code: /*
   * Copyright (c) 2016-present, RxJava Contributors.
   *
   * 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 io.reactivex.rxjava3.operators;
  
  import org.reactivestreams.Subscription;
  
  import io.reactivex.rxjava3.annotations.NonNull;
  
  /**
   * An interface extending {@link SimpleQueue} and {@link Subscription} and allows negotiating
   * the fusion mode between subsequent operators of the {@link io.reactivex.rxjava3.core.Flowable Flowable} base reactive type.
   * <p>
   * The negotiation happens in subscription time when the upstream
   * calls the {@code onSubscribe} with an instance of this interface. The
   * downstream has then the obligation to call {@link #requestFusion(int)}
   * with the appropriate mode before calling {@code request()}.
   * <p>
   * In <b>synchronous fusion</b>, all upstream values are either already available or is generated
   * when {@link #poll()} is called synchronously. When the {@link #poll()} returns null,
   * that is the indication if a terminated stream. Downstream should not call {@link #request(long)}
   * in this mode. In this mode, the upstream won't call the onXXX methods.
   * <p>
   * In <b>asynchronous fusion</b>, upstream values may become available to {@link #poll()} eventually.
   * Upstream signals {@code onError()} and {@code onComplete()} as usual, however,
   * {@code onNext} will be called with {@code null} instead of the actual value.
   * Downstream should treat such onNext as indication that {@link #poll()} can be called.
   * In this mode, the downstream still has to call {@link #request(long)}
   * to indicate it is prepared to receive more values.
   * <p>
   * The general rules for consuming the {@link SimpleQueue} interface:
   * <ul>
   * <li> {@link #poll()} and {@link #clear()} has to be called sequentially (from within a serializing drain-loop).</li>
   * <li>In addition, callers of {@link #poll()} should be prepared to catch exceptions.</li>
   * <li>Due to how computation attaches to the {@link #poll()}, {@link #poll()} may return
   * {@code null} even if a preceding {@link #isEmpty()} returned false.</li>
   * </ul>
   * <p>
   * Implementations should only allow calling the following methods and the rest of the
   * {@link SimpleQueue} interface methods should throw {@link UnsupportedOperationException}:
   * <ul>
   * <li>{@link #poll()}</li>
   * <li>{@link #isEmpty()}</li>
   * <li>{@link #clear()}</li>
   * </ul>
   * @param <T> the value type transmitted through the queue
   * @see QueueDisposable
   * @since 3.1.1
   */
  public interface QueueSubscription<@NonNull T> extends QueueFuseable<T>, Subscription {
  }
  