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.schedulers;
  
  import java.util.concurrent.*;
  import java.util.concurrent.atomic.AtomicReference;
  
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.internal.functions.Functions;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  /**
   * Wrapper for a regular task that gets immediately rescheduled when the task completed.
   */
  final class InstantPeriodicTask implements Callable<Void>, Disposable {
  
      final Runnable task;
  
      final AtomicReference<Future<?>> rest;
  
      final AtomicReference<Future<?>> first;
  
      final ExecutorService executor;
  
      Thread runner;
  
      static final FutureTask<Void> CANCELLED = new FutureTask<>(Functions.EMPTY_RUNNABLE, null);
  
      InstantPeriodicTask(Runnable task, ExecutorService executor) {
          super();
          this.task = task;
          this.first = new AtomicReference<>();
          this.rest = new AtomicReference<>();
          this.executor = executor;
      }
  
      @Override
      public Void call() {
          runner = Thread.currentThread();
          try {
              task.run();
              runner = null;
              setRest(executor.submit(this));
          } catch (Throwable ex) {
              // Exceptions.throwIfFatal(ex); nowhere to go
              runner = null;
              RxJavaPlugins.onError(ex);
              throw ex;
          }
          return null;
      }
  
      @Override
      public void dispose() {
          Future<?> current = first.getAndSet(CANCELLED);
          if (current != null && current != CANCELLED) {
              current.cancel(runner != Thread.currentThread());
          }
          current = rest.getAndSet(CANCELLED);
          if (current != null && current != CANCELLED) {
              current.cancel(runner != Thread.currentThread());
          }
      }
  
      @Override
      public boolean isDisposed() {
          return first.get() == CANCELLED;
      }
  
      void setFirst(Future<?> f) {
          for (;;) {
              Future<?> current = first.get();
              if (current == CANCELLED) {
                  f.cancel(runner != Thread.currentThread());
                  return;
              }
              if (first.compareAndSet(current, f)) {
                  return;
              }
          }
      }
  
      void setRest(Future<?> f) {
          for (;;) {
              Future<?> current = rest.get();
              if (current == CANCELLED) {
                  f.cancel(runner != Thread.currentThread());
                  return;
              }
              if (rest.compareAndSet(current, f)) {
                  return;
              }
          }
      }
  }
  