Timer: Ensure the timer thread is deleted before any static or global destructors run

In cases where there was a static or global instance of a class that
inherited from Timer, due to the order of events when a dll is unloaded
on windows, there was a risk of an infinite hang. Deleting the timer
thread before the dll is unloaded avoids this occurring.

modules/juce_events/timers/juce_Timer.cpp

@@ -35,21 +35,20 @@
 namespace juce
 {
 
-class Timer::TimerThread final : private Thread
+class Timer::TimerThread final : private Thread,
+                                 private DeletedAtShutdown
 {
 public:
-    using LockType = CriticalSection; // (mysteriously, using a SpinLock here causes problems on some XP machines..)
+    using LockType = CriticalSection;
 
-    TimerThread()  : Thread ("JUCE Timer")
-    {
-        timers.reserve (32);
-    }
+    JUCE_DECLARE_SINGLETON (TimerThread, true)
 
     ~TimerThread() override
     {
         signalThreadShouldExit();
         callbackArrived.signal();
         stopThread (-1);
+        clearSingletonInstance();
     }
 
     void run() override
@@ -216,8 +215,8 @@ private:
 
         void messageCallback() override
         {
-            if (auto instance = SharedResourcePointer<TimerThread>::getSharedObjectWithoutCreating())
-                (*instance)->callTimers();
+            if (auto* instance = TimerThread::getInstanceWithoutCreating())
+                instance->callTimers();
         }
     };
 
@@ -285,9 +284,14 @@ private:
         return timers.front().countdownMs;
     }
 
+    //==============================================================================
+    TimerThread()  : Thread ("JUCE Timer") { timers.reserve (32); }
+
     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (TimerThread)
 };
 
+JUCE_IMPLEMENT_SINGLETON (Timer::TimerThread)
+
 //==============================================================================
 Timer::Timer() noexcept {}
 Timer::Timer (const Timer&) noexcept {}
@@ -311,13 +315,16 @@ void Timer::startTimer (int interval) noexcept
     // running, then you're not going to get any timer callbacks!
     JUCE_ASSERT_MESSAGE_MANAGER_EXISTS
 
-    bool wasStopped = (timerPeriodMs == 0);
-    timerPeriodMs = jmax (1, interval);
+    if (auto* instance = TimerThread::getInstance())
+    {
+        bool wasStopped = (timerPeriodMs == 0);
+        timerPeriodMs = jmax (1, interval);
 
-    if (wasStopped)
-        timerThread->addTimer (this);
-    else
-        timerThread->resetTimerCounter (this);
+        if (wasStopped)
+            instance->addTimer (this);
+        else
+            instance->resetTimerCounter (this);
+    }
 }
 
 void Timer::startTimerHz (int timerFrequencyHz) noexcept
@@ -332,15 +339,17 @@ void Timer::stopTimer() noexcept
 {
     if (timerPeriodMs > 0)
     {
-        timerThread->removeTimer (this);
+        if (auto* instance = TimerThread::getInstanceWithoutCreating())
+            instance->removeTimer (this);
+
         timerPeriodMs = 0;
     }
 }
 
 void JUCE_CALLTYPE Timer::callPendingTimersSynchronously()
 {
-    if (auto instance = SharedResourcePointer<TimerThread>::getSharedObjectWithoutCreating())
-        (*instance)->callTimersSynchronously();
+    if (auto* instance = TimerThread::getInstanceWithoutCreating())
+        instance->callTimersSynchronously();
 }
 
 struct LambdaInvoker final : private Timer,

modules/juce_events/timers/juce_Timer.h

@@ -141,7 +141,6 @@ private:
     class TimerThread;
     size_t positionInQueue = (size_t) -1;
     int timerPeriodMs = 0;
-    SharedResourcePointer<TimerThread> timerThread;
 
     Timer& operator= (const Timer&) = delete;
 };