LUCENE-5390: Loosen assert in IW on pending event after close

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1556942 13f79535-47bb-0310-9956-ffa450edef68

lucene/core/src/java/org/apache/lucene/index/IndexWriter.java

@@ -930,10 +930,20 @@ public class IndexWriter implements Closeable, TwoPhaseCommit{
           closeInternal(waitForMerges, true);
         }
       }
-      assert eventQueue.isEmpty();
+      assert assertEventQueueAfterClose();
     }
   }
 
+  private boolean assertEventQueueAfterClose() {
+    if (eventQueue.isEmpty()) {
+      return true;
+    }
+    for (Event e : eventQueue) {
+      assert e instanceof DocumentsWriter.MergePendingEvent : e;
+    }
+    return true;
+  }
+
   // Returns true if this thread should attempt to close, or
   // false if IndexWriter is now closed; else, waits until
   // another thread finishes closing
@@ -2022,7 +2032,7 @@ public class IndexWriter implements Closeable, TwoPhaseCommit{
         rollbackInternal();
       }
     }
-    assert eventQueue.isEmpty() : eventQueue;
+    assert assertEventQueueAfterClose();
   }
 
   private void rollbackInternal() throws IOException {