squah-confluent commented on code in PR #20055:
URL: https://github.com/apache/kafka/pull/20055#discussion_r2174928086


##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/consumer/CurrentAssignmentBuilder.java:
##########
@@ -215,6 +278,64 @@ private boolean ownsRevokedPartitions(
         return false;
     }
 
+    /**
+     * Updates the current assignment, removing any partitions that are not 
part of the subscribed topics.
+     * This method is a lot faster than running the full reconciliation logic 
in computeNextAssignment.
+     *
+     * @param memberAssignedPartitions  The assigned partitions of the member 
to use.
+     * @return A new ConsumerGroupMember.
+     */
+    private ConsumerGroupMember updateCurrentAssignment(
+        Map<Uuid, Set<Integer>> memberAssignedPartitions
+    ) {
+        Set<Uuid> subscribedTopicIds = subscribedTopicIds();
+
+        // Reuse the original map if no topics need to be removed.
+        Map<Uuid, Set<Integer>> newAssignedPartitions = 
memberAssignedPartitions;
+        Map<Uuid, Set<Integer>> newPartitionsPendingRevocation = new 
HashMap<>(member.partitionsPendingRevocation());
+        for (Map.Entry<Uuid, Set<Integer>> entry : 
memberAssignedPartitions.entrySet()) {
+            if (!subscribedTopicIds.contains(entry.getKey())) {
+                if (newAssignedPartitions == memberAssignedPartitions) {
+                    newAssignedPartitions = new 
HashMap<>(memberAssignedPartitions);
+                    newPartitionsPendingRevocation = new 
HashMap<>(member.partitionsPendingRevocation());
+                }
+                newAssignedPartitions.remove(entry.getKey());
+                newPartitionsPendingRevocation.merge(
+                    entry.getKey(),
+                    entry.getValue(),
+                    (existing, additional) -> {
+                        existing = new HashSet<>(existing);
+                        existing.addAll(additional);
+                        return existing;
+                    }
+                );
+            }
+        }
+
+        if (newAssignedPartitions == memberAssignedPartitions) {
+            // If no partitions were removed, we can return the member as is.
+            return member;
+        }
+
+        if (ownsRevokedPartitions(newPartitionsPendingRevocation)) {

Review Comment:
   If we don't check this, then we run into the `UNREVOKED_PARTITIONS` -> 
`UNRELEASED_PARTITIONS` bug in the regex tests and have to heartbeat an extra 
time. It just makes the tests worse is all.
   
   Instead of transitioning
   ```
   STABLE --subscription update--> STABLE --regex and assignor update--> STABLE
   ```
   it goes
   ```
   STABLE --subscription update--> UNREVOKED_PARTITIONS --regex and assignor 
update--> UNRELEASED_PARTITIONS --> STABLE
   ```



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