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.internal.subscribers;
  
  import java.util.concurrent.atomic.*;
  
  import org.reactivestreams.*;
  
  import io.reactivex.rxjava3.core.FlowableSubscriber;
  import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
  import io.reactivex.rxjava3.internal.util.*;
  
  /**
   * Ensures that the event flow between the upstream and downstream follow
   * the Reactive-Streams 1.0 specification by honoring the 3 additional rules
   * (which are omitted in standard operators due to performance reasons).
   * <ul>
   * <li>§1.3: onNext should not be called concurrently until onSubscribe returns</li>
   * <li>§2.3: onError or onComplete must not call cancel</li>
   * <li>§3.9: negative requests should emit an onError(IllegalArgumentException)</li>
   * </ul>
   * In addition, if rule §2.12 (onSubscribe must be called at most once) is violated,
   * the sequence is cancelled an onError(IllegalStateException) is emitted.
   * @param <T> the value type
   * @since 2.0.7
   */
  public class StrictSubscriber<T>
  extends AtomicInteger
  implements FlowableSubscriber<T>, Subscription {
  
      private static final long serialVersionUID = -4945028590049415624L;
  
      final Subscriber<? super T> downstream;
  
      final AtomicThrowable error;
  
      final AtomicLong requested;
  
      final AtomicReference<Subscription> upstream;
  
      final AtomicBoolean once;
  
      volatile boolean done;
  
      public StrictSubscriber(Subscriber<? super T> downstream) {
          this.downstream = downstream;
          this.error = new AtomicThrowable();
          this.requested = new AtomicLong();
          this.upstream = new AtomicReference<>();
          this.once = new AtomicBoolean();
      }
  
      @Override
      public void request(long n) {
          if (n <= 0) {
              cancel();
              onError(new IllegalArgumentException("§3.9 violated: positive request amount required but it was " + n));
          } else {
              SubscriptionHelper.deferredRequest(upstream, requested, n);
          }
      }
  
      @Override
      public void cancel() {
          if (!done) {
              SubscriptionHelper.cancel(upstream);
          }
      }
  
      @Override
      public void onSubscribe(Subscription s) {
          if (once.compareAndSet(false, true)) {
  
              downstream.onSubscribe(this);
  
              SubscriptionHelper.deferredSetOnce(this.upstream, requested, s);
          } else {
              s.cancel();
              cancel();
              onError(new IllegalStateException("§2.12 violated: onSubscribe must be called at most once"));
          }
      }
  
      @Override
      public void onNext(T t) {
          HalfSerializer.onNext(downstream, t, this, error);
      }
  
      @Override
      public void onError(Throwable t) {
          done = true;
          HalfSerializer.onError(downstream, t, this, error);
      }
  
      @Override
      public void onComplete() {
          done = true;
          HalfSerializer.onComplete(downstream, this, error);
      }
  }
  