gh-98627: Add an Optional Check for Extension Module Subinterpreter Compatibility (gh-99040)

Enforcing (optionally) the restriction set by PEP 489 makes sense. Furthermore, this sets the stage for a potential restriction related to a per-interpreter GIL.

This change includes the following:

* add tests for extension module subinterpreter compatibility
* add _PyInterpreterConfig.check_multi_interp_extensions
* add Py_RTFLAGS_MULTI_INTERP_EXTENSIONS
* add _PyImport_CheckSubinterpIncompatibleExtensionAllowed()
* fail iff the module does not implement multi-phase init and the current interpreter is configured to check

https://github.com/python/cpython/issues/98627

Include/cpython/initconfig.h

@@ -248,6 +248,7 @@ typedef struct {
     int allow_exec;
     int allow_threads;
     int allow_daemon_threads;
+    int check_multi_interp_extensions;
 } _PyInterpreterConfig;
 
 #define _PyInterpreterConfig_INIT \
@@ -256,6 +257,7 @@ typedef struct {
         .allow_exec = 0, \
         .allow_threads = 1, \
         .allow_daemon_threads = 0, \
+        .check_multi_interp_extensions = 1, \
     }
 
 #define _PyInterpreterConfig_LEGACY_INIT \
@@ -264,6 +266,7 @@ typedef struct {
         .allow_exec = 1, \
         .allow_threads = 1, \
         .allow_daemon_threads = 1, \
+        .check_multi_interp_extensions = 0, \
     }
 
 /* --- Helper functions --------------------------------------- */

Include/cpython/pystate.h

@@ -11,6 +11,9 @@ is available in a given context.  For example, forking the process
 might not be allowed in the current interpreter (i.e. os.fork() would fail).
 */
 
+/* Set if import should check a module for subinterpreter support. */
+#define Py_RTFLAGS_MULTI_INTERP_EXTENSIONS (1UL << 8)
+
 /* Set if threads are allowed. */
 #define Py_RTFLAGS_THREADS (1UL << 10)
 

Include/internal/pycore_import.h

@@ -64,6 +64,7 @@ struct _import_state {
     /* override for config->use_frozen_modules (for tests)
        (-1: "off", 1: "on", 0: no override) */
     int override_frozen_modules;
+    int override_multi_interp_extensions_check;
 #ifdef HAVE_DLOPEN
     int dlopenflags;
 #endif
@@ -153,6 +154,10 @@ PyAPI_DATA(const struct _frozen *) _PyImport_FrozenStdlib;
 PyAPI_DATA(const struct _frozen *) _PyImport_FrozenTest;
 extern const struct _module_alias * _PyImport_FrozenAliases;
 
+PyAPI_FUNC(int) _PyImport_CheckSubinterpIncompatibleExtensionAllowed(
+    const char *name);
+
+
 // for testing
 PyAPI_FUNC(int) _PyImport_ClearExtension(PyObject *name, PyObject *filename);
 

Lib/test/support/import_helper.py

@@ -105,6 +105,24 @@ def frozen_modules(enabled=True):
         _imp._override_frozen_modules_for_tests(0)
 
 
+@contextlib.contextmanager
+def multi_interp_extensions_check(enabled=True):
+    """Force legacy modules to be allowed in subinterpreters (or not).
+
+    ("legacy" == single-phase init)
+
+    This only applies to modules that haven't been imported yet.
+    It overrides the PyInterpreterConfig.check_multi_interp_extensions
+    setting (see support.run_in_subinterp_with_config() and
+    _xxsubinterpreters.create()).
+    """
+    old = _imp._override_multi_interp_extensions_check(1 if enabled else -1)
+    try:
+        yield
+    finally:
+        _imp._override_multi_interp_extensions_check(old)
+
+
 def import_fresh_module(name, fresh=(), blocked=(), *,
                         deprecated=False,
                         usefrozen=False,

Lib/test/test_capi/check_config.py

@@ -0,0 +1,77 @@
+# This script is used by test_misc.
+
+import _imp
+import _testinternalcapi
+import json
+import os
+import sys
+
+
+def import_singlephase():
+    assert '_testsinglephase' not in sys.modules
+    try:
+        import _testsinglephase
+    except ImportError:
+        sys.modules.pop('_testsinglephase')
+        return False
+    else:
+        del sys.modules['_testsinglephase']
+        return True
+
+
+def check_singlephase(override):
+    # Check using the default setting.
+    settings_initial = _testinternalcapi.get_interp_settings()
+    allowed_initial = import_singlephase()
+    assert(_testinternalcapi.get_interp_settings() == settings_initial)
+
+    # Apply the override and check.
+    override_initial = _imp._override_multi_interp_extensions_check(override)
+    settings_after = _testinternalcapi.get_interp_settings()
+    allowed_after = import_singlephase()
+
+    # Apply the override again and check.
+    noop = {}
+    override_after = _imp._override_multi_interp_extensions_check(override)
+    settings_noop = _testinternalcapi.get_interp_settings()
+    if settings_noop != settings_after:
+        noop['settings_noop'] = settings_noop
+    allowed_noop = import_singlephase()
+    if allowed_noop != allowed_after:
+        noop['allowed_noop'] = allowed_noop
+
+    # Restore the original setting and check.
+    override_noop = _imp._override_multi_interp_extensions_check(override_initial)
+    if override_noop != override_after:
+        noop['override_noop'] = override_noop
+    settings_restored = _testinternalcapi.get_interp_settings()
+    allowed_restored = import_singlephase()
+
+    # Restore the original setting again.
+    override_restored = _imp._override_multi_interp_extensions_check(override_initial)
+    assert(_testinternalcapi.get_interp_settings() == settings_restored)
+
+    return dict({
+        'requested': override,
+        'override__initial': override_initial,
+        'override_after': override_after,
+        'override_restored': override_restored,
+        'settings__initial': settings_initial,
+        'settings_after': settings_after,
+        'settings_restored': settings_restored,
+        'allowed__initial': allowed_initial,
+        'allowed_after': allowed_after,
+        'allowed_restored': allowed_restored,
+    }, **noop)
+
+
+def run_singlephase_check(override, outfd):
+    with os.fdopen(outfd, 'w') as outfile:
+        sys.stdout = outfile
+        sys.stderr = outfile
+        try:
+            results = check_singlephase(override)
+            json.dump(results, outfile)
+        finally:
+            sys.stdout = sys.__stdout__
+            sys.stderr = sys.__stderr__

Lib/test/test_capi/test_misc.py

@@ -31,6 +31,10 @@ try:
     import _testmultiphase
 except ImportError:
     _testmultiphase = None
+try:
+    import _testsinglephase
+except ImportError:
+    _testsinglephase = None
 
 # Skip this test if the _testcapi module isn't available.
 _testcapi = import_helper.import_module('_testcapi')
@@ -1297,17 +1301,20 @@ class SubinterpreterTest(unittest.TestCase):
         """
         import json
 
+        EXTENSIONS = 1<<8
         THREADS = 1<<10
         DAEMON_THREADS = 1<<11
         FORK = 1<<15
         EXEC = 1<<16
 
-        features = ['fork', 'exec', 'threads', 'daemon_threads']
+        features = ['fork', 'exec', 'threads', 'daemon_threads', 'extensions']
         kwlist = [f'allow_{n}' for n in features]
+        kwlist[-1] = 'check_multi_interp_extensions'
         for config, expected in {
-            (True, True, True, True): FORK | EXEC | THREADS | DAEMON_THREADS,
+            (True, True, True, True, True):
-            (False, False, False, False): 0,
+                FORK | EXEC | THREADS | DAEMON_THREADS | EXTENSIONS,
-            (False, False, True, False): THREADS,
+            (False, False, False, False, False): 0,
+            (False, False, True, False, True): THREADS | EXTENSIONS,
         }.items():
             kwargs = dict(zip(kwlist, config))
             expected = {
@@ -1322,12 +1329,93 @@ class SubinterpreterTest(unittest.TestCase):
                         json.dump(settings, stdin)
                     ''')
                 with os.fdopen(r) as stdout:
-                    support.run_in_subinterp_with_config(script, **kwargs)
+                    ret = support.run_in_subinterp_with_config(script, **kwargs)
+                    self.assertEqual(ret, 0)
                     out = stdout.read()
                 settings = json.loads(out)
 
                 self.assertEqual(settings, expected)
 
+    @unittest.skipIf(_testsinglephase is None, "test requires _testsinglephase module")
+    @unittest.skipUnless(hasattr(os, "pipe"), "requires os.pipe()")
+    def test_overridden_setting_extensions_subinterp_check(self):
+        """
+        PyInterpreterConfig.check_multi_interp_extensions can be overridden
+        with PyInterpreterState.override_multi_interp_extensions_check.
+        This verifies that the override works but does not modify
+        the underlying setting.
+        """
+        import json
+
+        EXTENSIONS = 1<<8
+        THREADS = 1<<10
+        DAEMON_THREADS = 1<<11
+        FORK = 1<<15
+        EXEC = 1<<16
+        BASE_FLAGS = FORK | EXEC | THREADS | DAEMON_THREADS
+        base_kwargs = {
+            'allow_fork': True,
+            'allow_exec': True,
+            'allow_threads': True,
+            'allow_daemon_threads': True,
+        }
+
+        def check(enabled, override):
+            kwargs = dict(
+                base_kwargs,
+                check_multi_interp_extensions=enabled,
+            )
+            flags = BASE_FLAGS | EXTENSIONS if enabled else BASE_FLAGS
+            settings = {
+                'feature_flags': flags,
+            }
+
+            expected = {
+                'requested': override,
+                'override__initial': 0,
+                'override_after': override,
+                'override_restored': 0,
+                # The override should not affect the config or settings.
+                'settings__initial': settings,
+                'settings_after': settings,
+                'settings_restored': settings,
+                # These are the most likely values to be wrong.
+                'allowed__initial': not enabled,
+                'allowed_after': not ((override > 0) if override else enabled),
+                'allowed_restored': not enabled,
+            }
+
+            r, w = os.pipe()
+            script = textwrap.dedent(f'''
+                from test.test_capi.check_config import run_singlephase_check
+                run_singlephase_check({override}, {w})
+                ''')
+            with os.fdopen(r) as stdout:
+                ret = support.run_in_subinterp_with_config(script, **kwargs)
+                self.assertEqual(ret, 0)
+                out = stdout.read()
+            results = json.loads(out)
+
+            self.assertEqual(results, expected)
+
+        self.maxDiff = None
+
+        # setting: check disabled
+        with self.subTest('config: check disabled; override: disabled'):
+            check(False, -1)
+        with self.subTest('config: check disabled; override: use config'):
+            check(False, 0)
+        with self.subTest('config: check disabled; override: enabled'):
+            check(False, 1)
+
+        # setting: check enabled
+        with self.subTest('config: check enabled; override: disabled'):
+            check(True, -1)
+        with self.subTest('config: check enabled; override: use config'):
+            check(True, 0)
+        with self.subTest('config: check enabled; override: enabled'):
+            check(True, 1)
+
     def test_mutate_exception(self):
         """
         Exceptions saved in global module state get shared between

Lib/test/test_embed.py

@@ -1656,13 +1656,15 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
                                        api=API_PYTHON, env=env)
 
     def test_init_main_interpreter_settings(self):
+        EXTENSIONS = 1<<8
         THREADS = 1<<10
         DAEMON_THREADS = 1<<11
         FORK = 1<<15
         EXEC = 1<<16
         expected = {
             # All optional features should be enabled.
-            'feature_flags': FORK | EXEC | THREADS | DAEMON_THREADS,
+            'feature_flags':
+                FORK | EXEC | THREADS | DAEMON_THREADS,
         }
         out, err = self.run_embedded_interpreter(
             'test_init_main_interpreter_settings',

Lib/test/test_import/__init__.py

@@ -21,7 +21,7 @@ from unittest import mock
 from test.support import os_helper
 from test.support import (
     STDLIB_DIR, swap_attr, swap_item, cpython_only, is_emscripten,
-    is_wasi)
+    is_wasi, run_in_subinterp_with_config)
 from test.support.import_helper import (
     forget, make_legacy_pyc, unlink, unload, DirsOnSysPath, CleanImport)
 from test.support.os_helper import (
@@ -30,6 +30,14 @@ from test.support import script_helper
 from test.support import threading_helper
 from test.test_importlib.util import uncache
 from types import ModuleType
+try:
+    import _testsinglephase
+except ImportError:
+    _testsinglephase = None
+try:
+    import _testmultiphase
+except ImportError:
+    _testmultiphase = None
 
 
 skip_if_dont_write_bytecode = unittest.skipIf(
@@ -1392,6 +1400,216 @@ class CircularImportTests(unittest.TestCase):
             unwritable.x = 42
 
 
+class SubinterpImportTests(unittest.TestCase):
+
+    RUN_KWARGS = dict(
+        allow_fork=False,
+        allow_exec=False,
+        allow_threads=True,
+        allow_daemon_threads=False,
+    )
+
+    @unittest.skipUnless(hasattr(os, "pipe"), "requires os.pipe()")
+    def pipe(self):
+        r, w = os.pipe()
+        self.addCleanup(os.close, r)
+        self.addCleanup(os.close, w)
+        if hasattr(os, 'set_blocking'):
+            os.set_blocking(r, False)
+        return (r, w)
+
+    def import_script(self, name, fd, check_override=None):
+        override_text = ''
+        if check_override is not None:
+            override_text = f'''
+            import _imp
+            _imp._override_multi_interp_extensions_check({check_override})
+            '''
+        return textwrap.dedent(f'''
+            import os, sys
+            {override_text}
+            try:
+                import {name}
+            except ImportError as exc:
+                text = 'ImportError: ' + str(exc)
+            else:
+                text = 'okay'
+            os.write({fd}, text.encode('utf-8'))
+            ''')
+
+    def run_shared(self, name, *,
+                   check_singlephase_setting=False,
+                   check_singlephase_override=None,
+                   ):
+        """
+        Try importing the named module in a subinterpreter.
+
+        The subinterpreter will be in the current process.
+        The module will have already been imported in the main interpreter.
+        Thus, for extension/builtin modules, the module definition will
+        have been loaded already and cached globally.
+
+        "check_singlephase_setting" determines whether or not
+        the interpreter will be configured to check for modules
+        that are not compatible with use in multiple interpreters.
+
+        This should always return "okay" for all modules if the
+        setting is False (with no override).
+        """
+        __import__(name)
+
+        kwargs = dict(
+            **self.RUN_KWARGS,
+            check_multi_interp_extensions=check_singlephase_setting,
+        )
+
+        r, w = self.pipe()
+        script = self.import_script(name, w, check_singlephase_override)
+
+        ret = run_in_subinterp_with_config(script, **kwargs)
+        self.assertEqual(ret, 0)
+        return os.read(r, 100)
+
+    def check_compatible_shared(self, name, *, strict=False):
+        # Verify that the named module may be imported in a subinterpreter.
+        # (See run_shared() for more info.)
+        out = self.run_shared(name, check_singlephase_setting=strict)
+        self.assertEqual(out, b'okay')
+
+    def check_incompatible_shared(self, name):
+        # Differences from check_compatible_shared():
+        #  * verify that import fails
+        #  * "strict" is always True
+        out = self.run_shared(name, check_singlephase_setting=True)
+        self.assertEqual(
+            out.decode('utf-8'),
+            f'ImportError: module {name} does not support loading in subinterpreters',
+        )
+
+    def check_compatible_isolated(self, name, *, strict=False):
+        # Differences from check_compatible_shared():
+        #  * subinterpreter in a new process
+        #  * module has never been imported before in that process
+        #  * this tests importing the module for the first time
+        _, out, err = script_helper.assert_python_ok('-c', textwrap.dedent(f'''
+            import _testcapi, sys
+            assert (
+                {name!r} in sys.builtin_module_names or
+                {name!r} not in sys.modules
+            ), repr({name!r})
+            ret = _testcapi.run_in_subinterp_with_config(
+                {self.import_script(name, "sys.stdout.fileno()")!r},
+                **{self.RUN_KWARGS},
+                check_multi_interp_extensions={strict},
+            )
+            assert ret == 0, ret
+            '''))
+        self.assertEqual(err, b'')
+        self.assertEqual(out, b'okay')
+
+    def check_incompatible_isolated(self, name):
+        # Differences from check_compatible_isolated():
+        #  * verify that import fails
+        #  * "strict" is always True
+        _, out, err = script_helper.assert_python_ok('-c', textwrap.dedent(f'''
+            import _testcapi, sys
+            assert {name!r} not in sys.modules, {name!r}
+            ret = _testcapi.run_in_subinterp_with_config(
+                {self.import_script(name, "sys.stdout.fileno()")!r},
+                **{self.RUN_KWARGS},
+                check_multi_interp_extensions=True,
+            )
+            assert ret == 0, ret
+            '''))
+        self.assertEqual(err, b'')
+        self.assertEqual(
+            out.decode('utf-8'),
+            f'ImportError: module {name} does not support loading in subinterpreters',
+        )
+
+    def test_builtin_compat(self):
+        module = 'sys'
+        with self.subTest(f'{module}: not strict'):
+            self.check_compatible_shared(module, strict=False)
+        with self.subTest(f'{module}: strict, shared'):
+            self.check_compatible_shared(module, strict=True)
+
+    @cpython_only
+    def test_frozen_compat(self):
+        module = '_frozen_importlib'
+        if __import__(module).__spec__.origin != 'frozen':
+            raise unittest.SkipTest(f'{module} is unexpectedly not frozen')
+        with self.subTest(f'{module}: not strict'):
+            self.check_compatible_shared(module, strict=False)
+        with self.subTest(f'{module}: strict, shared'):
+            self.check_compatible_shared(module, strict=True)
+
+    @unittest.skipIf(_testsinglephase is None, "test requires _testsinglephase module")
+    def test_single_init_extension_compat(self):
+        module = '_testsinglephase'
+        with self.subTest(f'{module}: not strict'):
+            self.check_compatible_shared(module, strict=False)
+        with self.subTest(f'{module}: strict, shared'):
+            self.check_incompatible_shared(module)
+        with self.subTest(f'{module}: strict, isolated'):
+            self.check_incompatible_isolated(module)
+
+    @unittest.skipIf(_testmultiphase is None, "test requires _testmultiphase module")
+    def test_multi_init_extension_compat(self):
+        module = '_testmultiphase'
+        with self.subTest(f'{module}: not strict'):
+            self.check_compatible_shared(module, strict=False)
+        with self.subTest(f'{module}: strict, shared'):
+            self.check_compatible_shared(module, strict=True)
+        with self.subTest(f'{module}: strict, isolated'):
+            self.check_compatible_isolated(module, strict=True)
+
+    def test_python_compat(self):
+        module = 'threading'
+        if __import__(module).__spec__.origin == 'frozen':
+            raise unittest.SkipTest(f'{module} is unexpectedly frozen')
+        with self.subTest(f'{module}: not strict'):
+            self.check_compatible_shared(module, strict=False)
+        with self.subTest(f'{module}: strict, shared'):
+            self.check_compatible_shared(module, strict=True)
+        with self.subTest(f'{module}: strict, isolated'):
+            self.check_compatible_isolated(module, strict=True)
+
+    @unittest.skipIf(_testsinglephase is None, "test requires _testsinglephase module")
+    def test_singlephase_check_with_setting_and_override(self):
+        module = '_testsinglephase'
+
+        def check_compatible(setting, override):
+            out = self.run_shared(
+                module,
+                check_singlephase_setting=setting,
+                check_singlephase_override=override,
+            )
+            self.assertEqual(out, b'okay')
+
+        def check_incompatible(setting, override):
+            out = self.run_shared(
+                module,
+                check_singlephase_setting=setting,
+                check_singlephase_override=override,
+            )
+            self.assertNotEqual(out, b'okay')
+
+        with self.subTest('config: check enabled; override: enabled'):
+            check_incompatible(True, 1)
+        with self.subTest('config: check enabled; override: use config'):
+            check_incompatible(True, 0)
+        with self.subTest('config: check enabled; override: disabled'):
+            check_compatible(True, -1)
+
+        with self.subTest('config: check disabled; override: enabled'):
+            check_incompatible(False, 1)
+        with self.subTest('config: check disabled; override: use config'):
+            check_compatible(False, 0)
+        with self.subTest('config: check disabled; override: disabled'):
+            check_compatible(False, -1)
+
+
 if __name__ == '__main__':
     # Test needs to be a package, so we can do relative imports.
     unittest.main()

Lib/test/test_threading.py

@@ -1347,6 +1347,7 @@ class SubinterpThreadingTests(BaseTestCase):
                 allow_exec=True,
                 allow_threads={allowed},
                 allow_daemon_threads={daemon_allowed},
+                check_multi_interp_extensions=False,
             )
             """)
         with test.support.SuppressCrashReport():

Misc/NEWS.d/next/Core and Builtins/2022-11-02-20-23-47.gh-issue-98627.VJkdRM.rst

@@ -0,0 +1,5 @@
+When an interpreter is configured to check (and only then), importing an
+extension module will now fail when the extension does not support multiple
+interpreters (i.e. doesn't implement PEP 489 multi-phase init). This does
+not apply to the main interpreter, nor to subinterpreters created with
+``Py_NewInterpreter()``.

Modules/_testcapimodule.c

@@ -1618,6 +1618,7 @@ run_in_subinterp_with_config(PyObject *self, PyObject *args, PyObject *kwargs)
     int allow_exec = -1;
     int allow_threads = -1;
     int allow_daemon_threads = -1;
+    int check_multi_interp_extensions = -1;
     int r;
     PyThreadState *substate, *mainstate;
     /* only initialise 'cflags.cf_flags' to test backwards compatibility */
@@ -1628,11 +1629,13 @@ run_in_subinterp_with_config(PyObject *self, PyObject *args, PyObject *kwargs)
                              "allow_exec",
                              "allow_threads",
                              "allow_daemon_threads",
+                             "check_multi_interp_extensions",
                              NULL};
     if (!PyArg_ParseTupleAndKeywords(args, kwargs,
-                    "s$pppp:run_in_subinterp_with_config", kwlist,
+                    "s$ppppp:run_in_subinterp_with_config", kwlist,
                     &code, &allow_fork, &allow_exec,
-                    &allow_threads, &allow_daemon_threads)) {
+                    &allow_threads, &allow_daemon_threads,
+                    &check_multi_interp_extensions)) {
         return NULL;
     }
     if (allow_fork < 0) {
@@ -1651,6 +1654,10 @@ run_in_subinterp_with_config(PyObject *self, PyObject *args, PyObject *kwargs)
         PyErr_SetString(PyExc_ValueError, "missing allow_daemon_threads");
         return NULL;
     }
+    if (check_multi_interp_extensions < 0) {
+        PyErr_SetString(PyExc_ValueError, "missing check_multi_interp_extensions");
+        return NULL;
+    }
 
     mainstate = PyThreadState_Get();
 
@@ -1661,6 +1668,7 @@ run_in_subinterp_with_config(PyObject *self, PyObject *args, PyObject *kwargs)
         .allow_exec = allow_exec,
         .allow_threads = allow_threads,
         .allow_daemon_threads = allow_daemon_threads,
+        .check_multi_interp_extensions = check_multi_interp_extensions,
     };
     substate = _Py_NewInterpreterFromConfig(&config);
     if (substate == NULL) {

Python/clinic/import.c.h

@@ -442,6 +442,37 @@ exit:
     return return_value;
 }
 
+PyDoc_STRVAR(_imp__override_multi_interp_extensions_check__doc__,
+"_override_multi_interp_extensions_check($module, override, /)\n"
+"--\n"
+"\n"
+"(internal-only) Override PyInterpreterConfig.check_multi_interp_extensions.\n"
+"\n"
+"(-1: \"never\", 1: \"always\", 0: no override)");
+
+#define _IMP__OVERRIDE_MULTI_INTERP_EXTENSIONS_CHECK_METHODDEF    \
+    {"_override_multi_interp_extensions_check", (PyCFunction)_imp__override_multi_interp_extensions_check, METH_O, _imp__override_multi_interp_extensions_check__doc__},
+
+static PyObject *
+_imp__override_multi_interp_extensions_check_impl(PyObject *module,
+                                                  int override);
+
+static PyObject *
+_imp__override_multi_interp_extensions_check(PyObject *module, PyObject *arg)
+{
+    PyObject *return_value = NULL;
+    int override;
+
+    override = _PyLong_AsInt(arg);
+    if (override == -1 && PyErr_Occurred()) {
+        goto exit;
+    }
+    return_value = _imp__override_multi_interp_extensions_check_impl(module, override);
+
+exit:
+    return return_value;
+}
+
 #if defined(HAVE_DYNAMIC_LOADING)
 
 PyDoc_STRVAR(_imp_create_dynamic__doc__,
@@ -617,4 +648,4 @@ exit:
 #ifndef _IMP_EXEC_DYNAMIC_METHODDEF
     #define _IMP_EXEC_DYNAMIC_METHODDEF
 #endif /* !defined(_IMP_EXEC_DYNAMIC_METHODDEF) */
-/*[clinic end generated code: output=806352838c3f7008 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=b18d46e0036eff49 input=a9049054013a1b77]*/

Python/import.c

@@ -74,6 +74,8 @@ static struct _inittab *inittab_copy = NULL;
     (interp)->imports.modules_by_index
 #define IMPORTLIB(interp) \
     (interp)->imports.importlib
+#define OVERRIDE_MULTI_INTERP_EXTENSIONS_CHECK(interp) \
+    (interp)->imports.override_multi_interp_extensions_check
 #define OVERRIDE_FROZEN_MODULES(interp) \
     (interp)->imports.override_frozen_modules
 #ifdef HAVE_DLOPEN
@@ -816,6 +818,38 @@ _extensions_cache_clear_all(void)
     Py_CLEAR(EXTENSIONS);
 }
 
+
+static bool
+check_multi_interp_extensions(PyInterpreterState *interp)
+{
+    int override = OVERRIDE_MULTI_INTERP_EXTENSIONS_CHECK(interp);
+    if (override < 0) {
+        return false;
+    }
+    else if (override > 0) {
+        return true;
+    }
+    else if (_PyInterpreterState_HasFeature(
+                interp, Py_RTFLAGS_MULTI_INTERP_EXTENSIONS)) {
+        return true;
+    }
+    return false;
+}
+
+int
+_PyImport_CheckSubinterpIncompatibleExtensionAllowed(const char *name)
+{
+    PyInterpreterState *interp = _PyInterpreterState_Get();
+    if (check_multi_interp_extensions(interp)) {
+        assert(!_Py_IsMainInterpreter(interp));
+        PyErr_Format(PyExc_ImportError,
+                     "module %s does not support loading in subinterpreters",
+                     name);
+        return -1;
+    }
+    return 0;
+}
+
 static int
 fix_up_extension(PyObject *mod, PyObject *name, PyObject *filename)
 {
@@ -3297,6 +3331,34 @@ _imp__override_frozen_modules_for_tests_impl(PyObject *module, int override)
     Py_RETURN_NONE;
 }
 
+/*[clinic input]
+_imp._override_multi_interp_extensions_check
+
+    override: int
+    /
+
+(internal-only) Override PyInterpreterConfig.check_multi_interp_extensions.
+
+(-1: "never", 1: "always", 0: no override)
+[clinic start generated code]*/
+
+static PyObject *
+_imp__override_multi_interp_extensions_check_impl(PyObject *module,
+                                                  int override)
+/*[clinic end generated code: output=3ff043af52bbf280 input=e086a2ea181f92ae]*/
+{
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    if (_Py_IsMainInterpreter(interp)) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "_imp._override_multi_interp_extensions_check() "
+                        "cannot be used in the main interpreter");
+        return NULL;
+    }
+    int oldvalue = OVERRIDE_MULTI_INTERP_EXTENSIONS_CHECK(interp);
+    OVERRIDE_MULTI_INTERP_EXTENSIONS_CHECK(interp) = override;
+    return PyLong_FromLong(oldvalue);
+}
+
 #ifdef HAVE_DYNAMIC_LOADING
 
 /*[clinic input]
@@ -3329,18 +3391,23 @@ _imp_create_dynamic_impl(PyObject *module, PyObject *spec, PyObject *file)
 
     PyThreadState *tstate = _PyThreadState_GET();
     mod = import_find_extension(tstate, name, path);
-    if (mod != NULL || PyErr_Occurred()) {
+    if (mod != NULL) {
-        Py_DECREF(name);
+        const char *name_buf = PyUnicode_AsUTF8(name);
-        Py_DECREF(path);
+        assert(name_buf != NULL);
-        return mod;
+        if (_PyImport_CheckSubinterpIncompatibleExtensionAllowed(name_buf) < 0) {
+            Py_DECREF(mod);
+            mod = NULL;
+        }
+        goto finally;
+    }
+    else if (PyErr_Occurred()) {
+        goto finally;
     }
 
     if (file != NULL) {
         fp = _Py_fopen_obj(path, "r");
         if (fp == NULL) {
-            Py_DECREF(name);
+            goto finally;
-            Py_DECREF(path);
-            return NULL;
         }
     }
     else
@@ -3348,10 +3415,12 @@ _imp_create_dynamic_impl(PyObject *module, PyObject *spec, PyObject *file)
 
     mod = _PyImport_LoadDynamicModuleWithSpec(spec, fp);
 
-    Py_DECREF(name);
-    Py_DECREF(path);
     if (fp)
         fclose(fp);
+
+finally:
+    Py_DECREF(name);
+    Py_DECREF(path);
     return mod;
 }
 
@@ -3436,6 +3505,7 @@ static PyMethodDef imp_methods[] = {
     _IMP_IS_FROZEN_METHODDEF
     _IMP__FROZEN_MODULE_NAMES_METHODDEF
     _IMP__OVERRIDE_FROZEN_MODULES_FOR_TESTS_METHODDEF
+    _IMP__OVERRIDE_MULTI_INTERP_EXTENSIONS_CHECK_METHODDEF
     _IMP_CREATE_DYNAMIC_METHODDEF
     _IMP_EXEC_DYNAMIC_METHODDEF
     _IMP_EXEC_BUILTIN_METHODDEF

Python/importdl.c

@@ -3,6 +3,7 @@
 
 #include "Python.h"
 #include "pycore_call.h"
+#include "pycore_import.h"
 #include "pycore_pystate.h"
 #include "pycore_runtime.h"
 
@@ -203,6 +204,10 @@ _PyImport_LoadDynamicModuleWithSpec(PyObject *spec, FILE *fp)
 
     /* Fall back to single-phase init mechanism */
 
+    if (_PyImport_CheckSubinterpIncompatibleExtensionAllowed(name_buf) < 0) {
+        goto error;
+    }
+
     if (hook_prefix == nonascii_prefix) {
         /* don't allow legacy init for non-ASCII module names */
         PyErr_Format(

Python/pylifecycle.c

@@ -565,6 +565,10 @@ init_interp_settings(PyInterpreterState *interp, const _PyInterpreterConfig *con
     if (config->allow_daemon_threads) {
         interp->feature_flags |= Py_RTFLAGS_DAEMON_THREADS;
     }
+
+    if (config->check_multi_interp_extensions) {
+        interp->feature_flags |= Py_RTFLAGS_MULTI_INTERP_EXTENSIONS;
+    }
 }