[FLINK-20764] In BatchGroupedReduceOperator, fix timer logic

This closes #14489

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

@@ -69,9 +69,13 @@ public class BatchGroupedReduceOperator<IN, KEY>
 		IN value = element.getValue();
 		IN currentValue = values.value();
 
-		if (currentValue != null) {
-			value = userFunction.reduce(currentValue, value);
+		if (currentValue == null) {
+			// register a timer for emitting the result at the end when this is the
+			// first input for this key
 			timerService.registerEventTimeTimer(VoidNamespace.INSTANCE, Long.MAX_VALUE);
+		} else {
+			// otherwise, reduce things
+			value = userFunction.reduce(currentValue, value);
 		}
 		values.update(value);
 	}

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

+/*
+ * 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
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * 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.functions.ReduceFunction;
+import org.apache.flink.api.common.typeinfo.BasicTypeInfo;
+import org.apache.flink.api.common.typeutils.base.StringSerializer;
+import org.apache.flink.streaming.api.watermark.Watermark;
+import org.apache.flink.streaming.runtime.streamrecord.StreamRecord;
+import org.apache.flink.streaming.util.KeyedOneInputStreamOperatorTestHarness;
+import org.apache.flink.util.TestLogger;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+
+import java.util.ArrayDeque;
+
+import static org.hamcrest.Matchers.contains;
+import static org.hamcrest.Matchers.empty;
+import static org.junit.Assert.assertThat;
+
+/**
+ * Tests {@link BatchGroupedReduceOperator}.
+ */
+public class BatchGroupedReduceOperatorTest extends TestLogger {
+
+	@Rule
+	public ExpectedException expectedException = ExpectedException.none();
+
+	@Test
+	public void noIncrementalResults() throws Exception {
+		KeyedOneInputStreamOperatorTestHarness<String, String, String> testHarness = createTestHarness();
+
+		testHarness.processElement(new StreamRecord<>("hello"));
+		testHarness.processElement(new StreamRecord<>("hello"));
+		testHarness.processElement(new StreamRecord<>("ciao"));
+		testHarness.processElement(new StreamRecord<>("ciao"));
+
+		assertThat(
+				testHarness.getOutput(),
+				empty());
+	}
+
+	@Test
+	public void resultsOnMaxWatermark() throws Exception {
+		KeyedOneInputStreamOperatorTestHarness<String, String, String> testHarness = createTestHarness();
+
+		testHarness.processElement(new StreamRecord<>("hello"));
+		testHarness.processElement(new StreamRecord<>("hello"));
+		testHarness.processElement(new StreamRecord<>("ciao"));
+		testHarness.processElement(new StreamRecord<>("ciao"));
+		testHarness.processElement(new StreamRecord<>("ciao"));
+
+		testHarness.processWatermark(Long.MAX_VALUE);
+
+		ArrayDeque<Object> expectedOutput = new ArrayDeque<>();
+		expectedOutput.add(new StreamRecord<>("hellohello", Long.MAX_VALUE));
+		expectedOutput.add(new StreamRecord<>("ciaociaociao", Long.MAX_VALUE));
+		expectedOutput.add(new Watermark(Long.MAX_VALUE));
+
+		assertThat(
+				testHarness.getOutput(),
+				contains(expectedOutput.toArray()));
+	}
+
+	@Test
+	public void resultForSingleInput() throws Exception {
+		KeyedOneInputStreamOperatorTestHarness<String, String, String> testHarness = createTestHarness();
+
+		testHarness.processElement(new StreamRecord<>("hello"));
+		testHarness.processElement(new StreamRecord<>("ciao"));
+
+		testHarness.processWatermark(Long.MAX_VALUE);
+
+		ArrayDeque<Object> expectedOutput = new ArrayDeque<>();
+		expectedOutput.add(new StreamRecord<>("hello", Long.MAX_VALUE));
+		expectedOutput.add(new StreamRecord<>("ciao", Long.MAX_VALUE));
+		expectedOutput.add(new Watermark(Long.MAX_VALUE));
+
+		assertThat(
+				testHarness.getOutput(),
+				contains(expectedOutput.toArray()));
+	}
+
+	private KeyedOneInputStreamOperatorTestHarness<String, String, String> createTestHarness() throws Exception {
+		BatchGroupedReduceOperator<String, Object> operator =
+				new BatchGroupedReduceOperator<>(new Concatenator(), StringSerializer.INSTANCE);
+
+		KeyedOneInputStreamOperatorTestHarness<String, String, String> testHarness =
+				new KeyedOneInputStreamOperatorTestHarness<>(
+						operator,
+						in -> in,
+						BasicTypeInfo.STRING_TYPE_INFO);
+
+		testHarness.setup();
+		testHarness.open();
+
+		return testHarness;
+	}
+
+	static class Concatenator implements ReduceFunction<String> {
+		@Override
+		public String reduce(String value1, String value2) throws Exception {
+			return value1 + value2;
+		}
+	}
+}