[FLINK-4307] [streaming API] Restore ListState behavior for user-faci…

…ng ListStates

flink-streaming-java/src/main/java/org/apache/flink/streaming/api/operators/StreamingRuntimeContext.java

@@ -141,7 +141,8 @@ public <T> ListState<T> getListState(ListStateDescriptor<T> stateProperties) {
 		requireNonNull(stateProperties, "The state properties must not be null");
 		try {
 			stateProperties.initializeSerializerUnlessSet(getExecutionConfig());
-			return operator.getPartitionedState(stateProperties);
+			ListState<T> originalState = operator.getPartitionedState(stateProperties);
+			return new UserFacingListState<T>(originalState);
 		} catch (Exception e) {
 			throw new RuntimeException("Error while getting state", e);
 		}

flink-streaming-java/src/main/java/org/apache/flink/streaming/api/operators/UserFacingListState.java

@@ -0,0 +1,57 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.streaming.api.operators;
+
+import org.apache.flink.api.common.state.ListState;
+
+import java.util.Collections;
+
+/**
+ * Simple wrapper list state that exposes empty state properly as an empty list.
+ * 
+ * @param <T> The type of elements in the list state.
+ */
+class UserFacingListState<T> implements ListState<T> {
+
+	private final ListState<T> originalState;
+
+	private final Iterable<T> emptyState = Collections.emptyList();
+
+	UserFacingListState(ListState<T> originalState) {
+		this.originalState = originalState;
+	}
+
+	// ------------------------------------------------------------------------
+
+	@Override
+	public Iterable<T> get() throws Exception {
+		Iterable<T> original = originalState.get();
+		return original != null ? original : emptyState;
+	}
+
+	@Override
+	public void add(T value) throws Exception {
+		originalState.add(value);
+	}
+
+	@Override
+	public void clear() {
+		originalState.clear();
+	}
+}

flink-streaming-java/src/test/java/org/apache/flink/streaming/api/operators/StreamingRuntimeContextTest.java

@@ -22,15 +22,19 @@
 import org.apache.flink.api.common.TaskInfo;
 import org.apache.flink.api.common.accumulators.Accumulator;
 import org.apache.flink.api.common.functions.ReduceFunction;
+import org.apache.flink.api.common.state.ListState;
 import org.apache.flink.api.common.state.ListStateDescriptor;
 import org.apache.flink.api.common.state.ReducingStateDescriptor;
 import org.apache.flink.api.common.state.StateDescriptor;
 import org.apache.flink.api.common.state.ValueStateDescriptor;
 import org.apache.flink.api.common.typeutils.TypeSerializer;
+import org.apache.flink.api.common.typeutils.base.StringSerializer;
+import org.apache.flink.api.common.typeutils.base.VoidSerializer;
 import org.apache.flink.api.java.typeutils.runtime.kryo.KryoSerializer;
 import org.apache.flink.core.fs.Path;
 import org.apache.flink.runtime.execution.Environment;
 
+import org.apache.flink.runtime.state.memory.MemListState;
 import org.junit.Test;
 
 import org.mockito.invocation.InvocationOnMock;
@@ -54,7 +58,7 @@ public void testValueStateInstantiation() throws Exception {
 		final AtomicReference<Object> descriptorCapture = new AtomicReference<>();
 
 		StreamingRuntimeContext context = new StreamingRuntimeContext(
-				createMockOp(descriptorCapture, config),
+				createDescriptorCapturingMockOp(descriptorCapture, config),
 				createMockEnvironment(),
 				Collections.<String, Accumulator<?, ?>>emptyMap());
 
@@ -78,7 +82,7 @@ public void testReduceingStateInstantiation() throws Exception {
 		final AtomicReference<Object> descriptorCapture = new AtomicReference<>();
 
 		StreamingRuntimeContext context = new StreamingRuntimeContext(
-				createMockOp(descriptorCapture, config),
+				createDescriptorCapturingMockOp(descriptorCapture, config),
 				createMockEnvironment(),
 				Collections.<String, Accumulator<?, ?>>emptyMap());
 
@@ -107,7 +111,7 @@ public void testListStateInstantiation() throws Exception {
 		final AtomicReference<Object> descriptorCapture = new AtomicReference<>();
 
 		StreamingRuntimeContext context = new StreamingRuntimeContext(
-				createMockOp(descriptorCapture, config),
+				createDescriptorCapturingMockOp(descriptorCapture, config),
 				createMockEnvironment(),
 				Collections.<String, Accumulator<?, ?>>emptyMap());
 
@@ -121,13 +125,29 @@ public void testListStateInstantiation() throws Exception {
 		assertTrue(serializer instanceof KryoSerializer);
 		assertTrue(((KryoSerializer<?>) serializer).getKryo().getRegistration(Path.class).getId() > 0);
 	}
+
+	@Test
+	public void testListStateReturnsEmptyListByDefault() throws Exception {
+
+		StreamingRuntimeContext context = new StreamingRuntimeContext(
+				createPlainMockOp(),
+				createMockEnvironment(),
+				Collections.<String, Accumulator<?, ?>>emptyMap());
+
+		ListStateDescriptor<String> descr = new ListStateDescriptor<>("name", String.class);
+		ListState<String> state = context.getListState(descr);
+
+		Iterable<String> value = state.get();
+		assertNotNull(value);
+		assertFalse(value.iterator().hasNext());
+	}
 
 	// ------------------------------------------------------------------------
 	//  
 	// ------------------------------------------------------------------------
 
 	@SuppressWarnings("unchecked")
-	private static AbstractStreamOperator<?> createMockOp(
+	private static AbstractStreamOperator<?> createDescriptorCapturingMockOp(
 			final AtomicReference<Object> ref, final ExecutionConfig config) throws Exception {
 
 		AbstractStreamOperator<?> operatorMock = mock(AbstractStreamOperator.class);
@@ -145,6 +165,27 @@ public Object answer(InvocationOnMock invocationOnMock) throws Throwable {
 
 		return operatorMock;
 	}
+
+	@SuppressWarnings("unchecked")
+	private static AbstractStreamOperator<?> createPlainMockOp() throws Exception {
+
+		AbstractStreamOperator<?> operatorMock = mock(AbstractStreamOperator.class);
+		when(operatorMock.getExecutionConfig()).thenReturn(new ExecutionConfig());
+
+		when(operatorMock.getPartitionedState(any(ListStateDescriptor.class))).thenAnswer(
+				new Answer<ListState<String>>() {
+
+					@Override
+					public ListState<String> answer(InvocationOnMock invocationOnMock) throws Throwable {
+						ListStateDescriptor<String> descr =
+							(ListStateDescriptor<String>) invocationOnMock.getArguments()[0];
+						return new MemListState<String, Void, String>(
+								StringSerializer.INSTANCE, VoidSerializer.INSTANCE, descr);
+					}
+				});
+
+		return operatorMock;
+	}
 
 	private static Environment createMockEnvironment() {
 		Environment env = mock(Environment.class);