/*
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * 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.
 */

#pragma once

#include <folly/io/async/DelayedDestructionBase.h>

namespace folly {

/**
 * DelayedDestruction is a helper class to ensure objects are not deleted
 * while they still have functions executing in a higher stack frame.
 *
 * This is useful for objects that invoke callback functions, to ensure that a
 * callback does not destroy the calling object.
 *
 * Classes needing this functionality should:
 * - derive from DelayedDestruction
 * - make their destructor private or protected, so it cannot be called
 *   directly
 * - create a DestructorGuard object on the stack in each public method that
 *   may invoke a callback
 *
 * DelayedDestruction does not perform any locking.  It is intended to be used
 * only from a single thread.
 */
class DelayedDestruction : public DelayedDestructionBase {
 public:
  /**
   * destroy() requests destruction of the object.
   *
   * This method will destroy the object after it has no more functions running
   * higher up on the stack.  (i.e., No more DestructorGuard objects exist for
   * this object.)  This method must be used instead of the destructor.
   */
  virtual void destroy() {
    // If guardCount_ is not 0, just set destroyPending_ to delay
    // actual destruction.
    if (getDestructorGuardCount() != 0) {
      destroyPending_ = true;
    } else {
      onDelayedDestroy(false);
    }
  }

  /**
   * Helper class to allow DelayedDestruction classes to be used with
   * std::shared_ptr.
   *
   * This class can be specified as the destructor argument when creating the
   * shared_ptr, and it will destroy the guarded class properly when all
   * shared_ptr references are released.
   */
  class Destructor {
   public:
    void operator()(DelayedDestruction* dd) const { dd->destroy(); }
  };

  bool getDestroyPending() const { return destroyPending_; }

 protected:
  /**
   * Protected destructor.
   *
   * Making this protected ensures that users cannot delete DelayedDestruction
   * objects directly, and that everyone must use destroy() instead.
   * Subclasses of DelayedDestruction must also define their destructors as
   * protected or private in order for this to work.
   *
   * This also means that DelayedDestruction objects cannot be created
   * directly on the stack; they must always be dynamically allocated on the
   * heap.
   *
   * In order to use a DelayedDestruction object with a shared_ptr, create the
   * shared_ptr using a DelayedDestruction::Destructor as the second argument
   * to the shared_ptr constructor.
   */
  ~DelayedDestruction() override = default;

  DelayedDestruction() : destroyPending_(false) {}

 private:
  /**
   * destroyPending_ is set to true if destoy() is called while guardCount_ is
   * non-zero. It is set to false before the object is deleted.
   *
   * If destroyPending_ is true, the object will be destroyed the next time
   * guardCount_ drops to 0.
   */
  bool destroyPending_;

  void onDelayedDestroy(bool delayed) override {
    // check if it is ok to destroy now
    if (delayed && !destroyPending_) {
      return;
    }
    destroyPending_ = false;
    delete this;
  }
};
} // namespace folly
