[GitHub] [kafka] mjsax commented on a diff in pull request #10747: KAFKA-12446: Call subtractor before adder if key is the same

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

mjsax commented on code in PR #10747:
URL: https://github.com/apache/kafka/pull/10747#discussion_r1093813869


##########
streams/src/main/java/org/apache/kafka/streams/kstream/internals/ChangedSerializer.java:
##########
@@ -46,33 +45,40 @@ public void setIfUnset(final SerdeGetter getter) {
     }
 
     /**
-     * @throws StreamsException if both old and new values of data are null, or if
-     * both values are not null
+     * @throws StreamsException if both old and new values of data are null.
      */
     @Override
     public byte[] serialize(final String topic, final Headers headers, final Change<T> data) {
-        final byte[] serializedKey;
-
-        // only one of the old / new values would be not null
-        if (data.newValue != null) {
-            if (data.oldValue != null) {
-                throw new StreamsException("Both old and new values are not null (" + data.oldValue
-                    + " : " + data.newValue + ") in ChangeSerializer, which is not allowed.");
-            }
-
-            serializedKey = inner.serialize(topic, headers, data.newValue);
+        final boolean oldValueIsNull = data.oldValue == null;
+        final boolean newValueIsNull = data.newValue == null;
+
+        final byte[] newData = inner.serialize(topic, headers, data.newValue);
+        final byte[] oldData = inner.serialize(topic, headers, data.oldValue);
+
+        final int newDataLength = newValueIsNull ? 0 : newData.length;
+        final int oldDataLength = oldValueIsNull ? 0 : oldData.length;

Review Comment:
   The implicit contract is, that `inner.serialize` can only return `null` if `data.oldValue` is `null`. Otherwise, the serializer has a bug. Hence, we should never hit a NPE here (or accept to hit it, to surface the bug in the serializer)



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