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   * 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,
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   * See the License for the specific language governing permissions and
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  package org.apache.beam.runners.flink.batch;
  
  import java.util.Arrays;
  import java.util.Objects;
  import org.apache.beam.runners.flink.FlinkPipelineOptions;
  import org.apache.beam.runners.flink.FlinkTestPipeline;
  import org.apache.beam.sdk.Pipeline;
  import org.apache.beam.sdk.PipelineResult;
  import org.apache.beam.sdk.transforms.Create;
  import org.apache.beam.sdk.transforms.DoFn;
  import org.apache.beam.sdk.transforms.GroupByKey;
  import org.apache.beam.sdk.transforms.ParDo;
  import org.apache.beam.sdk.values.KV;
  import org.apache.flink.test.util.AbstractTestBase;
  import org.junit.Assert;
  import org.junit.Test;
  
  public class NonMergingGroupByKeyTest extends AbstractTestBase {
  
    private static class ReiterateDoFn<K, V> extends DoFn<KV<K, Iterable<V>>, Void> {
  
      @ProcessElement
      @SuppressWarnings("ReturnValueIgnored")
      public void processElement(@Element KV<K, Iterable<V>> el) {
        el.getValue().iterator();
        // this should throw an exception
        el.getValue().iterator();
      }
    }
  
    @Test
    public void testDisabledReIterationThrowsAnException() {
      final Pipeline p = FlinkTestPipeline.createForBatch();
      p.apply(Create.of(Arrays.asList(KV.of("a", 1), KV.of("b", 2), KV.of("c", 3))))
          .apply(GroupByKey.create())
          .apply(ParDo.of(new ReiterateDoFn<>()));
      Pipeline.PipelineExecutionException resultException = null;
      try {
        p.run().waitUntilFinish();
      } catch (Pipeline.PipelineExecutionException exception) {
        resultException = exception;
      }
      Assert.assertEquals(
          IllegalStateException.class, Objects.requireNonNull(resultException).getCause().getClass());
      Assert.assertTrue(
          resultException.getCause().getMessage().contains("GBK result is not re-iterable."));
    }
  
    @Test
    public void testEnabledReIterationDoesNotThrowAnException() {
      final Pipeline p = FlinkTestPipeline.createForBatch();
      p.getOptions().as(FlinkPipelineOptions.class).setReIterableGroupByKeyResult(true);
      p.apply(Create.of(Arrays.asList(KV.of("a", 1), KV.of("b", 2), KV.of("c", 3))))
          .apply(GroupByKey.create())
          .apply(ParDo.of(new ReiterateDoFn<>()));
      final PipelineResult.State state = p.run().waitUntilFinish();
      Assert.assertEquals(PipelineResult.State.DONE, state);
    }
  }
  