Pub/Sub: Add message abandonment (googleapis#4250)

* add abandonment

* add unit test for abandon

* update forget() to stop extending deadline

google-cloud-clients/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/AckReplyConsumer.java

@@ -29,4 +29,6 @@ public interface AckReplyConsumer {
    * message.
    */
   void nack();
+
+  void abandon();
 }

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

@@ -133,6 +133,7 @@ private class AckHandler implements ApiFutureCallback<AckReply> {
     private final int outstandingBytes;
     private final long receivedTimeMillis;
     private final Instant totalExpiration;
+    private boolean extending = true;
 
     AckHandler(String ackId, int outstandingBytes, Instant totalExpiration) {
       this.ackId = ackId;
@@ -151,6 +152,7 @@ private void forget() {
          */
         return;
       }
+      extending = false;
       flowController.release(1, outstandingBytes);
       messagesWaiter.incrementPendingMessages(-1);
       processOutstandingBatches();
@@ -417,6 +419,11 @@ public void ack() {
             public void nack() {
               response.set(AckReply.NACK);
             }
+
+            @Override
+            public void abandon() {
+              ackHandler.forget();
+            }
           };
       ApiFutures.addCallback(response, ackHandler, MoreExecutors.directExecutor());
       executor.execute(
@@ -471,6 +478,9 @@ void extendDeadlines() {
     Instant extendTo = now.plusSeconds(extendSeconds);
 
     for (Map.Entry<String, AckHandler> entry : pendingMessages.entrySet()) {
+      if (!entry.getValue().extending) {
+        continue;
+      }
       String ackId = entry.getKey();
       Instant totalExpiration = entry.getValue().totalExpiration;
       if (totalExpiration.isAfter(extendTo)) {

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

@@ -148,6 +148,14 @@ public void testNack() throws Exception {
     assertThat(sentModAcks).contains(ModAckItem.of(TEST_MESSAGE.getAckId(), 0));
   }
 
+  @Test
+  public void testAbandon() throws Exception {
+    dispatcher.processReceivedMessages(Collections.singletonList(TEST_MESSAGE), NOOP_RUNNABLE);
+    consumers.take().abandon();
+    dispatcher.extendDeadlines();
+    assertThat(sentModAcks).doesNotContain(TEST_MESSAGE.getAckId());
+  }
+
   @Test
   public void testExtension() throws Exception {
     dispatcher.processReceivedMessages(Collections.singletonList(TEST_MESSAGE), NOOP_RUNNABLE);