Enforce single module initialisation despite PEP 489 allowing to re-execute a module.

Cython/Compiler/ModuleNode.py

@@ -694,7 +694,7 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
         code.put(Nodes.branch_prediction_macros)
         code.putln('static CYTHON_INLINE void __Pyx_pretend_to_initialize(void* ptr) { (void)ptr; }')
         code.putln('')
-        code.putln('static PyObject *%s;' % env.module_cname)
+        code.putln('static PyObject *%s = NULL;' % env.module_cname)
         code.putln('static PyObject *%s;' % env.module_dict_cname)
         code.putln('static PyObject *%s;' % Naming.builtins_cname)
         code.putln('static PyObject *%s;' % Naming.cython_runtime_cname)
@@ -2274,6 +2274,15 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
             code.globalstate.use_utility_code(UtilityCode.load_cached("Profile", "Profile.c"))
 
         code.put_declare_refcount_context()
+        code.putln("#if CYTHON_PEP489_MULTI_PHASE_INIT")
+        # Hack: enforce single initialisation.
+        code.putln("if (%s && %s == %s) return 0;" % (
+            Naming.module_cname,
+            Naming.module_cname,
+            Naming.pymodinit_module_arg,
+        ))
+        code.putln("#endif")
+
         if profile or linetrace:
             tempdecl_code.put_trace_declarations()
             code.put_trace_frame_init()

Cython/Utility/ModuleSetupCode.c

@@ -753,6 +753,10 @@ static int __Pyx_copy_spec_to_module(PyObject *spec, PyObject *moddict, const ch
 static PyObject* ${pymodule_create_func_cname}(PyObject *spec, CYTHON_UNUSED PyModuleDef *def) {
     PyObject *module = NULL, *moddict, *modname;
 
+    // For now, we only have exactly one module instance.
+    if (${module_cname})
+        return __Pyx_NewRef({module_cname});
+
     modname = PyObject_GetAttrString(spec, "name");
     if (unlikely(!modname)) goto bad;
 

runtests.py

@@ -350,8 +350,6 @@ VER_DEP_MODULES = {
                                          'run.mod__spec__',
                                          'run.pep526_variable_annotations',  # typing module
                                          ]),
-    (99,): (operator.lt, lambda x: x in ['run.mod__spec__',  # actually Py3.5, but currently disabled
-                                         ]),
 }
 
 INCLUDE_DIRS = [ d for d in os.getenv('INCLUDE', '').split(os.pathsep) if d ]

tests/bugs.txt

@@ -12,6 +12,9 @@ temp_sideeffects_T654    # not really a bug, Cython warns about it
 inherited_final_method
 cimport_alias_subclass
 
+# PEP-489 is currently disabled
+run.mod__spec__
+
 # CPython regression tests that don't current work:
 pyregr.test_signal
 pyregr.test_capi