Fix concurrency issue with oneshot_broadcast (sigp#3596)

## Issue Addressed

NA

## Proposed Changes

Fixes an issue found during testing with sigp#3595.

## Additional Info

NA

common/oneshot_broadcast/src/lib.rs

@@ -3,7 +3,7 @@
 //!
 //! This implementation may not be blazingly fast but it should be simple enough to be reliable.
 use parking_lot::{Condvar, Mutex};
-use std::sync::{Arc, Weak};
+use std::sync::Arc;
 
 #[derive(Copy, Clone, Debug, PartialEq)]
 pub enum Error {
@@ -13,9 +13,10 @@ pub enum Error {
 enum Future<T> {
     /// The future is ready and the item may be consumed.
     Ready(T),
-    /// Future is not ready. The contained `Weak` is a reference to the `Sender` that may be used to
-    /// detect when the channel is disconnected.
-    NotReady(Weak<()>),
+    /// Future is not ready.
+    NotReady,
+    /// The sender has been dropped without sending a message.
+    SenderDropped,
 }
 
 struct MutexCondvar<T> {
@@ -24,7 +25,7 @@ struct MutexCondvar<T> {
 }
 
 /// The sending pair of the `oneshot` channel.
-pub struct Sender<T>(Arc<MutexCondvar<T>>, Option<Arc<()>>);
+pub struct Sender<T>(Arc<MutexCondvar<T>>);
 
 impl<T> Sender<T> {
     /// Send a message, consuming `self` and delivering the message to *all* receivers.
@@ -35,11 +36,15 @@ impl<T> Sender<T> {
 }
 
 impl<T> Drop for Sender<T> {
-    /// Drop the `Arc` and notify all receivers so they can't upgrade their `Weak`s and know that
-    /// the sender has been dropped.
+    /// Flag the sender as dropped and notify all receivers.
     fn drop(&mut self) {
-        self.1 = None;
+        let mut lock = self.0.mutex.lock();
+        if !matches!(*lock, Future::Ready(_)) {
+            *lock = Future::SenderDropped
+        }
         self.0.condvar.notify_all();
+        // The lock must be held whilst the condvar is notified.
+        drop(lock);
     }
 }
 
@@ -59,8 +64,8 @@ impl<T: Clone> Receiver<T> {
     pub fn try_recv(&self) -> Result<Option<T>, Error> {
         match &*self.0.mutex.lock() {
             Future::Ready(item) => Ok(Some(item.clone())),
-            Future::NotReady(weak) if weak.upgrade().is_some() => Ok(None),
-            Future::NotReady(_) => Err(Error::SenderDropped),
+            Future::NotReady => Ok(None),
+            Future::SenderDropped => Err(Error::SenderDropped),
         }
     }
 
@@ -71,10 +76,8 @@ impl<T: Clone> Receiver<T> {
         loop {
             match &*lock {
                 Future::Ready(item) => return Ok(item.clone()),
-                Future::NotReady(weak) if weak.upgrade().is_some() => {
-                    self.0.condvar.wait(&mut lock)
-                }
-                Future::NotReady(_) => return Err(Error::SenderDropped),
+                Future::NotReady => self.0.condvar.wait(&mut lock),
+                Future::SenderDropped => return Err(Error::SenderDropped),
             }
         }
     }
@@ -84,13 +87,12 @@ impl<T: Clone> Receiver<T> {
 ///
 /// The sender may send *only one* message which will be received by *all* receivers.
 pub fn oneshot<T: Clone>() -> (Sender<T>, Receiver<T>) {
-    let sender_ref = Arc::new(());
     let mutex_condvar = Arc::new(MutexCondvar {
-        mutex: Mutex::new(Future::NotReady(Arc::downgrade(&sender_ref))),
+        mutex: Mutex::new(Future::NotReady),
         condvar: Condvar::new(),
     });
     let receiver = Receiver(mutex_condvar.clone());
-    let sender = Sender(mutex_condvar, Some(sender_ref));
+    let sender = Sender(mutex_condvar);
     (sender, receiver)
 }