alirezazamani commented on a change in pull request #741: Fix 
ConcurrentModification exception in Workflow Garbage Collection
URL: https://github.com/apache/helix/pull/741#discussion_r383097458
 
 

 ##########
 File path: helix-core/src/main/java/org/apache/helix/task/TaskUtil.java
 ##########
 @@ -1043,23 +1043,40 @@ public static void purgeExpiredJobs(String workflow, 
WorkflowConfig workflowConf
    * @param dataProvider
    * @param manager
    */
-  public static void workflowGarbageCollection(WorkflowControllerDataProvider 
dataProvider,
+  public static void workflowGarbageCollection(final 
WorkflowControllerDataProvider dataProvider,
       final HelixManager manager) {
     // Garbage collections for conditions where workflow context exists but 
config is missing.
-    Map<String, ZNRecord> contexts = dataProvider.getContexts();
-    HelixDataAccessor accessor = manager.getHelixDataAccessor();
-    HelixPropertyStore<ZNRecord> propertyStore = 
manager.getHelixPropertyStore();
 
+    Set<String> existingContexts;
+    /*
+     * Here try-catch is used to avoid concurrent modification exception while 
doing deep copy.
+     * Map.keySet() can produce concurrent modification exception.
+     * Reason: If the map is modified while an iteration over the set is in 
progress, concurrent
+     * modification exception will be thrown.
+     */
+    try {
+      existingContexts = new HashSet<>(dataProvider.getContexts().keySet());
 
 Review comment:
   dataProvider.getContexts() is outside the scope of this class and perhaps 
there are performance concerns regarding this. Maybe we can consider this in 
next design of TF.

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