[GitHub] [kafka] cmccabe commented on a diff in pull request #13653: KAFKA-14946: fix NPE when merging the deltatable

["cmccabe (via GitHub)" <gi...@apache.org>]

cmccabe commented on code in PR #13653:
URL: https://github.com/apache/kafka/pull/13653#discussion_r1182836931


##########
server-common/src/main/java/org/apache/kafka/timeline/SnapshottableHashTable.java:
##########
@@ -113,7 +113,7 @@ public void mergeFrom(long epoch, Delta source) {
             HashTier<T> other = (HashTier<T>) source;
             // As an optimization, the deltaTable might not exist for a new key
             // as there is no previous value
-            if (other.deltaTable != null) {
+            if (deltaTable != null && other.deltaTable != null) {

Review Comment:
   Hmm, if our deltaTable doesn't exist here, we should create it like this, right?
   ```
               if (other.deltaTable != null) {
                   List<T> list = new ArrayList<>();
                   Object[] otherElements = other.deltaTable.baseElements();
                   for (int slot = 0; slot < otherElements.length; slot++) {
                       BaseHashTable.unpackSlot(list, otherElements, slot);
                       for (T element : list) {
                           // When merging in a later hash tier, we want to keep only the elements
                           // that were present at our epoch.
                           if (element.startEpoch() <= epoch) {
                               if (deltaTable == null) {
                                   deltaTable = new BaseHashTable<>(1);
                               }
                               deltaTable.baseAddOrReplace(element);
                           }
                       }
                   }
               }
   ```
   
   Otherwise we're not merging in all the stuff we should be from the later snapshot



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