add test and add comment

clients/src/main/java/org/apache/kafka/clients/consumer/internals/AbstractCoordinator.java

@@ -807,6 +807,8 @@ public void handle(SyncGroupResponse syncResponse,
                 } else if (error == Errors.REBALANCE_IN_PROGRESS) {
                     log.info("SyncGroup failed: The group began another rebalance. Need to re-join the group. " +
                                  "Sent generation was {}", sentGeneration);
+                    // consumer didn't get assignment in this generation, so we need to reset generation
+                    // to avoid joinGroup with out-of-data ownedPartitions in cooperative rebalance
                     resetStateOnResponseError(ApiKeys.SYNC_GROUP, error, false);
                     future.raise(error);
                 } else if (error == Errors.FENCED_INSTANCE_ID) {

clients/src/test/java/org/apache/kafka/clients/consumer/internals/AbstractCoordinatorTest.java

@@ -67,6 +67,7 @@
 import java.util.Map;
 import java.util.Optional;
 import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.Future;
@@ -488,6 +489,54 @@ public void testRetainMemberIdAfterSyncGroupDisconnect() {
         ensureActiveGroup(rejoinedGeneration, memberId);
     }
 
+    @Test
+    public void testResetGenerationIdAfterSyncGroupFailedWithRebalanceInProgress() throws InterruptedException, ExecutionException {
+        setupCoordinator();
+
+        String memberId = "memberId";
+        int generation = 5;
+
+        // Rebalance once to initialize the generation and memberId
+        mockClient.prepareResponse(groupCoordinatorResponse(node, Errors.NONE));
+        expectJoinGroup("", generation, memberId);
+        expectSyncGroup(generation, memberId);
+        ensureActiveGroup(generation, memberId);
+
+        // Force a rebalance
+        coordinator.requestRejoin("Manual test trigger");
+        assertTrue(coordinator.rejoinNeededOrPending());
+
+        ExecutorService executor = Executors.newFixedThreadPool(1);
+        try {
+            // Return RebalanceInProgress in syncGroup
+            int rejoinedGeneration = 10;
+            expectJoinGroup(memberId, rejoinedGeneration, memberId);
+            expectRebalanceInProgressForSyncGroup(rejoinedGeneration, memberId);
+            Future<Boolean> secondJoin = executor.submit(() ->
+                coordinator.ensureActiveGroup(mockTime.timer(Integer.MAX_VALUE)));
+
+            TestUtils.waitForCondition(() -> {
+                AbstractCoordinator.Generation currentGeneration = coordinator.generation();
+                return currentGeneration.generationId == AbstractCoordinator.Generation.NO_GENERATION.generationId &&
+                        currentGeneration.memberId.equals(memberId);
+            }, 2000, "Generation should be reset");
+
+            rejoinedGeneration = 20;
+            expectSyncGroup(rejoinedGeneration, memberId);
+            mockClient.respond(joinGroupFollowerResponse(
+                    rejoinedGeneration,
+                    memberId,
+                    "leaderId",
+                    Errors.NONE,
+                    PROTOCOL_TYPE
+            ));
+            assertTrue(secondJoin.get());
+        } finally {
+            executor.shutdownNow();
+            executor.awaitTermination(1000, TimeUnit.MILLISECONDS);
+        }
+    }
+
     @Test
     public void testRejoinReason() {
         setupCoordinator();
@@ -566,6 +615,22 @@ private void expectDisconnectInSyncGroup(
         }, null, true);
     }
 
+    private void expectRebalanceInProgressForSyncGroup(
+            int expectedGeneration,
+            String expectedMemberId
+    ) {
+        mockClient.prepareResponse(body -> {
+            if (!(body instanceof SyncGroupRequest)) {
+                return false;
+            }
+            SyncGroupRequestData syncGroupRequest = ((SyncGroupRequest) body).data();
+            return syncGroupRequest.generationId() == expectedGeneration
+                    && syncGroupRequest.memberId().equals(expectedMemberId)
+                    && syncGroupRequest.protocolType().equals(PROTOCOL_TYPE)
+                    && syncGroupRequest.protocolName().equals(PROTOCOL_NAME);
+        }, syncGroupResponse(Errors.REBALANCE_IN_PROGRESS, PROTOCOL_TYPE, PROTOCOL_NAME));
+    }
+
     private void expectDisconnectInJoinGroup(
         String expectedMemberId
     ) {