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.utils.concurrent;
  
  import java.util.concurrent.Executor;
  import java.util.function.Function;
  
  import javax.annotation.Nullable;
  
  import com.google.common.util.concurrent.AsyncFunction;
  import com.google.common.util.concurrent.ListenableFuture; // checkstyle: permit this import
  
  import accord.utils.async.AsyncResult;
  import io.netty.util.concurrent.GenericFutureListener;
  
  import static org.apache.cassandra.utils.concurrent.Awaitable.SyncAwaitable.waitUntil;
  
  /**
   * Netty's DefaultPromise uses a mutex to coordinate notifiers AND waiters between the eventLoop and the other threads.
   * Since we register cross-thread listeners, this has the potential to block internode messaging for an unknown
   * number of threads for an unknown period of time, if we are unlucky with the scheduler (which will certainly
   * happen, just with some unknown but low periodicity)
   *
   * At the same time, we manage some other efficiencies:
   *  - We save some space when registering listeners, especially if there is only one listener, as we perform no
   *    extra allocations in this case.
   *  - We permit efficient initial state declaration, avoiding unnecessary CAS or lock acquisitions when mutating
   *    a Promise we are ourselves constructing (and can easily add more; only those we use have been added)
   *
   * We can also make some guarantees about our behaviour here, although we primarily mirror Netty.
   * Specifically, we can guarantee that notifiers are always invoked in the order they are added (which may be true
   * for netty, but was unclear and is not declared).  This is useful for ensuring the correctness of some of our
   * behaviours in OutboundConnection without having to jump through extra hoops.
   *
   * The implementation loosely follows that of Netty's DefaultPromise, with some slight changes; notably that we have
   * no synchronisation on our listeners, instead using a CoW list that is cleared each time we notify listeners.
   *
   * We handle special values slightly differently.  We do not use a special value for null, instead using
   * a special value to indicate the result has not been set yet.  This means that once isSuccess() holds,
   * the result must be a correctly typed object (modulo generics pitfalls).
   * All special values are also instances of FailureHolder, which simplifies a number of the logical conditions.
   *
   * @param <V>
   */
  public class SyncFuture<V> extends AbstractFuture<V>
  {
      protected SyncFuture()
      {
          super();
      }
  
      protected SyncFuture(V immediateSuccess)
      {
          super(immediateSuccess);
      }
  
      protected SyncFuture(Throwable immediateFailure)
      {
          super(immediateFailure);
      }
  
      protected SyncFuture(FailureHolder initialState)
      {
          super(initialState);
      }
  
      protected SyncFuture(GenericFutureListener<? extends io.netty.util.concurrent.Future<? super V>> listener)
      {
          super(listener);
      }
  
      protected SyncFuture(FailureHolder initialState, GenericFutureListener<? extends io.netty.util.concurrent.Future<? super V>> listener)
      {
          super(initialState, listener);
      }
  
      /**
       * Support {@link com.google.common.util.concurrent.Futures#transform} natively
       *
       * See {@link #addListener(GenericFutureListener)} for ordering semantics.
       */
      @Override
      public <T> Future<T> map(Function<? super V, ? extends T> mapper, Executor executor)
      {
          return map(new SyncFuture<>(), mapper, executor);
      }
  
      /**
       * Support {@link com.google.common.util.concurrent.Futures#transformAsync(ListenableFuture, AsyncFunction, Executor)} natively
       *
       * See {@link #addListener(GenericFutureListener)} for ordering semantics.
       */
      @Override
      public <T> Future<T> flatMap(Function<? super V, ? extends AsyncResult<T>> flatMapper, @Nullable Executor executor)
      {
          return flatMap(new SyncFuture<>(), flatMapper, executor);
      }
  
      /**
       * Support {@link com.google.common.util.concurrent.Futures#transformAsync(ListenableFuture, AsyncFunction, Executor)} natively
       *
       * See {@link #addListener(GenericFutureListener)} for ordering semantics.
       */
      @Override
      public <T> Future<T> andThenAsync(Function<? super V, ? extends Future<T>> andThen, @Nullable Executor executor)
      {
          return andThenAsync(new SyncFuture<>(), andThen, executor);
      }
  
      /**
       * Shared implementation of various promise completion methods.
       * Updates the result if it is possible to do so, returning success/failure.
       *
       * If the promise is UNSET the new value will succeed;
       *          if it is UNCANCELLABLE it will succeed only if the new value is not CANCELLED
       *          otherwise it will fail, as isDone() is implied
       *
       * If the update succeeds, and the new state implies isDone(), any listeners and waiters will be notified
       */
      synchronized boolean trySet(Object v)
      {
          Object current = result;
          if (isDone(current) || (current == UNCANCELLABLE && (v == CANCELLED || v == UNCANCELLABLE)))
              return false;
  
          resultUpdater.lazySet(this, v);
          if (v != UNCANCELLABLE)
          {
              notifyListeners();
              notifyAll();
          }
          return true;
      }
  
      public synchronized boolean awaitUntil(long deadline) throws InterruptedException
      {
          if (isDone())
              return true;
  
          waitUntil(this, deadline);
          return isDone();
      }
  
      public synchronized Future<V> await() throws InterruptedException
      {
          while (!isDone())
              wait();
          return this;
      }
  
      /**
       * Logically append {@code newListener} to {@link #listeners}
       * (at this stage it is a stack, so we actually prepend)
       */
      synchronized void appendListener(ListenerList<V> newListener)
      {
          ListenerList.pushExclusive(listenersUpdater, this, newListener);
          if (isDone())
              notifyListeners();
      }
  
      private void notifyListeners()
      {
          ListenerList.notifyExclusive(listeners, this);
          listenersUpdater.lazySet(this, null);
      }
  }
  