@Test
    public void testFetcherConcurrency() throws Exception {
        int numPartitions = 20;
        Set<TopicPartition> topicPartitions = new HashSet<>();
        for (int i = 0; i < numPartitions; i++)
            topicPartitions.add(new TopicPartition(topicName, i));

        LogContext logContext = new LogContext();
        buildDependencies(new MetricConfig(), Long.MAX_VALUE, new SubscriptionState(logContext, OffsetResetStrategy.EARLIEST), logContext);

        IsolationLevel isolationLevel = IsolationLevel.READ_UNCOMMITTED;

        offsetFetcher = new OffsetFetcher(logContext,
                consumerClient,
                metadata,
                subscriptions,
                time,
                retryBackoffMs,
                requestTimeoutMs,
                isolationLevel,
                apiVersions);

        Deserializers<byte[], byte[]> deserializers = new Deserializers<>(new ByteArrayDeserializer(), new ByteArrayDeserializer());
        FetchConfig fetchConfig = new FetchConfig(
                minBytes,
                maxBytes,
                maxWaitMs,
                fetchSize,
                2 * numPartitions,
                true, // check crcs
                CommonClientConfigs.DEFAULT_CLIENT_RACK,
                isolationLevel);
        fetcher = new Fetcher<byte[], byte[]>(
                logContext,
                consumerClient,
                metadata,
                subscriptions,
                fetchConfig,
                deserializers,
                metricsManager,
                time,
                apiVersions) {
            @Override
            protected FetchSessionHandler sessionHandler(int node) {
                final FetchSessionHandler handler = super.sessionHandler(node);
                if (handler == null)
                    return null;
                else {
                    return new FetchSessionHandler(new LogContext(), node) {
                        @Override
                        public Builder newBuilder() {
                            verifySessionPartitions();
                            return handler.newBuilder();
                        }

                        @Override
                        public boolean handleResponse(FetchResponse response, short version) {
                            verifySessionPartitions();
                            return handler.handleResponse(response, version);
                        }

                        @Override
                        public void handleError(Throwable t) {
                            verifySessionPartitions();
                            handler.handleError(t);
                        }
                        
                        @Override
                        public Map<Uuid, String> sessionTopicNames() {
                            return handler.sessionTopicNames();
                        }

                        // Verify that session partitions can be traversed safely.
                        private void verifySessionPartitions() {
                            try {
                                Field field = FetchSessionHandler.class.getDeclaredField("sessionPartitions");
                                field.setAccessible(true);
                                LinkedHashMap<?, ?> sessionPartitions =
                                        (LinkedHashMap<?, ?>) field.get(handler);
                                for (Map.Entry<?, ?> entry : sessionPartitions.entrySet()) {
                                    // If `sessionPartitions` are modified on another thread, Thread.yield will increase the
                                    // possibility of ConcurrentModificationException if appropriate synchronization is not used.
                                    Thread.yield();
                                }
                            } catch (Exception e) {
                                throw new RuntimeException(e);
                            }
                        }
                    };
                }
            }
        };

        MetadataResponse initialMetadataResponse = RequestTestUtils.metadataUpdateWithIds(1,
                singletonMap(topicName, numPartitions), tp -> validLeaderEpoch, topicIds);
        client.updateMetadata(initialMetadataResponse);
        fetchSize = 10000;

        assignFromUser(topicPartitions);
        topicPartitions.forEach(tp -> subscriptions.seek(tp, 0L));

        AtomicInteger fetchesRemaining = new AtomicInteger(400);
        executorService = Executors.newSingleThreadExecutor();
        Future<?> future = executorService.submit(() -> {
            while (fetchesRemaining.get() > 0) {
                synchronized (consumerClient) {
                    if (!client.requests().isEmpty()) {
                        ClientRequest request = client.requests().peek();
                        FetchRequest fetchRequest = (FetchRequest) request.requestBuilder().build();
                        LinkedHashMap<TopicIdPartition, FetchResponseData.PartitionData> responseMap = new LinkedHashMap<>();
                        for (Map.Entry<TopicIdPartition, FetchRequest.PartitionData> entry : fetchRequest.fetchData(topicNames).entrySet()) {
                            TopicIdPartition tp = entry.getKey();
                            long offset = entry.getValue().fetchOffset;
                            responseMap.put(tp, new FetchResponseData.PartitionData()
                                    .setPartitionIndex(tp.topicPartition().partition())
                                    .setHighWatermark(offset + 2)
                                    .setLastStableOffset(offset + 2)
                                    .setLogStartOffset(0)
                                    .setRecords(buildRecords(offset, 2, offset)));
                        }
                        client.respondToRequest(request, FetchResponse.of(Errors.NONE, 0, 123, responseMap));
                        consumerClient.poll(time.timer(0));
                    }
                }
            }
            return fetchesRemaining.get();
        });
        Map<TopicPartition, Long> nextFetchOffsets = topicPartitions.stream()
                .collect(Collectors.toMap(Function.identity(), t -> 0L));
        while (fetchesRemaining.get() > 0 && !future.isDone()) {
            if (sendFetches() == 1) {
                synchronized (consumerClient) {
                    consumerClient.poll(time.timer(0));
                }
            }
            if (fetcher.hasCompletedFetches()) {
                Map<TopicPartition, List<ConsumerRecord<byte[], byte[]>>> fetchedRecords = fetchRecords();
                if (!fetchedRecords.isEmpty()) {
                    fetchesRemaining.decrementAndGet();
                    fetchedRecords.forEach((tp, records) -> {
                        assertEquals(2, records.size());
                        long nextOffset = nextFetchOffsets.get(tp);
                        assertEquals(nextOffset, records.get(0).offset());
                        assertEquals(nextOffset + 1, records.get(1).offset());
                        nextFetchOffsets.put(tp, nextOffset + 2);
                    });
                }
            }
        }
        assertEquals(0, future.get());
    }
Supplies parameters to {@link #testHandleFetchResponseError(Errors, long, boolean, boolean)}.
clients/src/test/java/org/apache/kafka/clients/consumer/internals/FetcherTest.java