handle duplicate PyType_Ready call

src/capi/typeobject.cpp

@@ -3404,6 +3404,12 @@ extern "C" void PyType_GiveHcAttrsDictDescr(PyTypeObject* cls) noexcept {
 extern "C" int PyType_Ready(PyTypeObject* cls) noexcept {
     ASSERT(!cls->is_pyston_class, "should not call this on Pyston classes");
 
+    // if this type is already in ready state we are finished.
+    if (cls->tp_flags & Py_TPFLAGS_READY) {
+        assert(cls->tp_dict != NULL);
+        return 0;
+    }
+
     gc::registerNonheapRootObject(cls, sizeof(PyTypeObject));
 
     // unhandled fields:

test/tests/import_test.py

@@ -29,3 +29,9 @@ print y
 print __name__
 
 print __import__("import_target") is import_target
+
+import sys
+import _multiprocessing
+del _multiprocessing
+del sys.modules["_multiprocessing"]
+import _multiprocessing