add Timer::close()

An aid to orderly shutdown

src/misc/pv/timer.h

@@ -75,6 +75,8 @@ public:
      */
     Timer(std::string threadName, ThreadPriority priority);
     virtual ~Timer();
+    //! Prevent new callbacks from being scheduled, and cancel pending callbacks
+    void close();
     /**
      * schedule a callback after a delay.
      * @param timerCallback the timerCallback instance.

src/misc/timer.cpp

@@ -160,8 +160,14 @@ void Timer::run()
 }
 
 Timer::~Timer() {
+    close();
+}
+
+void Timer::close() {
     {
          Lock xx(mutex);
+         if(!alive)
+             return; // already closed
          alive = false;
     }
     waitForWork.signal();