@Test
    public void testHeaders() {
        buildFetcher();

        MemoryRecordsBuilder builder = MemoryRecords.builder(ByteBuffer.allocate(1024), Compression.NONE, TimestampType.CREATE_TIME, 1L);
        builder.append(0L, "key".getBytes(), "value-1".getBytes());

        Header[] headersArray = new Header[1];
        headersArray[0] = new RecordHeader("headerKey", "headerValue".getBytes(StandardCharsets.UTF_8));
        builder.append(0L, "key".getBytes(), "value-2".getBytes(), headersArray);

        Header[] headersArray2 = new Header[2];
        headersArray2[0] = new RecordHeader("headerKey", "headerValue".getBytes(StandardCharsets.UTF_8));
        headersArray2[1] = new RecordHeader("headerKey", "headerValue2".getBytes(StandardCharsets.UTF_8));
        builder.append(0L, "key".getBytes(), "value-3".getBytes(), headersArray2);

        MemoryRecords memoryRecords = builder.build();

        List<ConsumerRecord<byte[], byte[]>> records;
        assignFromUser(singleton(tp0));
        subscriptions.seek(tp0, 1);

        client.prepareResponse(matchesOffset(tidp0, 1), fullFetchResponse(tidp0, memoryRecords, Errors.NONE, 100L, 0));

        assertEquals(1, sendFetches());
        networkClientDelegate.poll(time.timer(0));
        Map<TopicPartition, List<ConsumerRecord<byte[], byte[]>>> recordsByPartition = fetchRecords();
        records = recordsByPartition.get(tp0);

        assertEquals(3, records.size());

        Iterator<ConsumerRecord<byte[], byte[]>> recordIterator = records.iterator();

        ConsumerRecord<byte[], byte[]> record = recordIterator.next();
        assertNull(record.headers().lastHeader("headerKey"));

        record = recordIterator.next();
        assertEquals("headerValue", new String(record.headers().lastHeader("headerKey").value(), StandardCharsets.UTF_8));
        assertEquals("headerKey", record.headers().lastHeader("headerKey").key());

        record = recordIterator.next();
        assertEquals("headerValue2", new String(record.headers().lastHeader("headerKey").value(), StandardCharsets.UTF_8));
        assertEquals("headerKey", record.headers().lastHeader("headerKey").key());
    }
This validates that no records are fetched for partitions that are waiting for the
onPartitionsAssigned callback to complete. Note that this markPendingOnAssignedCallback
functionality is only used from the {@link AsyncKafkaConsumer}.
clients/src/test/java/org/apache/kafka/clients/consumer/internals/FetchRequestManagerTest.java