content: /*
   * 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.cassandra.tcm;
  
  import java.util.concurrent.atomic.AtomicReference;
  import java.util.function.Supplier;
  
  import com.github.benmanes.caffeine.cache.Cache;
  import com.github.benmanes.caffeine.cache.Caffeine;
  import org.apache.cassandra.concurrent.ScheduledExecutors;
  import org.apache.cassandra.utils.Closeable;
  import org.apache.cassandra.utils.concurrent.Future;
  import org.apache.cassandra.utils.concurrent.ImmediateFuture;
  
  import static java.util.concurrent.TimeUnit.MILLISECONDS;
  import static org.apache.cassandra.config.DatabaseDescriptor.getCmsAwaitTimeout;
  import static org.apache.cassandra.config.DatabaseDescriptor.getEpochAwareDebounceInFlightTrackerMaxSize;
  
  /**
   * When debouncing from a replica we know exactly which epoch we need, so to avoid retries we
   * keep track of which epoch we are currently debouncing, and if a request for a newer epoch
   * comes in, we create a new future. If a request for a newer epoch comes in, we simply
   * swap out the current future reference for a new one which is requesting the newer epoch.
   */
  public class EpochAwareDebounce implements Closeable
  {
      public static final EpochAwareDebounce instance = new EpochAwareDebounce();
  
      private final AtomicReference<EpochAwareFuture> currentFuture = new AtomicReference<>();
      private final Cache<Epoch, Future<ClusterMetadata>> inflight = Caffeine.newBuilder()
                                                                             .executor(ScheduledExecutors.scheduledFastTasks)
                                                                             .maximumSize(getEpochAwareDebounceInFlightTrackerMaxSize())
                                                                             .expireAfterWrite(getCmsAwaitTimeout().to(MILLISECONDS),
                                                                                               MILLISECONDS)
                                                                             .build();
  
      private EpochAwareDebounce()
      {
      }
  
      /**
       * Deduplicate requests to catch up log state based on the desired epoch. Callers supply a target epoch and
       * a function obtain the ClusterMetadata that corresponds with it. It is expected that this function will make rpc
       * calls to peers, retrieving a LogState which can be applied locally to produce the necessary {@code
       * ClusterMetadata}. 
       *
       * @param fetchFunction supplies the future that, when dereferenced, will yield metadata for the desired epoch
       * @param epoch the desired epoch
       * @return
       */
      public Future<ClusterMetadata> getAsync(Supplier<Future<ClusterMetadata>> fetchFunction, Epoch epoch)
      {
          while (true)
          {
              EpochAwareFuture running = currentFuture.get();
              // Someone else is about to install a new future
              if (running == SENTINEL)
                  continue;
  
              // The inflight future is sufficient, or we are shutting down the debouncer
              if ((running != null && !running.future.isDone() && running.epoch.isEqualOrAfter(epoch)) || running == CLOSED)
                  return running.future;
  
              if (currentFuture.compareAndSet(running, SENTINEL))
              {
                  Future<ClusterMetadata> cmFuture = fetchFunction.get();
                  cmFuture.addListener(() -> inflight.asMap().remove(epoch));
                  EpochAwareFuture promise = new EpochAwareFuture(epoch, cmFuture);
                  EpochAwareFuture current = currentFuture.compareAndExchange(SENTINEL, promise);
                  // we have to explicitly check here as close() unconditionally sets currentFuture
                  if (current == CLOSED)
                      return CLOSED.future;
  
                  inflight.put(epoch, cmFuture);
                  return cmFuture;
              }
          }
      }
  
      private static final EpochAwareFuture SENTINEL = new EpochAwareFuture(Epoch.EMPTY, null);
      private static final EpochAwareFuture CLOSED = new EpochAwareFuture(Epoch.EMPTY, ImmediateFuture.cancelled());
  
      @Override
      public void close()
      {
          EpochAwareFuture future = currentFuture.getAndSet(CLOSED);
          if (future != null && future != SENTINEL && future != CLOSED)
              future.future.cancel(true);
          for (Future<ClusterMetadata> toCancel : inflight.asMap().values())
              toCancel.cancel(true);
      }
  
      public long inflightTrackerSize()
      {
          return inflight.estimatedSize();
      }
  
      private static class EpochAwareFuture
      {
          private final Epoch epoch;
          private final Future<ClusterMetadata> future;
          public EpochAwareFuture(Epoch epoch, Future<ClusterMetadata> future)
          {
              this.epoch = epoch;
              this.future = future;
          }
      }
  }
  