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
//! Split a single value implementing `AsyncRead + AsyncWrite` into separate
//! `AsyncRead` and `AsyncWrite` handles.
//!
//! To restore this read/write object from its `split::ReadHalf` and
//! `split::WriteHalf` use `unsplit`.

use crate::io::{AsyncRead, AsyncWrite, ReadBuf};

use std::cell::UnsafeCell;
use std::fmt;
use std::io;
use std::pin::Pin;
use std::sync::atomic::AtomicBool;
use std::sync::atomic::Ordering::{Acquire, Release};
use std::sync::Arc;
use std::task::{Context, Poll};

cfg_io_util! {
    /// The readable half of a value returned from [`split`](split()).
    pub struct ReadHalf<T> {
        inner: Arc<Inner<T>>,
    }

    /// The writable half of a value returned from [`split`](split()).
    pub struct WriteHalf<T> {
        inner: Arc<Inner<T>>,
    }

    /// Splits a single value implementing `AsyncRead + AsyncWrite` into separate
    /// `AsyncRead` and `AsyncWrite` handles.
    ///
    /// To restore this read/write object from its `ReadHalf` and
    /// `WriteHalf` use [`unsplit`](ReadHalf::unsplit()).
    pub fn split<T>(stream: T) -> (ReadHalf<T>, WriteHalf<T>)
    where
        T: AsyncRead + AsyncWrite,
    {
        let inner = Arc::new(Inner {
            locked: AtomicBool::new(false),
            stream: UnsafeCell::new(stream),
        });

        let rd = ReadHalf {
            inner: inner.clone(),
        };

        let wr = WriteHalf { inner };

        (rd, wr)
    }
}

struct Inner<T> {
    locked: AtomicBool,
    stream: UnsafeCell<T>,
}

struct Guard<'a, T> {
    inner: &'a Inner<T>,
}

impl<T> ReadHalf<T> {
    /// Checks if this `ReadHalf` and some `WriteHalf` were split from the same
    /// stream.
    pub fn is_pair_of(&self, other: &WriteHalf<T>) -> bool {
        other.is_pair_of(self)
    }

    /// Reunites with a previously split `WriteHalf`.
    ///
    /// # Panics
    ///
    /// If this `ReadHalf` and the given `WriteHalf` do not originate from the
    /// same `split` operation this method will panic.
    /// This can be checked ahead of time by comparing the stream ID
    /// of the two halves.
    pub fn unsplit(self, wr: WriteHalf<T>) -> T {
        if self.is_pair_of(&wr) {
            drop(wr);

            let inner = Arc::try_unwrap(self.inner)
                .ok()
                .expect("`Arc::try_unwrap` failed");

            inner.stream.into_inner()
        } else {
            panic!("Unrelated `split::Write` passed to `split::Read::unsplit`.")
        }
    }
}

impl<T> WriteHalf<T> {
    /// Checks if this `WriteHalf` and some `ReadHalf` were split from the same
    /// stream.
    pub fn is_pair_of(&self, other: &ReadHalf<T>) -> bool {
        Arc::ptr_eq(&self.inner, &other.inner)
    }
}

impl<T: AsyncRead> AsyncRead for ReadHalf<T> {
    fn poll_read(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &mut ReadBuf<'_>,
    ) -> Poll<io::Result<()>> {
        let mut inner = ready!(self.inner.poll_lock(cx));
        inner.stream_pin().poll_read(cx, buf)
    }
}

impl<T: AsyncWrite> AsyncWrite for WriteHalf<T> {
    fn poll_write(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<Result<usize, io::Error>> {
        let mut inner = ready!(self.inner.poll_lock(cx));
        inner.stream_pin().poll_write(cx, buf)
    }

    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
        let mut inner = ready!(self.inner.poll_lock(cx));
        inner.stream_pin().poll_flush(cx)
    }

    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
        let mut inner = ready!(self.inner.poll_lock(cx));
        inner.stream_pin().poll_shutdown(cx)
    }
}

impl<T> Inner<T> {
    fn poll_lock(&self, cx: &mut Context<'_>) -> Poll<Guard<'_, T>> {
        if self
            .locked
            .compare_exchange(false, true, Acquire, Acquire)
            .is_ok()
        {
            Poll::Ready(Guard { inner: self })
        } else {
            // Spin... but investigate a better strategy

            std::thread::yield_now();
            cx.waker().wake_by_ref();

            Poll::Pending
        }
    }
}

impl<T> Guard<'_, T> {
    fn stream_pin(&mut self) -> Pin<&mut T> {
        // safety: the stream is pinned in `Arc` and the `Guard` ensures mutual
        // exclusion.
        unsafe { Pin::new_unchecked(&mut *self.inner.stream.get()) }
    }
}

impl<T> Drop for Guard<'_, T> {
    fn drop(&mut self) {
        self.inner.locked.store(false, Release);
    }
}

unsafe impl<T: Send> Send for ReadHalf<T> {}
unsafe impl<T: Send> Send for WriteHalf<T> {}
unsafe impl<T: Sync> Sync for ReadHalf<T> {}
unsafe impl<T: Sync> Sync for WriteHalf<T> {}

impl<T: fmt::Debug> fmt::Debug for ReadHalf<T> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("split::ReadHalf").finish()
    }
}

impl<T: fmt::Debug> fmt::Debug for WriteHalf<T> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("split::WriteHalf").finish()
    }
}