feat: adding support for dead letter queues (#60)

* feat: Adding support for DLQs

Adding delivery attempt count to PubsubMessages as a message attribute,
and creating helper function to allow users to get the count without
knowing implementation details.

* Fix formatting

* fix: making changes requested in pull request

google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/MessageDispatcher.java

@@ -341,10 +341,17 @@ private void processBatch(List<OutstandingMessage> batch) {
         // This should be a blocking flow controller and never throw an exception.
         throw new IllegalStateException("Flow control unexpected exception", unexpectedException);
       }
-      processOutstandingMessage(message.receivedMessage.getMessage(), message.ackHandler);
+      processOutstandingMessage(addDeliveryInfoCount(message.receivedMessage), message.ackHandler);
     }
   }
 
+  private PubsubMessage addDeliveryInfoCount(ReceivedMessage receivedMessage) {
+    return PubsubMessage.newBuilder(receivedMessage.getMessage())
+        .putAttributes(
+            "googclient_deliveryattempt", Integer.toString(receivedMessage.getDeliveryAttempt()))
+        .build();
+  }
+
   private void processOutstandingMessage(final PubsubMessage message, final AckHandler ackHandler) {
     final SettableApiFuture<AckReply> response = SettableApiFuture.create();
     final AckReplyConsumer consumer =

google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/Subscriber.java

@@ -44,6 +44,7 @@
 import com.google.common.base.Preconditions;
 import com.google.common.util.concurrent.ThreadFactoryBuilder;
 import com.google.pubsub.v1.ProjectSubscriptionName;
+import com.google.pubsub.v1.PubsubMessage;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.List;
@@ -205,6 +206,11 @@ public static Builder newBuilder(String subscription, MessageReceiver receiver)
     return new Builder(subscription, receiver);
   }
 
+  /** Returns the delivery attempt count for a received {@link PubsubMessage} */
+  public static int getDeliveryAttempt(PubsubMessage message) {
+    return Integer.parseInt(message.getAttributesOrDefault("googclient_deliveryattempt", "0"));
+  }
+
   /** Subscription which the subscriber is subscribed to. */
   public String getSubscriptionNameString() {
     return subscriptionName;

google-cloud-pubsub/src/test/java/com/google/cloud/pubsub/v1/MessageDispatcherTest.java

@@ -37,10 +37,13 @@
 import org.threeten.bp.Duration;
 
 public class MessageDispatcherTest {
+  private static final ByteString MESSAGE_DATA = ByteString.copyFromUtf8("message-data");
+  private static final int DELIVERY_INFO_COUNT = 3;
   private static final ReceivedMessage TEST_MESSAGE =
       ReceivedMessage.newBuilder()
           .setAckId("ackid")
-          .setMessage(PubsubMessage.newBuilder().setData(ByteString.EMPTY).build())
+          .setMessage(PubsubMessage.newBuilder().setData(MESSAGE_DATA).build())
+          .setDeliveryAttempt(DELIVERY_INFO_COUNT)
           .build();
   private static final Runnable NOOP_RUNNABLE =
       new Runnable() {
@@ -78,6 +81,9 @@ public void setUp() {
         new MessageReceiver() {
           @Override
           public void receiveMessage(final PubsubMessage message, final AckReplyConsumer consumer) {
+            assertThat(message.getData()).isEqualTo(MESSAGE_DATA);
+            assertThat(message.getAttributesOrThrow("googclient_deliveryattempt"))
+                .isEqualTo(Integer.toString(DELIVERY_INFO_COUNT));
             consumers.add(consumer);
           }
         };

google-cloud-pubsub/src/test/java/com/google/cloud/pubsub/v1/SubscriberTest.java

@@ -84,6 +84,19 @@ public void tearDown() throws Exception {
     testChannel.shutdown();
   }
 
+  @Test
+  public void testDeliveryAttemptHelper() {
+    int deliveryAttempt = 3;
+    PubsubMessage message =
+        PubsubMessage.newBuilder()
+            .putAttributes("googclient_deliveryattempt", Integer.toString(deliveryAttempt))
+            .build();
+    assertEquals(Subscriber.getDeliveryAttempt(message), deliveryAttempt);
+
+    PubsubMessage emptyMessage = PubsubMessage.newBuilder().build();
+    assertEquals(Subscriber.getDeliveryAttempt(emptyMessage), 0);
+  }
+
   @Test
   public void testOpenedChannels() throws Exception {
     int expectedChannelCount = 1;