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 #ifndef FWCore_Concurrency_WaitingTaskList_h
 #define FWCore_Concurrency_WaitingTaskList_h
 // -*- C++ -*-
 //
 // Package:     Concurrency
 // Class  :     WaitingTaskList
 //
 /**\class WaitingTaskList WaitingTaskList.h FWCore/Concurrency/interface/WaitingTaskList.h
 
  Description: Handles starting tasks once some resource becomes available.
 
  Usage:
     This class can be used to have tasks wait to be spawned until a resource is available.
  Tasks that want to use the resource are added to the list by calling add(oneapi::tbb::task*).
  When the resource becomes available one calls doneWaiting() and then any waiting tasks will
  be spawned. If a call to add() is made after doneWaiting() the newly added task will
  immediately be spawned.
  The class can be reused by calling reset(). However, reset() is not thread-safe so one 
  must be certain neither add(...) nor doneWaiting() is called while reset() is running.
  
  An example usage would be if you had a task doing a long calculation (the resource) and
  then several other tasks have been created in a different thread and before running those
  new tasks you need the result of the long calculation.
  \code
  class CalcTask : public edm::WaitingTask {
     public:
     CalcTask(edm::WaitingTaskList* iWL, Value* v):
     m_waitList(iWL), m_output(v) {}
  
     oneapi::tbb::task* execute() {
      std::exception_ptr ptr;
      try {
        *m_output = doCalculation();
      } catch(...) {
        ptr = std::current_exception();
      }
      m_waitList.doneWaiting(ptr);
      return nullptr;
     }
     private:
      edm::WaitingTaskList* m_waitList;
      Value* m_output;
  };
  \endcode
  
  In one part of the code we can setup the shared resource
  \code
  WaitingTaskList waitList;
  Value v;
  \endcode
 
  In another part we can start the calculation
  \code
  oneapi::tbb::task* calc = new(oneapi::tbb::task::allocate_root()) CalcTask(&waitList,&v);
  oneapi::tbb::task::spawn(calc);
  \endcode
  
  Finally in some unrelated part of the code we can create tasks that need the calculation
  \code
  oneapi::tbb::task* t1 = makeTask1(v);
  waitList.add(t1);
  oneapi::tbb::task* t2 = makeTask2(v);
  waitList.add(t2);
  \endcode
 
 */
 //
 // Original Author:  Chris Jones
 //         Created:  Thu Feb 21 13:46:31 CST 2013
 // $Id$
 //
 
 // system include files
 #include <atomic>
 
 // user include files
 #include "FWCore/Concurrency/interface/WaitingTask.h"
 #include "FWCore/Concurrency/interface/WaitingTaskHolder.h"
 #include "FWCore/Utilities/interface/thread_safety_macros.h"
 
 // forward declarations
 
 namespace edm {
   class WaitingTaskList {
   public:
     ///Constructor
     /**The WaitingTaskList is initial set to waiting.
        * \param[in] iInitialSize specifies the initial size of the cache used to hold waiting tasks.
        * The value is only useful for optimization as the object can resize itself.
        */
     explicit WaitingTaskList(unsigned int iInitialSize = 2);
     WaitingTaskList(const WaitingTaskList&) = delete;                   // stop default
     const WaitingTaskList& operator=(const WaitingTaskList&) = delete;  // stop default
     ~WaitingTaskList() = default;
 
     // ---------- member functions ---------------------------
 
     /** Use in the case where you need to inform the parent task of a
        failure before some other child task which may be run later reports
        a different, but related failure. You must later call doneWaiting
        with same exception later in the same thread.
        */
     void presetTaskAsFailed(std::exception_ptr iExcept);
 
     ///Adds task to the waiting list
     /**If doneWaiting() has already been called then the added task will immediately be run.
        * If that is not the case then the task will be held until doneWaiting() is called and will
        * then be run.
        * Calls to add() and doneWaiting() can safely be done concurrently.
        */
     void add(oneapi::tbb::task_group*, WaitingTask*);
 
     ///Adds task to the waiting list
     /**Calls to add() and doneWaiting() can safely be done concurrently.
       */
     void add(WaitingTaskHolder);
 
     ///Signals that the resource is now available and tasks should be spawned
     /**The owner of the resource calls this function to allow the waiting tasks to
        * start accessing it.
        * If the task fails, a non 'null' std::exception_ptr should be used.
        * To have tasks wait again one must call reset().
        * Calls to add() and doneWaiting() can safely be done concurrently.
        */
     void doneWaiting(std::exception_ptr iPtr);
 
     ///Resets access to the resource so that added tasks will wait.
     /**The owner of the resouce calls reset() to make tasks wait.
        * Calling reset() is NOT thread safe. The system must guarantee that no tasks are
        * using the resource when reset() is called and neither add() nor doneWaiting() can
        * be called concurrently with reset().
        */
     void reset();
 
   private:
     /**Handles running the tasks,
        * safe to call from multiple threads
        */
     void announce();
 
     struct WaitNode {
       WaitingTask* m_task;
       oneapi::tbb::task_group* m_group;
       std::atomic<WaitNode*> m_next;
       bool m_fromCache;
 
       void setNextNode(WaitNode* iNext) { m_next = iNext; }
 
       WaitNode* nextNode() const { return m_next; }
     };
 
     WaitNode* createNode(oneapi::tbb::task_group* iGroup, WaitingTask* iTask);
 
     // ---------- member data --------------------------------
     std::atomic<WaitNode*> m_head;
     std::unique_ptr<WaitNode[]> m_nodeCache;
     CMS_THREAD_GUARD(m_waiting) std::exception_ptr m_exceptionPtr;
     unsigned int m_nodeCacheSize;
     std::atomic<unsigned int> m_lastAssignedCacheIndex;
     std::atomic<bool> m_waiting;
   };
 }  // namespace edm
 
 #endif

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