rajinisivaram commented on a change in pull request #9382:
URL: https://github.com/apache/kafka/pull/9382#discussion_r518198753



##########
File path: core/src/main/scala/kafka/server/AbstractFetcherThread.scala
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@@ -426,21 +451,34 @@ abstract class AbstractFetcherThread(name: String,
     warn(s"Partition $topicPartition marked as failed")
   }
 
-  def addPartitions(initialFetchStates: Map[TopicPartition, OffsetAndEpoch]): 
Set[TopicPartition] = {
+  /**
+   * Returns initial partition fetch state based on current state and the 
provided `initialFetchState`.
+   * From IBP 2.7 onwards, we can rely on truncation based on diverging data 
returned in fetch responses.
+   * For older versions, we can skip the truncation step iff the leader epoch 
matches the existing epoch.
+   */
+  private def partitionFetchState(tp: TopicPartition, initialFetchState: 
InitialFetchState, currentState: PartitionFetchState): PartitionFetchState = {
+    if (currentState != null && currentState.currentLeaderEpoch == 
initialFetchState.currentLeaderEpoch) {
+      currentState
+    } else if (isTruncationOnFetchSupported && initialFetchState.initOffset >= 
0 && initialFetchState.lastFetchedEpoch.nonEmpty &&
+              (currentState == null || currentState.state == Fetching)) {
+      PartitionFetchState(initialFetchState.initOffset, None, 
initialFetchState.currentLeaderEpoch,
+          state = Fetching, initialFetchState.lastFetchedEpoch)

Review comment:
       I couldn't come up with a case where `currentState` is non-null, so I 
removed that check. Haven't seen any test failures as a result, so hopefully 
that is ok. 




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