Implement safe-guard to prevent loading Cython modules into multiple subinterpreters.

Cython/Utility/ModuleSetupCode.c

@@ -906,6 +906,31 @@ PyEval_InitThreads();
 //@substitute: naming
 
 //#if CYTHON_PEP489_MULTI_PHASE_INIT
+static CYTHON_SMALL_CODE int __Pyx_check_single_interpreter(void) {
+    #if PY_VERSION_HEX >= 0x030700A1
+    static PY_INT64_T main_interpreter_id = -1;
+    PY_INT64_T current_id = PyInterpreterState_GetID(PyThreadState_Get()->interp);
+    if (main_interpreter_id == -1) {
+        main_interpreter_id = current_id;
+        return (unlikely(current_id == -1)) ? -1 : 0;
+    } else if (unlikely(main_interpreter_id != current_id))
+
+    #else
+    static PyInterpreterState *main_interpreter = NULL;
+    PyInterpreterState *current_interpreter = PyThreadState_Get()->interp;
+    if (!main_interpreter) {
+        main_interpreter = current_interpreter;
+    } else if (unlikely(main_interpreter != current_interpreter))
+    #endif
+
+    {
+        PyErr_SetString(
+            PyExc_RuntimeError, "Interpreter change detected - this module does not support subinterpreters.");
+        return -1;
+    }
+    return 0;
+}
+
 static CYTHON_SMALL_CODE int __Pyx_copy_spec_to_module(PyObject *spec, PyObject *moddict, const char* from_name, const char* to_name) {
     PyObject *value = PyObject_GetAttrString(spec, from_name);
     int result = 0;
@@ -924,6 +949,8 @@ static CYTHON_SMALL_CODE PyObject* ${pymodule_create_func_cname}(PyObject *spec,
     PyObject *module = NULL, *moddict, *modname;
 
     // For now, we only have exactly one module instance.
+    if (__Pyx_check_single_interpreter())
+        return NULL;
     if (${module_cname})
         return __Pyx_NewRef(${module_cname});