Clear the padding in the key and fix the test on Windows

The key is hashed and compared as raw bytes, so the uninitialized padding
of the header made lookups miss at random.

Import the extension module in a subprocess: it stays loaded, and on
Windows its file cannot be removed, so the temporary directory leaked.

Author: serhiy-storchaka

Lib/test/test_import/__init__.py

@@ -20,7 +20,6 @@
 import stat
 import subprocess
 import sys
-import tempfile
 import textwrap
 import threading
 import time
@@ -1271,27 +1270,28 @@ def test_import_from_undecodable_path(self):
         # gh-155247: the path of the extension module is not encodable
         # in UTF-8.
         origin = _testsinglephase.__file__
-        # The module is cached by its path, so restore it afterwards.
-        self.addCleanup(restore__testsinglephase)
-        tempdir = tempfile.mkdtemp()
-        # The copied extension module stays loaded, so on Windows it cannot
-        # be removed.
-        self.addCleanup(shutil.rmtree, tempdir, ignore_errors=True)
-        subdir = os.path.join(os.fsencode(tempdir),
-                              os_helper.TESTFN_UNDECODABLE)
-        try:
-            os.mkdir(subdir)
-        except OSError:
-            self.skipTest('undecodable paths are not supported')
-        path = os.path.join(subdir, os.fsencode(os.path.basename(origin)))
-        shutil.copyfile(origin, path)
-        path = os.fsdecode(path)
-        spec = importlib.util.spec_from_file_location('_testsinglephase', path)
-        module = importlib.util.module_from_spec(spec)
-        spec.loader.exec_module(module)
-        self.assertEqual(module.__name__, '_testsinglephase')
-        self.assertEqual(module.__file__, path)
-        _testinternalcapi.clear_extension('_testsinglephase', path)
+        with os_helper.temp_dir() as tempdir:
+            subdir = os.path.join(os.fsencode(tempdir),
+                                  os_helper.TESTFN_UNDECODABLE)
+            try:
+                os.mkdir(subdir)
+            except OSError:
+                self.skipTest('undecodable paths are not supported')
+            path = os.path.join(subdir, os.fsencode(os.path.basename(origin)))
+            shutil.copyfile(origin, path)
+            # Import it in a subprocess: the extension module stays loaded,
+            # and on Windows its file cannot be removed.
+            script = textwrap.dedent(f"""
+                import importlib.util
+                path = {os.fsdecode(path)!a}
+                spec = importlib.util.spec_from_file_location(
+                        '_testsinglephase', path)
+                module = importlib.util.module_from_spec(spec)
+                spec.loader.exec_module(module)
+                assert module.__name__ == '_testsinglephase', module.__name__
+                assert module.__file__ == path, module.__file__
+                """)
+            script_helper.assert_python_ok('-c', script)
 
     def test_create_builtin(self):
         class Spec:

Python/import.c

@@ -1308,6 +1308,8 @@ hashtable_key_from_2_strings(PyObject *str1, PyObject *str2)
         return NULL;
     }
 
+    /* Clear the padding: the key is hashed and compared as raw bytes. */
+    memset(key, 0, sizeof(struct hashtable_key));
     key->size = size;
     key->kind1 = (unsigned char)kind1;
     key->kind2 = (unsigned char)kind2;