Initialize Python function-local statics (singletons)

Singletons are created during Python module initialization in order to avoid
deadlocks with subsequent multi-threaded usage.
Fixes #1275 #1279

CHANGES.current

@@ -7,6 +7,11 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.0.0 (in progress)
 ===========================
 
+2018-07-17: petrmitrichev,wsfulton
+            [Python] #1275 #1279 Initialize function-local statics (singletons) that call Python
+            code during Python module initialization in order to avoid deadlocks with subsequent
+            multi-threaded usage.
+
 2018-06-15: wsfulton
             [Python] Fix seg fault using Python 2 when passing a Python string, containing
             invalid utf-8 content, to a wstring or wchar * parameter. A TypeError is thrown instead, eg:

Lib/python/pyinit.swg

@@ -379,6 +379,17 @@ SWIG_init(void) {
   assert(metatype);
 #endif
 
+  /* Create singletons now to avoid potential deadlocks with multi-threaded usage after module initialization */
+#ifdef SWIG_PYTHON_BUILD_NONE
+  SWIG_Py_None();
+#endif
+  SWIG_This();
+  SWIG_Python_TypeCache();
+  SwigPyPacked_type();
+#ifndef SWIGPYTHON_BUILTIN
+  SwigPyObject_type();
+#endif
+
   /* Fix SwigMethods to carry the callback ptrs when needed */
   SWIG_Python_FixMethods(SwigMethods, swig_const_table, swig_types, swig_type_initial);