1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
//! Asynchronous sinks
//!
//! This crate contains the `Sink` trait which allows values to be sent
//! asynchronously.

#![cfg_attr(not(feature = "std"), no_std)]
#![warn(missing_docs, missing_debug_implementations, 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))]
#![warn(clippy::all)]

#![doc(test(attr(deny(warnings), allow(dead_code, unused_assignments, unused_variables))))]

#![doc(html_root_url = "https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-alpha.19/futures_sink")]

#[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)
        }
    }
}