bpo-44050: Extension modules can share state when they don't support sub-interpreters. (GH-27794) (GH-28738)

Automerge-Triggered-By: GH:encukou
(cherry picked from commit b9bb748)

Co-authored-by: Hai Shi <shihai1992@gmail.com>

Author: miss-islington

Lib/test/test_capi.py

@@ -762,6 +762,37 @@ def test_mutate_exception(self):
 
         self.assertFalse(hasattr(binascii.Error, "foobar"))
 
+    def test_module_state_shared_in_global(self):
+        """
+        bpo-44050: Extension module state should be shared between interpreters
+        when it doesn't support sub-interpreters.
+        """
+        r, w = os.pipe()
+        self.addCleanup(os.close, r)
+        self.addCleanup(os.close, w)
+
+        script = textwrap.dedent(f"""
+            import importlib.machinery
+            import importlib.util
+            import os
+
+            fullname = '_test_module_state_shared'
+            origin = importlib.util.find_spec('_testmultiphase').origin
+            loader = importlib.machinery.ExtensionFileLoader(fullname, origin)
+            spec = importlib.util.spec_from_loader(fullname, loader)
+            module = importlib.util.module_from_spec(spec)
+            attr_id = str(id(module.Error)).encode()
+
+            os.write({w}, attr_id)
+            """)
+        exec(script)
+        main_attr_id = os.read(r, 100)
+
+        ret = support.run_in_subinterp(script)
+        self.assertEqual(ret, 0)
+        subinterp_attr_id = os.read(r, 100)
+        self.assertEqual(main_attr_id, subinterp_attr_id)
+
 
 class TestThreadState(unittest.TestCase):
 

Misc/NEWS.d/next/Core and Builtins/2021-09-08-00-30-09.bpo-44050.mFI15u.rst

@@ -0,0 +1,3 @@
+Extensions that indicate they use global state (by setting ``m_size`` to -1)
+can again be used in multiple interpreters. This reverts to behavior of
+Python 3.8.

Modules/_testmultiphase.c

@@ -844,6 +844,28 @@ PyInit__testmultiphase_meth_state_access(PyObject *spec)
     return PyModuleDef_Init(&def_meth_state_access);
 }
 
+static PyModuleDef def_module_state_shared = {
+    PyModuleDef_HEAD_INIT,
+    .m_name = "_test_module_state_shared",
+    .m_doc = PyDoc_STR("Regression Test module for single-phase init."),
+    .m_size = -1,
+};
+
+PyMODINIT_FUNC
+PyInit__test_module_state_shared(PyObject *spec)
+{
+    PyObject *module = PyModule_Create(&def_module_state_shared);
+    if (module == NULL) {
+        return NULL;
+    }
+
+    if (PyModule_AddObjectRef(module, "Error", PyExc_Exception) < 0) {
+        Py_DECREF(module);
+        return NULL;
+    }
+    return module;
+}
+
 
 /*** Helper for imp test ***/
 

Python/import.c

@@ -441,7 +441,9 @@ _PyImport_FixupExtensionObject(PyObject *mod, PyObject *name,
         return -1;
     }
 
-    if (_Py_IsMainInterpreter(tstate->interp)) {
+    // bpo-44050: Extensions and def->m_base.m_copy can be updated
+    // when the extension module doesn't support sub-interpreters.
+    if (_Py_IsMainInterpreter(tstate->interp) || def->m_size == -1) {
         if (def->m_size == -1) {
             if (def->m_base.m_copy) {
                 /* Somebody already imported the module,