Illustrate how max.poll.records affects buffer (#865)

* Illustrate how max.poll.records effects buffer
* Make test fail/succeed just for the max.poll.records

tests/src/test/scala/akka/kafka/scaladsl/RebalanceSpec.scala

@@ -0,0 +1,104 @@
+/*
+ * Copyright (C) 2014 - 2016 Softwaremill <http://softwaremill.com>
+ * Copyright (C) 2016 - 2019 Lightbend Inc. <http://www.lightbend.com>
+ */
+
+package akka.kafka.scaladsl
+
+import akka.Done
+import akka.kafka._
+import akka.kafka.testkit.scaladsl.TestcontainersKafkaLike
+import akka.stream.scaladsl.Keep
+import akka.stream.testkit.scaladsl.StreamTestKit.assertAllStagesStopped
+import akka.stream.testkit.scaladsl.TestSink
+import akka.testkit.TestProbe
+import org.apache.kafka.clients.consumer.ConsumerConfig
+import org.apache.kafka.common.TopicPartition
+import org.scalatest._
+
+import scala.concurrent.duration._
+import scala.util.Random
+
+class RebalanceSpec extends SpecBase with TestcontainersKafkaLike with Inside {
+
+  implicit val patience: PatienceConfig = PatienceConfig(30.seconds, 500.millis)
+
+  final val Numbers = (1 to 5000).map(_.toString)
+  final val partition1 = 1
+
+  "Fetched records" must {
+
+    // The `max.poll.records` controls how many records Kafka fetches internally during a poll.
+    // documented in https://github.com/akka/alpakka-kafka/pull/865
+    "actually show even if partition is revoked" in assertAllStagesStopped {
+      val count = 20L
+      // de-coupling consecutive test runs with crossScalaVersions on Travis
+      val topicSuffix = Random.nextInt()
+      val topic1 = createTopic(topicSuffix, partitions = 2)
+      val group1 = createGroupId(1)
+      val consumerSettings = consumerDefaults
+      // This test FAILS with the default value as messages are enqueue in the stage
+        .withProperty(ConsumerConfig.MAX_POLL_RECORDS_CONFIG, "1")
+        .withGroupId(group1)
+
+      awaitProduce(produce(topic1, 0 to count.toInt, partition1))
+
+      // Subscribe to the topic (without demand)
+      val probe1rebalanceActor = TestProbe()
+      val probe1subscription = Subscriptions.topics(topic1).withRebalanceListener(probe1rebalanceActor.ref)
+      val (control1, probe1) = Consumer
+        .plainSource(consumerSettings, probe1subscription)
+        .toMat(TestSink.probe)(Keep.both)
+        .run()
+
+      // Await initial partition assignment
+      probe1rebalanceActor.expectMsgClass(classOf[TopicPartitionsRevoked])
+      probe1rebalanceActor.expectMsg(
+        TopicPartitionsAssigned(probe1subscription,
+                                Set(new TopicPartition(topic1, partition0), new TopicPartition(topic1, partition1)))
+      )
+
+      // read one message from probe1 with partition 1
+      probe1.requestNext()
+
+      // Subscribe to the topic (without demand)
+      val probe2rebalanceActor = TestProbe()
+      val probe2subscription = Subscriptions.topics(topic1).withRebalanceListener(probe2rebalanceActor.ref)
+      val (control2, probe2) = Consumer
+        .plainSource(consumerSettings, probe2subscription)
+        .toMat(TestSink.probe)(Keep.both)
+        .run()
+
+      // Await a revoke to both consumers
+      probe1rebalanceActor.expectMsg(
+        TopicPartitionsRevoked(probe1subscription,
+                               Set(new TopicPartition(topic1, partition0), new TopicPartition(topic1, partition1)))
+      )
+      probe2rebalanceActor.expectMsgClass(classOf[TopicPartitionsRevoked])
+
+      // the rebalance finishes
+      probe1rebalanceActor.expectMsg(
+        TopicPartitionsAssigned(probe1subscription, Set(new TopicPartition(topic1, partition0)))
+      )
+      probe2rebalanceActor.expectMsg(
+        TopicPartitionsAssigned(probe2subscription, Set(new TopicPartition(topic1, partition1)))
+      )
+
+      probe1.request(count)
+      probe2.request(count)
+
+      val probe2messages = probe2.expectNextN(count)
+
+      // no further messages enqueued on probe1 as partition 1 is balanced away
+      probe1.expectNoMessage(500.millis)
+
+      probe2messages should have size (count)
+
+      probe1.cancel()
+      probe2.cancel()
+
+      control1.isShutdown.futureValue shouldBe Done
+      control2.isShutdown.futureValue shouldBe Done
+    }
+  }
+}