Issue #4692 - Move responsibility for engine reference from tasks to scheduler

src/Scheduler.h

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 48 matching lines @@
   /*
    * Utility function uses type inference to simplify expressions at point of use.
    */
   template<class T>
   HeapFunction<T> MakeHeapFunction(std::shared_ptr<T> body)
   {
     return HeapFunction<T>(body);
   }
 }
 
-/*
+/**
+ * Interface class for scheduled tasks.
+ */
+struct TaskFunctionInterface
+{
+  /** 
+   * The main function of the task,
+   *   the moral equivalent to the main function of the thread.
+   */
+  virtual void operator()() = 0;
+  /**
+   * Request that the task end itself early, without needing to complete.
+   * Reserved for future use.
+   */
+  virtual void Interrupt() {};
+};
+
+/**
  * Execute a task immediately in detached thread that's used only for this task.
  *
  * The present version is nothing more than a rewrite of the legacy behavior,
  *   which was to create a new thread for each task.
  */
 void StartImmediatelyInSingleUseDetachedThread(std::function<void()> task);
 
 /**
  * Active object to run arbitrary function objects.
  *
@@ skipping 190 matching lines @@
   {
     // The call to `Worker::Join()` is in another function,
     //   namely `JoinAndRemove()`, which also removes items from the task list.
     // Thus all we do here is to wait for the task list to empty.
     UniqueLockType ul(m);
     taskRemoveCv.wait(ul, [this]() -> bool { return taskList.empty(); });
   }
 };
 
 #endif