Address review feedback

Author: wangshuo128

core/src/main/java/org/apache/spark/SparkFirehoseListener.java

@@ -30,13 +30,17 @@
  */
 public class SparkFirehoseListener implements SparkListenerInterface {
 
+  protected volatile boolean dead;
+
   @Override
   public boolean dead() {
-    return false;
+    return dead;
   }
 
   @Override
-  public void dead_$eq(boolean dead) { }
+  public void dead_$eq(boolean dead) {
+    this.dead = dead;
+  }
 
   public void onEvent(SparkListenerEvent event) { }
 

core/src/main/scala/org/apache/spark/scheduler/AsyncEventQueue.scala

@@ -210,9 +210,9 @@ private class AsyncEventQueue(
 
   override def removeListenerOnError(listener: SparkListenerInterface): Unit = {
     if (bus.isInStop) {
-      // If bus is in the progress of stop, we just mark the listener as dead instead of removing
-      // via calling `bus.removeListener` to avoid race condition
-      // dead listeners will be removed eventually in `bus.stop`
+      // If we're in the middle of stopping the bus, we just mark the listener as dead,
+      // instead of removing, to avoid a deadlock.
+      // Dead listeners will be removed eventually in `bus.stop`.
       listener.dead = true
     } else {
       // the listener failed in an unrecoverably way, we want to remove it from the entire