } +/// } +/// +/// #[tokio::main] +/// # async fn _doc() {} +/// # #[tokio::main(flavor = "current_thread")] +/// async fn main() { +/// let (send, recv) = oneshot::channel(); +/// +/// let send_on_drop = SendOnDrop { sender: Some(send) }; +/// drop(send_on_drop); +/// +/// assert_eq!(recv.await, Ok("I got dropped!")); +/// } +/// ``` +/// +/// [`Option`]: std::option::Option +/// [`Option::take`]: std::option::Option::take +#[derive(Debug)] +pub struct Sender<T> { + inner: Option<Arc<Inner<T>>>, + #[cfg(all(tokio_unstable, feature = "tracing"))] + resource_span: tracing::Span, +} + +/// Receives a value from the associated [`Sender`]. +/// +/// A pair of both a [`Sender`] and a [`Receiver`] are created by the +/// [`channel`](fn@channel) function. +/// +/// This channel has no `recv` method because the receiver itself implements the +/// [`Future`] trait. To receive a value, `.await` the `Receiver` object directly. +/// +/// [`Future`]: trait@std::future::Future +/// +/// # Examples +/// +/// ``` +/// use tokio::sync::oneshot; +/// +/// #[tokio::main] +/// async fn main() { +/// let (tx, rx) = oneshot::channel(); +/// +/// tokio::spawn(async move { +/// if let Err(_) = tx.send(3) { +/// println!("the receiver dropped"); +/// } +/// }); +/// +/// match rx.await { +/// Ok(v) => println!("got = {:?}", v), +/// Err(_) => println!("the sender dropped"), +/// } +/// } +/// ``` +/// +/// If the sender is dropped without sending, the receiver will fail with +/// [`error::RecvError`]: +/// +/// ``` +/// use tokio::sync::oneshot; +/// +/// #[tokio::main] +/// async fn main() { +/// let (tx, rx) = oneshot::channel::<u32>(); +/// +/// tokio::spawn(async move { +/// drop(tx); +/// }); +/// +/// match rx.await { +/// Ok(_) => panic!("This doesn't happen"), +/// Err(_) => println!("the sender dropped"), +/// } +/// } +/// ``` +/// +/// To use a `Receiver` in a `tokio::select!` loop, add `&mut` in front of the +/// channel. +/// +/// ``` +/// use tokio::sync::oneshot; +/// use tokio::time::{interval, sleep, Duration}; +/// +/// #[tokio::main] +/// # async fn _doc() {} +/// # #[tokio::main(flavor = "current_thread", start_paused = true)] +/// async fn main() { +/// let (send, mut recv) = oneshot::channel(); +/// let mut interval = interval(Duration::from_millis(100)); +/// +/// # let handle = +/// tokio::spawn(async move { +/// sleep(Duration::from_secs(1)).await; +/// send.send("shut down").unwrap(); +/// }); +/// +/// loop { +/// tokio::select! { +/// _ = interval.tick() => println!("Another 100ms"), +/// msg = &mut recv => { +/// println!("Got message: {}", msg.unwrap()); +/// break; +/// } +/// } +/// } +/// # handle.await.unwrap(); +/// } +/// ``` +#[derive(Debug)] +pub struct Receiver<T> { + inner: Option<Arc<Inner<T>>>, + #[cfg(all(tokio_unstable, feature = "tracing"))] + resource_span: tracing::Span, + #[cfg(all(tokio_unstable, feature = "tracing"))] + async_op_span: tracing::Span, + #[cfg(all(tokio_unstable, feature = "tracing"))] + async_op_poll_span: tracing::Span, +} + +pub mod error { + //! Oneshot error types. + + use std::fmt; + + /// Error returned by the `Future` implementation for `Receiver`. + #[derive(Debug, Eq, PartialEq)] + pub struct RecvError(pub(super) ()); + + /// Error returned by the `try_recv` function on `Receiver`. + #[derive(Debug, Eq, PartialEq)] + pub enum TryRecvError { + /// The send half of the channel has not yet sent a value. + Empty, + + /// The send half of the channel was dropped without sending a value. + Closed, + } + + // ===== impl RecvError ===== + + impl fmt::Display for RecvError { + fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { + write!(fmt, "channel closed") + } + } + + impl std::error::Error for RecvError {} + + // ===== impl TryRecvError ===== + + impl fmt::Display for TryRecvError { + fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { + match self { + TryRecvError::Empty => write!(fmt, "channel empty"), + TryRecvError::Closed => write!(fmt, "channel closed"), + } + } + } + + impl std::error::Error for TryRecvError {} +} + +use self::error::*; + +struct Inner<T> { + /// Manages the state of the inner cell. + state: AtomicUsize, + + /// The value. This is set by `Sender` and read by `Receiver`. The state of + /// the cell is tracked by `state`. + value: UnsafeCell<Option<T>>, + + /// The task to notify when the receiver drops without consuming the value. + /// + /// ## Safety + /// + /// The `TX_TASK_SET` bit in the `state` field is set if this field is + /// initialized. If that bit is unset, this field may be uninitialized. + tx_task: Task, + + /// The task to notify when the value is sent. + /// + /// ## Safety + /// + /// The `RX_TASK_SET` bit in the `state` field is set if this field is + /// initialized. If that bit is unset, this field may be uninitialized. + rx_task: Task, +} + +struct Task(UnsafeCell<MaybeUninit<Waker>>); + +impl Task { + unsafe fn will_wake(&self, cx: &mut Context<'_>) -> bool { + self.with_task(|w| w.will_wake(cx.waker())) + } + + unsafe fn with_task<F, R>(&self, f: F) -> R + where + F: FnOnce(&Waker) -> R, + { + self.0.with(|ptr| { + let waker: *const Waker = (&*ptr).as_ptr(); + f(&*waker) + }) + } + + unsafe fn drop_task(&self) { + self.0.with_mut(|ptr| { + let ptr: *mut Waker = (&mut *ptr).as_mut_ptr(); + ptr.drop_in_place(); + }); + } + + unsafe fn set_task(&self, cx: &mut Context<'_>) { + self.0.with_mut(|ptr| { + let ptr: *mut Waker = (&mut *ptr).as_mut_ptr(); + ptr.write(cx.waker().clone()); + }); + } +} + +#[derive(Clone, Copy)] +struct State(usize); + +/// Creates a new one-shot channel for sending single values across asynchronous +/// tasks. +/// +/// The function returns separate "send" and "receive" handles. The `Sender` +/// handle is used by the producer to send the value. The `Receiver` handle is +/// used by the consumer to receive the value. +/// +/// Each handle can be used on separate tasks. +/// +/// # Examples +/// +/// ``` +/// use tokio::sync::oneshot; +/// +/// #[tokio::main] +/// async fn main() { +/// let (tx, rx) = oneshot::channel(); +/// +/// tokio::spawn(async move { +/// if let Err(_) = tx.send(3) { +/// println!("the receiver dropped"); +/// } +/// }); +/// +/// match rx.await { +/// Ok(v) => println!("got = {:?}", v), +/// Err(_) => println!("the sender dropped"), +/// } +/// } +/// ``` +#[track_caller] +pub fn channel<T>() -> (Sender<T>, Receiver<T>) { + #[cfg(all(tokio_unstable, feature = "tracing"))] + let resource_span = { + let location = std::panic::Location::caller(); + + let resource_span = tracing::trace_span!( + "runtime.resource", + concrete_type = "Sender|Receiver", + kind = "Sync", + loc.file = location.file(), + loc.line = location.line(), + loc.col = location.column(), + ); + + resource_span.in_scope(|| { + tracing::trace!( + target: "runtime::resource::state_update", + tx_dropped = false, + tx_dropped.op = "override", + ) + }); + + resource_span.in_scope(|| { + tracing::trace!( + target: "runtime::resource::state_update", + rx_dropped = false, + rx_dropped.op = "override", + ) + }); + + resource_span.in_scope(|| { + tracing::trace!( + target: "runtime::resource::state_update", + value_sent = false, + value_sent.op = "override", + ) + }); + + resource_span.in_scope(|| { + tracing::trace!( + target: "runtime::resource::state_update", + value_received = false, + value_received.op = "override", + ) + }); + + resource_span + }; + + let inner = Arc::new(Inner { + state: AtomicUsize::new(State::new().as_usize()), + value: UnsafeCell::new(None), + tx_task: Task(UnsafeCell::new(MaybeUninit::uninit())), + rx_task: Task(UnsafeCell::new(MaybeUninit::uninit())), + }); + + let tx = Sender { + inner: Some(inner.clone()), + #[cfg(all(tokio_unstable, feature = "tracing"))] + resource_span: resource_span.clone(), + }; + + #[cfg(all(tokio_unstable, feature = "tracing"))] + let async_op_span = resource_span + .in_scope(|| tracing::trace_span!("runtime.resource.async_op", source = "Receiver::await")); + + #[cfg(all(tokio_unstable, feature = "tracing"))] + let async_op_poll_span = + async_op_span.in_scope(|| tracing::trace_span!("runtime.resource.async_op.poll")); + + let rx = Receiver { + inner: Some(inner), + #[cfg(all(tokio_unstable, feature = "tracing"))] + resource_span: resource_span, + #[cfg(all(tokio_unstable, feature = "tracing"))] + async_op_span, + #[cfg(all(tokio_unstable, feature = "tracing"))] + async_op_poll_span, + }; + + (tx, rx) +} + +impl<T> Sender<T> { + /// Attempts to send a value on this channel, returning it back if it could + /// not be sent. + /// + /// This method consumes `self` as only one value may ever be sent on a oneshot + /// channel. It is not marked async because sending a message to an oneshot + /// channel never requires any form of waiting. Because of this, the `send` + /// method can be used in both synchronous and asynchronous code without + /// problems. + /// + /// A successful send occurs when it is determined that the other end of the + /// channel has not hung up already. An unsuccessful send would be one where + /// the corresponding receiver has already been deallocated. Note that a + /// return value of `Err` means that the data will never be received, but + /// a return value of `Ok` does *not* mean that the data will be received. + /// It is possible for the corresponding receiver to hang up immediately + /// after this function returns `Ok`. + /// + /// # Examples + /// + /// Send a value to another task + /// + /// ``` + /// use tokio::sync::oneshot; + /// + /// #[tokio::main] + /// async fn main() { + /// let (tx, rx) = oneshot::channel(); + /// + /// tokio::spawn(async move { + /// if let Err(_) = tx.send(3) { + /// println!("the receiver dropped"); + /// } + /// }); + /// + /// match rx.await { + /// Ok(v) => println!("got = {:?}", v), + /// Err(_) => println!("the sender dropped"), + /// } + /// } + /// ``` + pub fn send(mut self, t: T) -> Result<(), T> { + let inner = self.inner.take().unwrap(); + + inner.value.with_mut(|ptr| unsafe { + // SAFETY: The receiver will not access the `UnsafeCell` unless the + // channel has been marked as "complete" (the `VALUE_SENT` state bit + // is set). + // That bit is only set by the sender later on in this method, and + // calling this method consumes `self`. Therefore, if it was possible to + // call this method, we know that the `VALUE_SENT` bit is unset, and + // the receiver is not currently accessing the `UnsafeCell`. + *ptr = Some(t); + }); + + if !inner.complete() { + unsafe { + // SAFETY: The receiver will not access the `UnsafeCell` unless + // the channel has been marked as "complete". Calling + // `complete()` will return true if this bit is set, and false + // if it is not set. Thus, if `complete()` returned false, it is + // safe for us to access the value, because we know that the + // receiver will not. + return Err(inner.consume_value().unwrap()); + } + } + + #[cfg(all(tokio_unstable, feature = "tracing"))] + self.resource_span.in_scope(|| { + tracing::trace!( + target: "runtime::resource::state_update", + value_sent = true, + value_sent.op = "override", + ) + }); + + Ok(()) + } + + /// Waits for the associated [`Receiver`] handle to close. + /// + /// A [`Receiver`] is closed by either calling [`close`] explicitly or the + /// [`Receiver`] value is dropped. + /// + /// This function is useful when paired with `select!` to abort a + /// computation when the receiver is no longer interested in the result. + /// + /// # Return + /// + /// Returns a `Future` which must be awaited on. + /// + /// [`Receiver`]: Receiver + /// [`close`]: Receiver::close + /// + /// # Examples + /// + /// Basic usage + /// + /// ``` + /// use tokio::sync::oneshot; + /// + /// #[tokio::main] + /// async fn main() { + /// let (mut tx, rx) = oneshot::channel::<()>(); + /// + /// tokio::spawn(async move { + /// drop(rx); + /// }); + /// + /// tx.closed().await; + /// println!("the receiver dropped"); + /// } + /// ``` + /// + /// Paired with select + /// + /// ``` + /// use tokio::sync::oneshot; + /// use tokio::time::{self, Duration}; + /// + /// async fn compute() -> String { + /// // Complex computation returning a `String` + /// # "hello".to_string() + /// } + /// + /// #[tokio::main] + /// async fn main() { + /// let (mut tx, rx) = oneshot::channel(); + /// + /// tokio::spawn(async move { + /// tokio::select! { + /// _ = tx.closed() => { + /// // The receiver dropped, no need to do any further work + /// } + /// value = compute() => { + /// // The send can fail if the channel was closed at the exact same + /// // time as when compute() finished, so just ignore the failure. + /// let _ = tx.send(value); + /// } + /// } + /// }); + /// + /// // Wait for up to 10 seconds + /// let _ = time::timeout(Duration::from_secs(10), rx).await; + /// } + /// ``` + pub async fn closed(&mut self) { + use crate::future::poll_fn; + + #[cfg(all(tokio_unstable, feature = "tracing"))] + let resource_span = self.resource_span.clone(); + #[cfg(all(tokio_unstable, feature = "tracing"))] + let closed = trace::async_op( + || poll_fn(|cx| self.poll_closed(cx)), + resource_span, + "Sender::closed", + "poll_closed", + false, + ); + #[cfg(not(all(tokio_unstable, feature = "tracing")))] + let closed = poll_fn(|cx| self.poll_closed(cx)); + + closed.await + } + + /// Returns `true` if the associated [`Receiver`] handle has been dropped. + /// + /// A [`Receiver`] is closed by either calling [`close`] explicitly or the + /// [`Receiver`] value is dropped. + /// + /// If `true` is returned, a call to `send` will always result in an error. + /// + /// [`Receiver`]: Receiver + /// [`close`]: Receiver::close + /// + /// # Examples + /// + /// ``` + /// use tokio::sync::oneshot; + /// + /// #[tokio::main] + /// async fn main() { + /// let (tx, rx) = oneshot::channel(); + /// + /// assert!(!tx.is_closed()); + /// + /// drop(rx); + /// + /// assert!(tx.is_closed()); + /// assert!(tx.send("never received").is_err()); + /// } + /// ``` + pub fn is_closed(&self) -> bool { + let inner = self.inner.as_ref().unwrap(); + + let state = State::load(&inner.state, Acquire); + state.is_closed() + } + + /// Checks whether the oneshot channel has been closed, and if not, schedules the + /// `Waker` in the provided `Context` to receive a notification when the channel is + /// closed. + /// + /// A [`Receiver`] is closed by either calling [`close`] explicitly, or when the + /// [`Receiver`] value is dropped. + /// + /// Note that on multiple calls to poll, only the `Waker` from the `Context` passed + /// to the most recent call will be scheduled to receive a wakeup. + /// + /// [`Receiver`]: struct@crate::sync::oneshot::Receiver + /// [`close`]: fn@crate::sync::oneshot::Receiver::close + /// + /// # Return value + /// + /// This function returns: + /// + /// * `Poll::Pending` if the channel is still open. + /// * `Poll::Ready(())` if the channel is closed. + /// + /// # Examples + /// + /// ``` + /// use tokio::sync::oneshot; + /// + /// use futures::future::poll_fn; + /// + /// #[tokio::main] + /// async fn main() { + /// let (mut tx, mut rx) = oneshot::channel::<()>(); + /// + /// tokio::spawn(async move { + /// rx.close(); + /// }); + /// + /// poll_fn(|cx| tx.poll_closed(cx)).await; + /// + /// println!("the receiver dropped"); + /// } + /// ``` + pub fn poll_closed(&mut self, cx: &mut Context<'_>) -> Poll<()> { + // Keep track of task budget + let coop = ready!(crate::coop::poll_proceed(cx)); + + let inner = self.inner.as_ref().unwrap(); + + let mut state = State::load(&inner.state, Acquire); + + if state.is_closed() { + coop.made_progress(); + return Poll::Ready(()); + } + + if state.is_tx_task_set() { + let will_notify = unsafe { inner.tx_task.will_wake(cx) }; + + if !will_notify { + state = State::unset_tx_task(&inner.state); + + if state.is_closed() { + // Set the flag again so that the waker is released in drop + State::set_tx_task(&inner.state); + coop.made_progress(); + return Ready(()); + } else { + unsafe { inner.tx_task.drop_task() }; + } + } + } + + if !state.is_tx_task_set() { + // Attempt to set the task + unsafe { + inner.tx_task.set_task(cx); + } + + // Update the state + state = State::set_tx_task(&inner.state); + + if state.is_closed() { + coop.made_progress(); + return Ready(()); + } + } + + Pending + } +} + +impl<T> Drop for Sender<T> { + fn drop(&mut self) { + if let Some(inner) = self.inner.as_ref() { + inner.complete(); + #[cfg(all(tokio_unstable, feature = "tracing"))] + self.resource_span.in_scope(|| { + tracing::trace!( + target: "runtime::resource::state_update", + tx_dropped = true, + tx_dropped.op = "override", + ) + }); + } + } +} + +impl<T> Receiver<T> { + /// Prevents the associated [`Sender`] handle from sending a value. + /// + /// Any `send` operation which happens after calling `close` is guaranteed + /// to fail. After calling `close`, [`try_recv`] should be called to + /// receive a value if one was sent **before** the call to `close` + /// completed. + /// + /// This function is useful to perform a graceful shutdown and ensure that a + /// value will not be sent into the channel and never received. + /// + /// `close` is no-op if a message is already received or the channel + /// is already closed. + /// + /// [`Sender`]: Sender + /// [`try_recv`]: Receiver::try_recv + /// + /// # Examples + /// + /// Prevent a value from being sent + /// + /// ``` + /// use tokio::sync::oneshot; + /// use tokio::sync::oneshot::error::TryRecvError; + /// + /// #[tokio::main] + /// async fn main() { + /// let (tx, mut rx) = oneshot::channel(); + /// + /// assert!(!tx.is_closed()); + /// + /// rx.close(); + /// + /// assert!(tx.is_closed()); + /// assert!(tx.send("never received").is_err()); + /// + /// match rx.try_recv() { + /// Err(TryRecvError::Closed) => {} + /// _ => unreachable!(), + /// } + /// } + /// ``` + /// + /// Receive a value sent **before** calling `close` + /// + /// ``` + /// use tokio::sync::oneshot; + /// + /// #[tokio::main] + /// async fn main() { + /// let (tx, mut rx) = oneshot::channel(); + /// + /// assert!(tx.send("will receive").is_ok()); + /// + /// rx.close(); + /// + /// let msg = rx.try_recv().unwrap(); + /// assert_eq!(msg, "will receive"); + /// } + /// ``` + pub fn close(&mut self) { + if let Some(inner) = self.inner.as_ref() { + inner.close(); + #[cfg(all(tokio_unstable, feature = "tracing"))] + self.resource_span.in_scope(|| { + tracing::trace!( + target: "runtime::resource::state_update", + rx_dropped = true, + rx_dropped.op = "override", + ) + }); + } + } + + /// Attempts to receive a value. + /// + /// If a pending value exists in the channel, it is returned. If no value + /// has been sent, the current task **will not** be registered for + /// future notification. + /// + /// This function is useful to call from outside the context of an + /// asynchronous task. + /// + /// # Return + /// + /// - `Ok(T)` if a value is pending in the channel. + /// - `Err(TryRecvError::Empty)` if no value has been sent yet. + /// - `Err(TryRecvError::Closed)` if the sender has dropped without sending + /// a value. + /// + /// # Examples + /// + /// `try_recv` before a value is sent, then after. + /// + /// ``` + /// use tokio::sync::oneshot; + /// use tokio::sync::oneshot::error::TryRecvError; + /// + /// #[tokio::main] + /// async fn main() { + /// let (tx, mut rx) = oneshot::channel(); + /// + /// match rx.try_recv() { + /// // The channel is currently empty + /// Err(TryRecvError::Empty) => {} + /// _ => unreachable!(), + /// } + /// + /// // Send a value + /// tx.send("hello").unwrap(); + /// + /// match rx.try_recv() { + /// Ok(value) => assert_eq!(value, "hello"), + /// _ => unreachable!(), + /// } + /// } + /// ``` + /// + /// `try_recv` when the sender dropped before sending a value + /// + /// ``` + /// use tokio::sync::oneshot; + /// use tokio::sync::oneshot::error::TryRecvError; + /// + /// #[tokio::main] + /// async fn main() { + /// let (tx, mut rx) = oneshot::channel::<()>(); + /// + /// drop(tx); + /// + /// match rx.try_recv() { + /// // The channel will never receive a value. + /// Err(TryRecvError::Closed) => {} + /// _ => unreachable!(), + /// } + /// } + /// ``` + pub fn try_recv(&mut self) -> Result<T, TryRecvError> { + let result = if let Some(inner) = self.inner.as_ref() { + let state = State::load(&inner.state, Acquire); + + if state.is_complete() { + // SAFETY: If `state.is_complete()` returns true, then the + // `VALUE_SENT` bit has been set and the sender side of the + // channel will no longer attempt to access the inner + // `UnsafeCell`. Therefore, it is now safe for us to access the + // cell. + match unsafe { inner.consume_value() } { + Some(value) => { + #[cfg(all(tokio_unstable, feature = "tracing"))] + self.resource_span.in_scope(|| { + tracing::trace!( + target: "runtime::resource::state_update", + value_received = true, + value_received.op = "override", + ) + }); + Ok(value) + } + None => Err(TryRecvError::Closed), + } + } else if state.is_closed() { + Err(TryRecvError::Closed) + } else { + // Not ready, this does not clear `inner` + return Err(TryRecvError::Empty); + } + } else { + Err(TryRecvError::Closed) + }; + + self.inner = None; + result + } + + /// Blocking receive to call outside of asynchronous contexts. + /// + /// # Panics + /// + /// This function panics if called within an asynchronous execution + /// context. + /// + /// # Examples + /// + /// ``` + /// use std::thread; + /// use tokio::sync::oneshot; + /// + /// #[tokio::main] + /// async fn main() { + /// let (tx, rx) = oneshot::channel::<u8>(); + /// + /// let sync_code = thread::spawn(move || { + /// assert_eq!(Ok(10), rx.blocking_recv()); + /// }); + /// + /// let _ = tx.send(10); + /// sync_code.join().unwrap(); + /// } + /// ``` + #[cfg(feature = "sync")] + pub fn blocking_recv(self) -> Result<T, RecvError> { + crate::future::block_on(self) + } +} + +impl<T> Drop for Receiver<T> { + fn drop(&mut self) { + if let Some(inner) = self.inner.as_ref() { + inner.close(); + #[cfg(all(tokio_unstable, feature = "tracing"))] + self.resource_span.in_scope(|| { + tracing::trace!( + target: "runtime::resource::state_update", + rx_dropped = true, + rx_dropped.op = "override", + ) + }); + } + } +} + +impl<T> Future for Receiver<T> { + type Output = Result<T, RecvError>; + + fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { + // If `inner` is `None`, then `poll()` has already completed. + #[cfg(all(tokio_unstable, feature = "tracing"))] + let _res_span = self.resource_span.clone().entered(); + #[cfg(all(tokio_unstable, feature = "tracing"))] + let _ao_span = self.async_op_span.clone().entered(); + #[cfg(all(tokio_unstable, feature = "tracing"))] + let _ao_poll_span = self.async_op_poll_span.clone().entered(); + + let ret = if let Some(inner) = self.as_ref().get_ref().inner.as_ref() { + #[cfg(all(tokio_unstable, feature = "tracing"))] + let res = ready!(trace_poll_op!("poll_recv", inner.poll_recv(cx)))?; + + #[cfg(any(not(tokio_unstable), not(feature = "tracing")))] + let res = ready!(inner.poll_recv(cx))?; + + res + } else { + panic!("called after complete"); + }; + + self.inner = None; + Ready(Ok(ret)) + } +} + +impl<T> Inner<T> { + fn complete(&self) -> bool { + let prev = State::set_complete(&self.state); + + if prev.is_closed() { + return false; + } + + if prev.is_rx_task_set() { + // TODO: Consume waker? + unsafe { + self.rx_task.with_task(Waker::wake_by_ref); + } + } + + true + } + + fn poll_recv(&self, cx: &mut Context<'_>) -> Poll<Result<T, RecvError>> { + // Keep track of task budget + let coop = ready!(crate::coop::poll_proceed(cx)); + + // Load the state + let mut state = State::load(&self.state, Acquire); + + if state.is_complete() { + coop.made_progress(); + match unsafe { self.consume_value() } { + Some(value) => Ready(Ok(value)), + None => Ready(Err(RecvError(()))), + } + } else if state.is_closed() { + coop.made_progress(); + Ready(Err(RecvError(()))) + } else { + if state.is_rx_task_set() { + let will_notify = unsafe { self.rx_task.will_wake(cx) }; + + // Check if the task is still the same + if !will_notify { + // Unset the task + state = State::unset_rx_task(&self.state); + if state.is_complete() { + // Set the flag again so that the waker is released in drop + State::set_rx_task(&self.state); + + coop.made_progress(); + // SAFETY: If `state.is_complete()` returns true, then the + // `VALUE_SENT` bit has been set and the sender side of the + // channel will no longer attempt to access the inner + // `UnsafeCell`. Therefore, it is now safe for us to access the + // cell. + return match unsafe { self.consume_value() } { + Some(value) => Ready(Ok(value)), + None => Ready(Err(RecvError(()))), + }; + } else { + unsafe { self.rx_task.drop_task() }; + } + } + } + + if !state.is_rx_task_set() { + // Attempt to set the task + unsafe { + self.rx_task.set_task(cx); + } + + // Update the state + state = State::set_rx_task(&self.state); + + if state.is_complete() { + coop.made_progress(); + match unsafe { self.consume_value() } { + Some(value) => Ready(Ok(value)), + None => Ready(Err(RecvError(()))), + } + } else { + Pending + } + } else { + Pending + } + } + } + + /// Called by `Receiver` to indicate that the value will never be received. + fn close(&self) { + let prev = State::set_closed(&self.state); + + if prev.is_tx_task_set() && !prev.is_complete() { + unsafe { + self.tx_task.with_task(Waker::wake_by_ref); + } + } + } + + /// Consumes the value. This function does not check `state`. + /// + /// # Safety + /// + /// Calling this method concurrently on multiple threads will result in a + /// data race. The `VALUE_SENT` state bit is used to ensure that only the + /// sender *or* the receiver will call this method at a given point in time. + /// If `VALUE_SENT` is not set, then only the sender may call this method; + /// if it is set, then only the receiver may call this method. + unsafe fn consume_value(&self) -> Option<T> { + self.value.with_mut(|ptr| (*ptr).take()) + } +} + +unsafe impl<T: Send> Send for Inner<T> {} +unsafe impl<T: Send> Sync for Inner<T> {} + +fn mut_load(this: &mut AtomicUsize) -> usize { + this.with_mut(|v| *v) +} + +impl<T> Drop for Inner<T> { + fn drop(&mut self) { + let state = State(mut_load(&mut self.state)); + + if state.is_rx_task_set() { + unsafe { + self.rx_task.drop_task(); + } + } + + if state.is_tx_task_set() { + unsafe { + self.tx_task.drop_task(); + } + } + } +} + +impl<T: fmt::Debug> fmt::Debug for Inner<T> { + fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { + use std::sync::atomic::Ordering::Relaxed; + + fmt.debug_struct("Inner") + .field("state", &State::load(&self.state, Relaxed)) + .finish() + } +} + +/// Indicates that a waker for the receiving task has been set. +/// +/// # Safety +/// +/// If this bit is not set, the `rx_task` field may be uninitialized. +const RX_TASK_SET: usize = 0b00001; +/// Indicates that a value has been stored in the channel's inner `UnsafeCell`. +/// +/// # Safety +/// +/// This bit controls which side of the channel is permitted to access the +/// `UnsafeCell`. If it is set, the `UnsafeCell` may ONLY be accessed by the +/// receiver. If this bit is NOT set, the `UnsafeCell` may ONLY be accessed by +/// the sender. +const VALUE_SENT: usize = 0b00010; +const CLOSED: usize = 0b00100; + +/// Indicates that a waker for the sending task has been set. +/// +/// # Safety +/// +/// If this bit is not set, the `tx_task` field may be uninitialized. +const TX_TASK_SET: usize = 0b01000; + +impl State { + fn new() -> State { + State(0) + } + + fn is_complete(self) -> bool { + self.0 & VALUE_SENT == VALUE_SENT + } + + fn set_complete(cell: &AtomicUsize) -> State { + // This method is a compare-and-swap loop rather than a fetch-or like + // other `set_$WHATEVER` methods on `State`. This is because we must + // check if the state has been closed before setting the `VALUE_SENT` + // bit. + // + // We don't want to set both the `VALUE_SENT` bit if the `CLOSED` + // bit is already set, because `VALUE_SENT` will tell the receiver that + // it's okay to access the inner `UnsafeCell`. Immediately after calling + // `set_complete`, if the channel was closed, the sender will _also_ + // access the `UnsafeCell` to take the value back out, so if a + // `poll_recv` or `try_recv` call is occurring concurrently, both + // threads may try to access the `UnsafeCell` if we were to set the + // `VALUE_SENT` bit on a closed channel. + let mut state = cell.load(Ordering::Relaxed); + loop { + if State(state).is_closed() { + break; + } + // TODO: This could be `Release`, followed by an `Acquire` fence *if* + // the `RX_TASK_SET` flag is set. However, `loom` does not support + // fences yet. + match cell.compare_exchange_weak( + state, + state | VALUE_SENT, + Ordering::AcqRel, + Ordering::Acquire, + ) { + Ok(_) => break, + Err(actual) => state = actual, + } + } + State(state) + } + + fn is_rx_task_set(self) -> bool { + self.0 & RX_TASK_SET == RX_TASK_SET + } + + fn set_rx_task(cell: &AtomicUsize) -> State { + let val = cell.fetch_or(RX_TASK_SET, AcqRel); + State(val | RX_TASK_SET) + } + + fn unset_rx_task(cell: &AtomicUsize) -> State { + let val = cell.fetch_and(!RX_TASK_SET, AcqRel); + State(val & !RX_TASK_SET) + } + + fn is_closed(self) -> bool { + self.0 & CLOSED == CLOSED + } + + fn set_closed(cell: &AtomicUsize) -> State { + // Acquire because we want all later writes (attempting to poll) to be + // ordered after this. + let val = cell.fetch_or(CLOSED, Acquire); + State(val) + } + + fn set_tx_task(cell: &AtomicUsize) -> State { + let val = cell.fetch_or(TX_TASK_SET, AcqRel); + State(val | TX_TASK_SET) + } + + fn unset_tx_task(cell: &AtomicUsize) -> State { + let val = cell.fetch_and(!TX_TASK_SET, AcqRel); + State(val & !TX_TASK_SET) + } + + fn is_tx_task_set(self) -> bool { + self.0 & TX_TASK_SET == TX_TASK_SET + } + + fn as_usize(self) -> usize { + self.0 + } + + fn load(cell: &AtomicUsize, order: Ordering) -> State { + let val = cell.load(order); + State(val) + } +} + +impl fmt::Debug for State { + fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { + fmt.debug_struct("State") + .field("is_complete", &self.is_complete()) + .field("is_closed", &self.is_closed()) + .field("is_rx_task_set", &self.is_rx_task_set()) + .field("is_tx_task_set", &self.is_tx_task_set()) + .finish() + } +} |