sync: Ensure try_send() wakes up receivers

This branch of try_send() just forgot to wake up any receiver waiting for data.

Closes #15761

1 files changed, 7 insertions, 1 deletions

diff --git a/src/libsync/comm/sync.rs b/src/libsync/comm/sync.rs
index cc3c2197c13..1d5a7d6ed9f 100644
--- a/src/libsync/comm/sync.rs
+++ b/src/libsync/comm/sync.rs
@@ -218,9 +218,15 @@ impl<T: Send> Packet<T> {
             }
         } else {
             // If the buffer has some space and the capacity isn't 0, then we
-            // just enqueue the data for later retrieval.
+            // just enqueue the data for later retrieval, ensuring to wake up
+            // any blocked receiver if there is one.
             assert!(state.buf.size() < state.buf.cap());
             state.buf.enqueue(t);
+            match mem::replace(&mut state.blocker, NoneBlocked) {
+                BlockedReceiver(task) => wakeup(task, guard),
+                NoneBlocked => {}
+                BlockedSender(..) => unreachable!(),
+            }
             Ok(())
         }
     }