//! 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)

        }

    }

}