Guard all uses of basic_random_generator for thread safety

src/Mod/Part/App/Geometry.cpp

@@ -113,6 +113,8 @@
 #include <BRep_Tool.hxx>
 #include <TopoDS.hxx>
 #include <memory>
+#include <boost/thread/mutex.hpp>
+#include <boost/thread/thread.hpp>
 
 #include "Geometry.h"
 #include "ArcOfCirclePy.h"
@@ -455,8 +457,13 @@ void Geometry::deleteExtension(const std::string & name)
 void Geometry::createNewTag()
 {
     // Initialize a random number generator, to avoid Valgrind false positives.
+    // The random number generator is not threadsafe so we guard it.  See
+    // https://www.boost.org/doc/libs/1_62_0/libs/uuid/uuid.html#Design%20notes
     static boost::mt19937 ran;
     static bool seeded = false;
+    static boost::mutex random_number_mutex;
+
+    boost::lock_guard<boost::mutex> guard(random_number_mutex);
 
     if (!seeded) {
         ran.seed(static_cast<unsigned int>(std::time(nullptr)));