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//! Asynchronous sinks
//!
//! This crate contains the `Sink` trait which allows values to be sent
//! asynchronously.
#![cfg_attr(not(feature = "std"), no_std)]
#![warn(missing_debug_implementations, missing_docs, rust_2018_idioms, unreachable_pub)]
// It cannot be included in the published code because this lints have false positives in the minimum required version.
#![cfg_attr(test, warn(single_use_lifetimes))]
#![doc(test(
no_crate_inject,
attr(
deny(warnings, rust_2018_idioms, single_use_lifetimes),
allow(dead_code, unused_assignments, unused_variables)
)
))]
#[cfg(feature = "alloc")]
extern crate alloc;
use core::ops::DerefMut;
use core::pin::Pin;
use core::task::{Context, Poll};
/// A `Sink` is a value into which other values can be sent, asynchronously.
///
/// Basic examples of sinks include the sending side of:
///
/// - Channels
/// - Sockets
/// - Pipes
///
/// In addition to such "primitive" sinks, it's typical to layer additional
/// functionality, such as buffering, on top of an existing sink.
///
/// Sending to a sink is "asynchronous" in the sense that the value may not be
/// sent in its entirety immediately. Instead, values are sent in a two-phase
/// way: first by initiating a send, and then by polling for completion. This
/// two-phase setup is analogous to buffered writing in synchronous code, where
/// writes often succeed immediately, but internally are buffered and are
/// *actually* written only upon flushing.
///
/// In addition, the `Sink` may be *full*, in which case it is not even possible
/// to start the sending process.
///
/// As with `Future` and `Stream`, the `Sink` trait is built from a few core
/// required methods, and a host of default methods for working in a
/// higher-level way. The `Sink::send_all` combinator is of particular
/// importance: you can use it to send an entire stream to a sink, which is
/// the simplest way to ultimately consume a stream.
#[must_use = "sinks do nothing unless polled"]
pub trait Sink<Item> {
/// The type of value produced by the sink when an error occurs.
type Error;
/// Attempts to prepare the `Sink` to receive a value.
///
/// This method must be called and return `Poll::Ready(Ok(()))` prior to
/// each call to `start_send`.
///
/// This method returns `Poll::Ready` once the underlying sink is ready to
/// receive data. If this method returns `Poll::Pending`, the current task
/// is registered to be notified (via `cx.waker().wake_by_ref()`) when `poll_ready`
/// should be called again.
///
/// In most cases, if the sink encounters an error, the sink will
/// permanently be unable to receive items.
fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>;
/// Begin the process of sending a value to the sink.
/// Each call to this function must be preceded by a successful call to
/// `poll_ready` which returned `Poll::Ready(Ok(()))`.
///
/// As the name suggests, this method only *begins* the process of sending
/// the item. If the sink employs buffering, the item isn't fully processed
/// until the buffer is fully flushed. Since sinks are designed to work with
/// asynchronous I/O, the process of actually writing out the data to an
/// underlying object takes place asynchronously. **You *must* use
/// `poll_flush` or `poll_close` in order to guarantee completion of a
/// send**.
///
/// Implementations of `poll_ready` and `start_send` will usually involve
/// flushing behind the scenes in order to make room for new messages.
/// It is only necessary to call `poll_flush` if you need to guarantee that
/// *all* of the items placed into the `Sink` have been sent.
///
/// In most cases, if the sink encounters an error, the sink will
/// permanently be unable to receive items.
fn start_send(self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error>;
/// Flush any remaining output from this sink.
///
/// Returns `Poll::Ready(Ok(()))` when no buffered items remain. If this
/// value is returned then it is guaranteed that all previous values sent
/// via `start_send` have been flushed.
///
/// Returns `Poll::Pending` if there is more work left to do, in which
/// case the current task is scheduled (via `cx.waker().wake_by_ref()`) to wake up when
/// `poll_flush` should be called again.
///
/// In most cases, if the sink encounters an error, the sink will
/// permanently be unable to receive items.
fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>;
/// Flush any remaining output and close this sink, if necessary.
///
/// Returns `Poll::Ready(Ok(()))` when no buffered items remain and the sink
/// has been successfully closed.
///
/// Returns `Poll::Pending` if there is more work left to do, in which
/// case the current task is scheduled (via `cx.waker().wake_by_ref()`) to wake up when
/// `poll_close` should be called again.
///
/// If this function encounters an error, the sink should be considered to
/// have failed permanently, and no more `Sink` methods should be called.
fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>;
}
impl<S: ?Sized + Sink<Item> + Unpin, Item> Sink<Item> for &mut S {
type Error = S::Error;
fn poll_ready(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
Pin::new(&mut **self).poll_ready(cx)
}
fn start_send(mut self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error> {
Pin::new(&mut **self).start_send(item)
}
fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
Pin::new(&mut **self).poll_flush(cx)
}
fn poll_close(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
Pin::new(&mut **self).poll_close(cx)
}
}
impl<P, Item> Sink<Item> for Pin<P>
where
P: DerefMut + Unpin,
P::Target: Sink<Item>,
{
type Error = <P::Target as Sink<Item>>::Error;
fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
self.get_mut().as_mut().poll_ready(cx)
}
fn start_send(self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error> {
self.get_mut().as_mut().start_send(item)
}
fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
self.get_mut().as_mut().poll_flush(cx)
}
fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
self.get_mut().as_mut().poll_close(cx)
}
}
#[cfg(feature = "alloc")]
mod if_alloc {
use super::*;
use core::convert::Infallible as Never;
impl<T> Sink<T> for alloc::vec::Vec<T> {
type Error = Never;
fn poll_ready(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
Poll::Ready(Ok(()))
}
fn start_send(self: Pin<&mut Self>, item: T) -> Result<(), Self::Error> {
// TODO: impl<T> Unpin for Vec<T> {}
unsafe { self.get_unchecked_mut() }.push(item);
Ok(())
}
fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
Poll::Ready(Ok(()))
}
fn poll_close(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
Poll::Ready(Ok(()))
}
}
impl<T> Sink<T> for alloc::collections::VecDeque<T> {
type Error = Never;
fn poll_ready(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
Poll::Ready(Ok(()))
}
fn start_send(self: Pin<&mut Self>, item: T) -> Result<(), Self::Error> {
// TODO: impl<T> Unpin for Vec<T> {}
unsafe { self.get_unchecked_mut() }.push_back(item);
Ok(())
}
fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
Poll::Ready(Ok(()))
}
fn poll_close(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
Poll::Ready(Ok(()))
}
}
impl<S: ?Sized + Sink<Item> + Unpin, Item> Sink<Item> for alloc::boxed::Box<S> {
type Error = S::Error;
fn poll_ready(
mut self: Pin<&mut Self>,
cx: &mut Context<'_>,
) -> Poll<Result<(), Self::Error>> {
Pin::new(&mut **self).poll_ready(cx)
}
fn start_send(mut self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error> {
Pin::new(&mut **self).start_send(item)
}
fn poll_flush(
mut self: Pin<&mut Self>,
cx: &mut Context<'_>,
) -> Poll<Result<(), Self::Error>> {
Pin::new(&mut **self).poll_flush(cx)
}
fn poll_close(
mut self: Pin<&mut Self>,
cx: &mut Context<'_>,
) -> Poll<Result<(), Self::Error>> {
Pin::new(&mut **self).poll_close(cx)
}
}
}