HADOOP-14912. FairCallQueue may defer servicing calls. Contributed by Daryn Sharp

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/FairCallQueue.java

@@ -122,13 +122,15 @@ public FairCallQueue(int priorityLevels, int capacity, String ns,
   private E removeNextElement() {
     int priority = multiplexer.getAndAdvanceCurrentIndex();
     E e = queues.get(priority).poll();
-    if (e == null) {
+    // a semaphore permit has been acquired, so an element MUST be extracted
+    // or the semaphore and queued elements will go out of sync.  loop to
+    // avoid race condition if elements are added behind the current position,
+    // awakening other threads that poll the elements ahead of our position.
+    while (e == null) {
       for (int idx = 0; e == null && idx < queues.size(); idx++) {
         e = queues.get(idx).poll();
       }
     }
-    // guaranteed to find an element if caller acquired permit.
-    assert e != null : "consumer didn't acquire semaphore!";
     return e;
   }