sync: support mpsc send with `&self` (#2861)

Updates the mpsc channel to use the intrusive waker based sempahore.
This enables using `Sender` with `&self`.

Instead of using `Sender::poll_ready` to ensure capacity and updating
the `Sender` state, `async fn Sender::reserve()` is added. This function
returns a `Permit` value representing the reserved capacity.

Fixes: #2637
Refs: #2718 (intrusive waiters)

1 files changed, 36 insertions, 3 deletions

diff --git a/tokio/src/sync/mpsc/unbounded.rs b/tokio/src/sync/mpsc/unbounded.rs
index 6b2ca722..59456375 100644
--- a/tokio/src/sync/mpsc/unbounded.rs
+++ b/tokio/src/sync/mpsc/unbounded.rs
@@ -73,8 +73,7 @@ impl<T> UnboundedReceiver<T> {
         UnboundedReceiver { chan }
     }
 
-    #[doc(hidden)] // TODO: doc
-    pub fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<T>> {
+    fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<T>> {
         self.chan.recv(cx)
     }
 
@@ -174,7 +173,41 @@ impl<T> UnboundedSender<T> {
     /// [`close`]: UnboundedReceiver::close
     /// [`UnboundedReceiver`]: UnboundedReceiver
     pub fn send(&self, message: T) -> Result<(), SendError<T>> {
-        self.chan.send_unbounded(message)?;
+        if !self.inc_num_messages() {
+            return Err(SendError(message));
+        }
+
+        self.chan.send(message);
         Ok(())
     }
+
+    fn inc_num_messages(&self) -> bool {
+        use std::process;
+        use std::sync::atomic::Ordering::{AcqRel, Acquire};
+
+        let mut curr = self.chan.semaphore().load(Acquire);
+
+        loop {
+            if curr & 1 == 1 {
+                return false;
+            }
+
+            if curr == usize::MAX ^ 1 {
+                // Overflowed the ref count. There is no safe way to recover, so
+                // abort the process. In practice, this should never happen.
+                process::abort()
+            }
+
+            match self
+                .chan
+                .semaphore()
+                .compare_exchange(curr, curr + 2, AcqRel, Acquire)
+            {
+                Ok(_) => return true,
+                Err(actual) => {
+                    curr = actual;
+                }
+            }
+        }
+    }
 }