use a different list for waitNotEmpty

reusing getters seems to create a race condition

chronos/asyncsync.nim

@@ -52,6 +52,7 @@ type
     ## removed by "await get()".
     getters: seq[Future[void].Raising([CancelledError])]
     putters: seq[Future[void].Raising([CancelledError])]
+    waitersForNonEmpty: seq[Future[void].Raising([CancelledError])]
     queue: Deque[T]
     maxsize: int
 
@@ -267,14 +268,14 @@ proc waitNotEmpty*[T](aq: AsyncQueue[T]): Future[void] {.
   ## It merely ensures that the queue is not empty. After `waitNotEmpty` completes,
   ## use other queue operations to manipulate items.
   while aq.empty():
-    let getter =
+    let waiter =
       Future[void].Raising([CancelledError]).init("AsyncQueue.waitNotEmpty")
-    aq.getters.add(getter)
+    aq.waitersForNonEmpty.add(waiter)
     try:
-      await getter
+      await waiter
     except CancelledError as exc:
-      if not(aq.empty()) and not(getter.cancelled()):
-        aq.getters.wakeupNext()
+      if not(aq.empty()) and not(waiter.cancelled()):
+        aq.waitersForNonEmpty.wakeupNext()
       raise exc
 
 proc full*[T](aq: AsyncQueue[T]): bool {.inline.} =
@@ -294,10 +295,12 @@ proc empty*[T](aq: AsyncQueue[T]): bool {.inline.} =
 proc addFirstImpl[T](aq: AsyncQueue[T], item: T) =
   aq.queue.addFirst(item)
   aq.getters.wakeupNext()
+  aq.waitersForNonEmpty.wakeupNext()
 
 proc addLastImpl[T](aq: AsyncQueue[T], item: T) =
   aq.queue.addLast(item)
   aq.getters.wakeupNext()
+  aq.waitersForNonEmpty.wakeupNext()
 
 proc popFirstImpl[T](aq: AsyncQueue[T]): T =
   let res = aq.queue.popFirst()