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 com.datastax.oss.driver.internal.core.util.concurrent;
  
  import io.netty.util.concurrent.EventExecutor;
  import io.netty.util.concurrent.Future;
  import java.util.concurrent.Callable;
  import java.util.concurrent.CompletableFuture;
  import java.util.concurrent.CompletionStage;
  import java.util.function.Consumer;
  
  /**
   * Utility to run a task on a Netty event executor (i.e. thread). If we're already on the executor,
   * the task is submitted, otherwise it's scheduled.
   *
   * <p>Be careful when using this, always keep in mind that the task might be executed synchronously.
   * This can lead to subtle bugs when both the calling code and the callback manipulate a collection:
   *
   * <pre>{@code
   * List<CompletionStage<Foo>> futureFoos;
   *
   * // Scheduled on eventExecutor:
   * for (int i = 0; i < count; i++) {
   *   CompletionStage<Foo> futureFoo = FooFactory.init();
   *   futureFoos.add(futureFoo);
   *   // futureFoo happens to be complete by now, so callback gets executed immediately
   *   futureFoo.whenComplete(RunOrSchedule.on(eventExecutor, () -> callback(futureFoo)));
   * }
   *
   * private void callback(CompletionStage<Foo> futureFoo) {
   *    futureFoos.remove(futureFoo); // ConcurrentModificationException!!!
   * }
   * }</pre>
   *
   * For that kind of situation, it's better to use {@code futureFoo.whenCompleteAsync(theTask,
   * eventExecutor)}, so that the task is always scheduled.
   */
  public class RunOrSchedule {
  
    public static void on(EventExecutor executor, Runnable task) {
      if (executor.inEventLoop()) {
        task.run();
      } else {
        executor.submit(task).addListener(UncaughtExceptions::log);
      }
    }
  
    public static <T> Consumer<T> on(EventExecutor executor, Consumer<T> task) {
      return (t) -> {
        if (executor.inEventLoop()) {
          task.accept(t);
        } else {
          executor.submit(() -> task.accept(t)).addListener(UncaughtExceptions::log);
        }
      };
    }
  
    public static <T> CompletionStage<T> on(
        EventExecutor executor, Callable<CompletionStage<T>> task) {
      if (executor.inEventLoop()) {
        try {
          return task.call();
        } catch (Exception e) {
          return CompletableFutures.failedFuture(e);
        }
      } else {
        CompletableFuture<T> result = new CompletableFuture<>();
        executor
            .submit(task)
            .addListener(
                (Future<CompletionStage<T>> f) -> {
                  if (f.isSuccess()) {
                    CompletableFutures.completeFrom(f.getNow(), result);
                  } else {
                    result.completeExceptionally(f.cause());
                  }
                });
        return result;
      }
    }
  }
  