Simplify ThreadPool::isRunning (#1391)

* simplify ThreadPool::isRunning: it doesn't need to be static and to go through the global unique_ptr

* it's undefined behavior to access the threadpool from a shutting down thread, as the parent is being destroyed

src/support/threads.cpp

@@ -47,7 +47,6 @@ Thread::Thread(ThreadPool* parent) : parent(parent) {
 }
 
 Thread::~Thread() {
- assert(!parent->isRunning());
  {
  std::lock_guard<std::mutex> lock(mutex);
  // notify the thread that it can exit
@@ -194,7 +193,7 @@ size_t ThreadPool::size() {
 
 bool ThreadPool::isRunning() {
  DEBUG_POOL("check if running\n");
- return pool && pool->running;
+ return running;
 }
 
 void ThreadPool::notifyThreadIsReady() {

src/support/threads.h

@@ -104,7 +104,7 @@ class ThreadPool {
 
  size_t size();
 
- static bool isRunning();
+ bool isRunning();
 
  // Called by helper threads when they are free and ready.
  void notifyThreadIsReady();