Fixed race condition bug on multithreading

src/openalpr/alpr_impl.cpp

@@ -159,16 +159,13 @@ void plateAnalysisThread(void* arg)
   int loop_count = 0;
   while (true)
   {
-    
-    if (dispatcher->hasPlate() == false)
+    PlateRegion plateRegion;
+    if (dispatcher->nextPlate(&plateRegion) == false)
       break;
     
     if (dispatcher->config->debugGeneral)
       cout << "Thread: " << tthread::this_thread::get_id() << " loop " << ++loop_count << endl;
       
-    // Get a single plate region from the queue
-    PlateRegion plateRegion = dispatcher->nextPlate();
-    
     Mat img = dispatcher->getImageCopy();
     
     timespec platestarttime;

src/openalpr/alpr_impl.h

@@ -109,22 +109,18 @@ class PlateDispatcher
       return img;
     }
 
-    bool hasPlate()
-    {
-      bool plateAvailable;
-      mMutex.lock();
-      plateAvailable = plateRegions.size() > 0;
-      mMutex.unlock();
-      return plateAvailable;
-    }
-    PlateRegion nextPlate()
+    
+    bool nextPlate(PlateRegion* plateRegion)
     {
       tthread::lock_guard<tthread::mutex> guard(mMutex);
       
-      PlateRegion plateRegion = plateRegions[plateRegions.size() - 1];
+      if (plateRegions.size() == 0)
+	return false;
+      
+      *plateRegion = plateRegions[plateRegions.size() - 1];
       plateRegions.pop_back();
       
-      return plateRegion;
+      return true;
     }
     
     void appendPlate(PlateRegion plate)