Copilot commented on code in PR #23608:
URL: https://github.com/apache/kafka/pull/23608#discussion_r4123674615


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streams/integration-tests/src/test/java/org/apache/kafka/streams/integration/StreamsGroupWarmupTaskIntegrationTest.java:
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@@ -0,0 +1,322 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.streams.integration;
+
+import org.apache.kafka.clients.consumer.Consumer;
+import org.apache.kafka.clients.consumer.ConsumerConfig;
+import org.apache.kafka.clients.consumer.KafkaConsumer;
+import org.apache.kafka.clients.producer.KafkaProducer;
+import org.apache.kafka.clients.producer.Producer;
+import org.apache.kafka.clients.producer.ProducerConfig;
+import org.apache.kafka.clients.producer.ProducerRecord;
+import org.apache.kafka.common.TopicPartition;
+import org.apache.kafka.common.serialization.Serdes;
+import org.apache.kafka.common.serialization.StringDeserializer;
+import org.apache.kafka.common.serialization.StringSerializer;
+import org.apache.kafka.common.utils.Bytes;
+import org.apache.kafka.coordinator.group.GroupCoordinatorConfig;
+import org.apache.kafka.coordinator.group.streams.AssignmentRefinerImpl;
+import org.apache.kafka.streams.GroupProtocol;
+import org.apache.kafka.streams.KafkaStreams;
+import org.apache.kafka.streams.StreamsBuilder;
+import org.apache.kafka.streams.StreamsConfig;
+import org.apache.kafka.streams.TaskMetadata;
+import org.apache.kafka.streams.ThreadMetadata;
+import org.apache.kafka.streams.Topology;
+import org.apache.kafka.streams.integration.utils.EmbeddedKafkaCluster;
+import org.apache.kafka.streams.integration.utils.IntegrationTestUtils;
+import org.apache.kafka.streams.kstream.Materialized;
+import org.apache.kafka.streams.processor.StateRestoreListener;
+import org.apache.kafka.streams.processor.TaskId;
+import org.apache.kafka.streams.state.KeyValueStore;
+import org.apache.kafka.streams.state.Stores;
+import org.apache.kafka.test.NoRetryException;
+import org.apache.kafka.test.TestUtils;
+
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Tag;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.TestInfo;
+import org.junit.jupiter.api.Timeout;
+
+import java.io.IOException;
+import java.util.Collection;
+import java.util.Locale;
+import java.util.Properties;
+import java.util.Set;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Function;
+
+import static org.apache.kafka.common.utils.Utils.mkEntry;
+import static org.apache.kafka.common.utils.Utils.mkMap;
+import static org.apache.kafka.common.utils.Utils.mkObjectProperties;
+import static org.apache.kafka.common.utils.Utils.mkProperties;
+import static org.apache.kafka.streams.utils.TestUtils.safeUniqueTestName;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * Streams group protocol (KIP-1071) analog of {@link 
HighAvailabilityTaskAssignorIntegrationTest}: proves that
+ * scaling out a stateful app gets the new instance a warm-up task first, and 
only promotes it to active once
+ * caught up, reusing the warm-up's local state with no restoration.
+ *
+ * Unlike the classic protocol, warm-up derivation for streams groups happens 
broker-side via an
+ * {@code AssignmentRefiner} that is a {@code NoOp} by default, so the real 
{@link AssignmentRefinerImpl} is
+ * enabled explicitly via the (internal, testing-only) {@code 
group.streams.assignment.refiner.class} config.
+ */
+@Timeout(600)
+@Tag("integration")
+public class StreamsGroupWarmupTaskIntegrationTest {
+    private static final Properties BROKER_CONFIG = mkProperties(mkMap(
+        
mkEntry(GroupCoordinatorConfig.STREAMS_GROUP_ASSIGNMENT_REFINER_CLASS_CONFIG, 
AssignmentRefinerImpl.class.getName()),
+        
mkEntry(GroupCoordinatorConfig.STREAMS_GROUP_ACCEPTABLE_RECOVERY_LAG_CONFIG, 
"0")
+    ));
+
+    public static final EmbeddedKafkaCluster CLUSTER = new 
EmbeddedKafkaCluster(3, BROKER_CONFIG);
+
+    @BeforeAll
+    public static void startCluster() throws IOException {
+        CLUSTER.start();
+    }
+
+    @AfterAll
+    public static void closeCluster() {
+        CLUSTER.stop();
+    }
+
+    @Test
+    public void shouldScaleOutWithWarmupTasksAndInMemoryStores(final TestInfo 
testInfo) throws InterruptedException {
+        shouldScaleOutWithWarmupTasks(storeName -> 
Materialized.as(Stores.inMemoryKeyValueStore(storeName)), testInfo);
+    }
+
+    @Test
+    public void shouldScaleOutWithWarmupTasksAndPersistentStores(final 
TestInfo testInfo) throws InterruptedException {
+        shouldScaleOutWithWarmupTasks(storeName -> 
Materialized.as(Stores.persistentKeyValueStore(storeName)), testInfo);
+    }
+
+    private void shouldScaleOutWithWarmupTasks(final Function<String, 
Materialized<Object, Object, KeyValueStore<Bytes, byte[]>>> 
materializedFunction,
+                                               final TestInfo testInfo) throws 
InterruptedException {
+        final String testId = safeUniqueTestName(testInfo);
+        final String appId = "appId_" + System.currentTimeMillis() + "_" + 
testId;
+        final String inputTopic = "input" + testId;
+        final Set<TopicPartition> inputTopicPartitions = Set.of(
+            new TopicPartition(inputTopic, 0),
+            new TopicPartition(inputTopic, 1)
+        );
+
+        final String storeName = "store" + testId;
+        final String storeChangelog = appId + "-store" + testId + "-changelog";
+        final Set<TopicPartition> changelogTopicPartitions = Set.of(
+            new TopicPartition(storeChangelog, 0),
+            new TopicPartition(storeChangelog, 1)
+        );
+
+        IntegrationTestUtils.cleanStateBeforeTest(CLUSTER, 2, 2, inputTopic, 
storeChangelog);
+
+        final StreamsBuilder builder = new StreamsBuilder();
+        builder.table(inputTopic, materializedFunction.apply(storeName));
+        final Topology topology = builder.build();
+
+        final int numberOfRecords = 500;
+
+        produceTestData(inputTopic, numberOfRecords);
+
+        try (final KafkaStreams kafkaStreams0 = new KafkaStreams(topology, 
streamsProperties(appId));
+             final KafkaStreams kafkaStreams1 = new KafkaStreams(topology, 
streamsProperties(appId));
+             final Consumer<String, String> consumer = new 
KafkaConsumer<>(getConsumerProperties())) {
+            kafkaStreams0.start();
+
+            // sanity check: just make sure we actually wrote all the input 
records
+            TestUtils.waitForCondition(
+                () -> getEndOffsetSum(inputTopicPartitions, consumer) == 
numberOfRecords,
+                120_000L,
+                () -> "Input records haven't all been written to the input 
topic: " + getEndOffsetSum(inputTopicPartitions, consumer)
+            );
+
+            // wait until all the input records are in the changelog
+            TestUtils.waitForCondition(
+                () -> getEndOffsetSum(changelogTopicPartitions, consumer) == 
numberOfRecords,
+                120_000L,
+                () -> "Input records haven't all been written to the 
changelog: " + getEndOffsetSum(changelogTopicPartitions, consumer)
+            );
+
+            final AtomicLong instance1TotalRestored = new AtomicLong(-1);
+            final AtomicLong instance1NumRestored = new AtomicLong(-1);
+            final CountDownLatch restoreCompleteLatch = new CountDownLatch(1);
+            kafkaStreams1.setGlobalStateRestoreListener(new 
StateRestoreListener() {
+                @Override
+                public void onRestoreStart(final TopicPartition topicPartition,
+                                           final String storeName,
+                                           final long startingOffset,
+                                           final long endingOffset) {
+                }
+
+                @Override
+                public void onBatchRestored(final TopicPartition 
topicPartition,
+                                            final String storeName,
+                                            final long batchEndOffset,
+                                            final long numRestored) {
+                    instance1NumRestored.accumulateAndGet(
+                        numRestored,
+                        (prev, restored) -> prev == -1 ? restored : prev + 
restored
+                    );
+                }
+
+                @Override
+                public void onRestoreEnd(final TopicPartition topicPartition,
+                                         final String storeName,
+                                         final long totalRestored) {
+                    instance1TotalRestored.accumulateAndGet(
+                        totalRestored,
+                        (prev, restored) -> prev == -1 ? restored : prev + 
restored
+                    );
+                    restoreCompleteLatch.countDown();
+                }
+            });
+
+            kafkaStreams1.start();
+
+            // the new instance has no caught-up copy of any stateful task, so 
the target assignment's mismatch
+            // should be staged behind a warm-up task rather than promoted to 
active right away.
+            final AtomicReference<TaskId> warmupTaskId = new 
AtomicReference<>();
+            TestUtils.waitForCondition(
+                () -> {
+                    final TaskId standbyTaskId = 
findStandbyTaskId(kafkaStreams1);
+                    if (standbyTaskId == null) {
+                        return false;
+                    }
+                    assertFalseNoRetry(
+                        isActiveTask(kafkaStreams1, standbyTaskId),
+                        "a task that was just handed out as a warm-up should 
not already be active on the new instance."
+                    );
+                    warmupTaskId.set(standbyTaskId);

Review Comment:
   Promotion is asynchronous, so the second metadata read can legitimately 
observe this task as active immediately after `findStandbyTaskId` observed it 
as a standby. In that valid warm-up→promotion race, `NoRetryException` makes 
the test fail instead of accepting the warm-up observation. Remove this second 
read (the observed standby already establishes the intermediate state), or 
inspect both roles from one metadata snapshot.



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