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
//! Reference counter for channels.

use std::boxed::Box;
use std::isize;
use std::ops;
use std::process;
use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};

/// Reference counter internals.
struct Counter<C> {
    /// The number of senders associated with the channel.
    senders: AtomicUsize,

    /// The number of receivers associated with the channel.
    receivers: AtomicUsize,

    /// Set to `true` if the last sender or the last receiver reference deallocates the channel.
    destroy: AtomicBool,

    /// The internal channel.
    chan: C,
}

/// Wraps a channel into the reference counter.
pub(crate) fn new<C>(chan: C) -> (Sender<C>, Receiver<C>) {
    let counter = Box::into_raw(Box::new(Counter {
        senders: AtomicUsize::new(1),
        receivers: AtomicUsize::new(1),
        destroy: AtomicBool::new(false),
        chan,
    }));
    let s = Sender { counter };
    let r = Receiver { counter };
    (s, r)
}

/// The sending side.
pub(crate) struct Sender<C> {
    counter: *mut Counter<C>,
}

impl<C> Sender<C> {
    /// Returns the internal `Counter`.
    fn counter(&self) -> &Counter<C> {
        unsafe { &*self.counter }
    }

    /// Acquires another sender reference.
    pub(crate) fn acquire(&self) -> Sender<C> {
        let count = self.counter().senders.fetch_add(1, Ordering::Relaxed);

        // Cloning senders and calling `mem::forget` on the clones could potentially overflow the
        // counter. It's very difficult to recover sensibly from such degenerate scenarios so we
        // just abort when the count becomes very large.
        if count > isize::MAX as usize {
            process::abort();
        }

        Sender {
            counter: self.counter,
        }
    }

    /// Releases the sender reference.
    ///
    /// Function `disconnect` will be called if this is the last sender reference.
    pub(crate) unsafe fn release<F: FnOnce(&C) -> bool>(&self, disconnect: F) {
        if self.counter().senders.fetch_sub(1, Ordering::AcqRel) == 1 {
            disconnect(&self.counter().chan);

            if self.counter().destroy.swap(true, Ordering::AcqRel) {
                drop(Box::from_raw(self.counter));
            }
        }
    }
}

impl<C> ops::Deref for Sender<C> {
    type Target = C;

    fn deref(&self) -> &C {
        &self.counter().chan
    }
}

impl<C> PartialEq for Sender<C> {
    fn eq(&self, other: &Sender<C>) -> bool {
        self.counter == other.counter
    }
}

/// The receiving side.
pub(crate) struct Receiver<C> {
    counter: *mut Counter<C>,
}

impl<C> Receiver<C> {
    /// Returns the internal `Counter`.
    fn counter(&self) -> &Counter<C> {
        unsafe { &*self.counter }
    }

    /// Acquires another receiver reference.
    pub(crate) fn acquire(&self) -> Receiver<C> {
        let count = self.counter().receivers.fetch_add(1, Ordering::Relaxed);

        // Cloning receivers and calling `mem::forget` on the clones could potentially overflow the
        // counter. It's very difficult to recover sensibly from such degenerate scenarios so we
        // just abort when the count becomes very large.
        if count > isize::MAX as usize {
            process::abort();
        }

        Receiver {
            counter: self.counter,
        }
    }

    /// Releases the receiver reference.
    ///
    /// Function `disconnect` will be called if this is the last receiver reference.
    pub(crate) unsafe fn release<F: FnOnce(&C) -> bool>(&self, disconnect: F) {
        if self.counter().receivers.fetch_sub(1, Ordering::AcqRel) == 1 {
            disconnect(&self.counter().chan);

            if self.counter().destroy.swap(true, Ordering::AcqRel) {
                drop(Box::from_raw(self.counter));
            }
        }
    }
}

impl<C> ops::Deref for Receiver<C> {
    type Target = C;

    fn deref(&self) -> &C {
        &self.counter().chan
    }
}

impl<C> PartialEq for Receiver<C> {
    fn eq(&self, other: &Receiver<C>) -> bool {
        self.counter == other.counter
    }
}