Fixed a deadlock when tasks are scheduled while the worker threads ha…

…ven't reached m_signal_work.wait() yet

Fixes #44

core/parallel.cpp

@@ -11,21 +11,24 @@ class Parallel
 {
 public:
     Parallel(uint32_t num_workers) {
+        std::unique_lock<std::mutex> ul(m_mutex);
+
         // create worker threads
         for (uint32_t worker_id = 0; worker_id < num_workers; worker_id++) {
             m_workers.emplace_back(std::thread(&Parallel::do_work, this, worker_id));
         }
+
+        // give workers an empty task and wait for them to finish it to
+        // make sure they're ready
+        m_task = [](uint32_t){};
+        m_workers_active = m_workers.size();
+        m_signal_done.wait(ul);
     }
 
     ~Parallel() {
         // wait for all workers to finish their current work
         wait();
 
-        // set a dummy task in case nothing has been done yet
-        if (!m_task) {
-            m_task = [](uint32_t) {};
-        }
-
         // exit worker main loops
         m_accept_work = false;
         m_signal_work.notify_all();
@@ -66,12 +69,14 @@ class Parallel
     void do_work(int32_t worker_id) {
         std::unique_lock<std::mutex> ul(m_mutex);
         while (m_accept_work) {
-            m_signal_work.wait(ul);
             ul.unlock();
-            m_task(worker_id);
+            if (m_accept_work) {
+                m_task(worker_id);
+            }
             ul.lock();
             m_workers_active--;
             m_signal_done.notify_all();
+            m_signal_work.wait(ul);
         }
     }