1 files changed, 137 insertions, 0 deletions
diff --git a/library/std/src/sync/mpmc/counter.rs b/library/std/src/sync/mpmc/counter.rs
new file mode 100644
index 000000000..a5a6bdc67
--- /dev/null
+++ b/library/std/src/sync/mpmc/counter.rs
@@ -0,0 +1,137 @@
+use crate::ops;
+use crate::process;
+use crate::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
+    }
+}