change unpickling to use import rather than retrieving from sys.modules

Author: tjb900

Misc/NEWS.d/next/Library/2018-09-05-03-02-32.bpo-34572.ayisd2.rst

@@ -0,0 +1,3 @@
+Fix C implementation of pickle.loads to use importlib's locking
+mechanisms, and thereby avoid using partially-loaded modules.
+Patch by Tim Burgess.

Modules/_pickle.c

@@ -6623,14 +6623,9 @@ _pickle_Unpickler_find_class_impl(UnpicklerObject *self,
         }
     }
 
-    module = PyImport_GetModule(module_name);
-    if (module == NULL) {
-        if (PyErr_Occurred())
-            return NULL;
-        module = PyImport_Import(module_name);
-        if (module == NULL)
-            return NULL;
-    }
+    module = PyImport_Import(module_name);
+    if (module == NULL)
+        return NULL;
     global = getattribute(module, global_name, self->proto >= 4);
     Py_DECREF(module);
     return global;