Optimze away WindmillWatermarkHold::clear when the cached hold is empty

runners/google-cloud-dataflow-java/worker/src/main/java/org/apache/beam/runners/dataflow/worker/windmill/state/WindmillWatermarkHold.java

@@ -72,9 +72,13 @@ public class WindmillWatermarkHold extends WindmillState implements WatermarkHol
 
   @Override
   public void clear() {
+    localAdditions = null;
+    if (cachedValue != null && !cachedValue.isPresent()) {
+      // No need to clear the backend as it is known empty.
+      return;
+    }
     cleared = true;
     cachedValue = Optional.absent();
-    localAdditions = null;
   }
 
   @Override

runners/google-cloud-dataflow-java/worker/src/test/java/org/apache/beam/runners/dataflow/worker/StreamingDataflowWorkerTest.java

@@ -2374,9 +2374,6 @@ public void testMergeSessionWindows_singleLateWindow() throws Exception {
             new Action(
                     buildSessionInput(
                         1, 40, 0, Collections.singletonList(1L), Collections.EMPTY_LIST))
-                .withHolds(
-                    buildHold("/gAAAAAAAAAsK/+uhold", -1, true),
-                    buildHold("/gAAAAAAAAAsK/+uextra", -1, true))
                 .withTimers(buildWatermarkTimer("/s/gAAAAAAAAAsK/+0", 3600010))));
   }
 
@@ -2404,10 +2401,7 @@ public void testMergeSessionWindows() throws Exception {
                         0,
                         Collections.EMPTY_LIST,
                         Collections.singletonList(buildWatermarkTimer("/s/gAAAAAAAAAsK/+0", 10))))
-                .withTimers(buildWatermarkTimer("/s/gAAAAAAAAAsK/+0", 3600010))
-                .withHolds(
-                    buildHold("/gAAAAAAAAAsK/+uhold", -1, true),
-                    buildHold("/gAAAAAAAAAsK/+uextra", -1, true)),
+                .withTimers(buildWatermarkTimer("/s/gAAAAAAAAAsK/+0", 3600010)),
             new Action(
                     buildSessionInput(
                         3, 30, 0, Collections.singletonList(8L), Collections.EMPTY_LIST))
@@ -2436,8 +2430,8 @@ public void testMergeSessionWindows() throws Exception {
                 .withHolds(
                     buildHold("/gAAAAAAAACkK/+uhold", -1, true),
                     buildHold("/gAAAAAAAACkK/+uextra", -1, true),
-                    buildHold("/gAAAAAAAAAsK/+uhold", 40, true),
-                    buildHold("/gAAAAAAAAAsK/+uextra", 3600040, true)),
+                    buildHold("/gAAAAAAAAAsK/+uhold", 40, false),
+                    buildHold("/gAAAAAAAAAsK/+uextra", 3600040, false)),
             new Action(
                     buildSessionInput(
                         6,

runners/google-cloud-dataflow-java/worker/src/test/java/org/apache/beam/runners/dataflow/worker/windmill/state/WindmillStateInternalsTest.java

@@ -3228,6 +3228,40 @@ public void testNewWatermarkNoFetch() throws Exception {
     Mockito.verifyNoInteractions(mockReader);
   }
 
+  @Test
+  public void testWatermarkClearNoOp() throws Exception {
+    StateTag<WatermarkHoldState> addr =
+        StateTags.watermarkStateInternal("watermark", TimestampCombiner.EARLIEST);
+    WatermarkHoldState hold = underTestNewKey.state(NAMESPACE, addr);
+
+    hold.clear();
+
+    Windmill.WorkItemCommitRequest.Builder commitBuilder =
+        Windmill.WorkItemCommitRequest.newBuilder();
+    underTestNewKey.persist(commitBuilder);
+
+    assertEquals(0, commitBuilder.getWatermarkHoldsCount());
+    assertBuildable(commitBuilder);
+  }
+
+  @Test
+  public void testWatermarkClearWithLocalAdditionsNoop() throws Exception {
+    StateTag<WatermarkHoldState> addr =
+        StateTags.watermarkStateInternal("watermark", TimestampCombiner.EARLIEST);
+    WatermarkHoldState hold = underTestNewKey.state(NAMESPACE, addr);
+
+    hold.add(new Instant(500));
+
+    hold.clear();
+
+    Windmill.WorkItemCommitRequest.Builder commitBuilder =
+        Windmill.WorkItemCommitRequest.newBuilder();
+    underTestNewKey.persist(commitBuilder);
+
+    assertEquals(0, commitBuilder.getWatermarkHoldsCount());
+    assertBuildable(commitBuilder);
+  }
+
   @Test
   public void testValueSetBeforeRead() throws Exception {
     StateTag<ValueState<String>> addr = StateTags.value("value", StringUtf8Coder.of());