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Beam IO for Google Cloud Pub/Sub Lite

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/*
 * Copyright 2021 Google LLC
 *
 * Licensed 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 com.google.cloud.pubsublite.beam;

import com.google.api.gax.rpc.ApiException;
import com.google.cloud.pubsublite.Offset;
import com.google.cloud.pubsublite.proto.ComputeMessageStatsResponse;
import java.util.HashMap;
import java.util.Map;
import javax.annotation.concurrent.GuardedBy;
import org.apache.beam.sdk.transforms.SerializableFunction;

public class ManagedBacklogReaderFactoryImpl implements ManagedBacklogReaderFactory {
  private final SerializableFunction newReader;

  @GuardedBy("this")
  private final Map readers = new HashMap<>();

  ManagedBacklogReaderFactoryImpl(
      SerializableFunction newReader) {
    this.newReader = newReader;
  }

  private static final class NonCloseableTopicBacklogReader implements TopicBacklogReader {
    private final TopicBacklogReader underlying;

    NonCloseableTopicBacklogReader(TopicBacklogReader underlying) {
      this.underlying = underlying;
    }

    @Override
    public ComputeMessageStatsResponse computeMessageStats(Offset offset) throws ApiException {
      return underlying.computeMessageStats(offset);
    }

    @Override
    public void close() {
      throw new IllegalArgumentException(
          "Cannot call close() on a reader returned from ManagedBacklogReaderFactory.");
    }
  }

  @Override
  public synchronized TopicBacklogReader newReader(SubscriptionPartition subscriptionPartition) {
    return new NonCloseableTopicBacklogReader(
        readers.computeIfAbsent(subscriptionPartition, newReader::apply));
  }

  @Override
  public synchronized void close() {
    readers.values().forEach(TopicBacklogReader::close);
  }
}