gh-127350: Add more tests for Py_fopen()

Lib/test/test_capi/test_file.py

@@ -5,6 +5,8 @@
 
 _testcapi = import_helper.import_module('_testcapi')
 
+NULL = None
+
 
 class CAPIFileTest(unittest.TestCase):
     def test_py_fopen(self):
@@ -25,15 +27,22 @@ def test_py_fopen(self):
             os_helper.TESTFN,
             os.fsencode(os_helper.TESTFN),
         ]
-        # TESTFN_UNDECODABLE cannot be used to create a file on macOS/WASI.
+        if os_helper.TESTFN_UNDECODABLE is not None:
+            filenames.append(os_helper.TESTFN_UNDECODABLE)
+            filenames.append(os.fsdecode(os_helper.TESTFN_UNDECODABLE))
         if os_helper.TESTFN_UNENCODABLE is not None:
             filenames.append(os_helper.TESTFN_UNENCODABLE)
         for filename in filenames:
             with self.subTest(filename=filename):
                 try:
                     with open(filename, "wb") as fp:
                         fp.write(source)
-
+                except OSError:
+                    # TESTFN_UNDECODABLE cannot be used to create a file
+                    # on macOS/WASI.
+                    filename = None
+                    continue
+                try:
                     data = _testcapi.py_fopen(filename, "rb")
                     self.assertEqual(data, source[:256])
                 finally:
@@ -47,7 +56,13 @@ def test_py_fopen(self):
 
         # non-ASCII mode failing with "Invalid argument"
         with self.assertRaises(OSError):
-            _testcapi.py_fopen(__file__, "\xe9")
+            _testcapi.py_fopen(__file__, b"\xc2\x80")
+        with self.assertRaises(OSError):
+            # \x98 is invalid in cp1250, cp1251, cp1257
+            # \x9d is invalid in cp1252-cp1255, cp1258
+            _testcapi.py_fopen(__file__, b"\xc2\x98\xc2\x9d")
+        with self.assertRaises((UnicodeDecodeError, OSError)):
+            _testcapi.py_fopen(__file__, b"\x98\x9d")
 
         # invalid filename type
         for invalid_type in (123, object()):
@@ -60,7 +75,8 @@ def test_py_fopen(self):
                 # On Windows, the file mode is limited to 10 characters
                 _testcapi.py_fopen(__file__, "rt+, ccs=UTF-8")
 
-        # CRASHES py_fopen(__file__, None)
+        # CRASHES _testcapi.py_fopen(NULL, 'rb')
+        # CRASHES _testcapi.py_fopen(__file__, NULL)
 
 
 if __name__ == "__main__":

Modules/_testcapi/file.c

@@ -14,16 +14,18 @@ module _testcapi
 _testcapi.py_fopen
 
     path: object
-    mode: str
+    mode: str(zeroes=True, accept={robuffer, str, NoneType})
     /
 
 Call Py_fopen(), fread(256) and Py_fclose(). Return read bytes.
 [clinic start generated code]*/
 
 static PyObject *
-_testcapi_py_fopen_impl(PyObject *module, PyObject *path, const char *mode)
-/*[clinic end generated code: output=5a900af000f759de input=d7e7b8f0fd151953]*/
+_testcapi_py_fopen_impl(PyObject *module, PyObject *path, const char *mode,
+                        Py_ssize_t mode_length)
+/*[clinic end generated code: output=69840d0cfd8b7fbb input=f3a579dd7eb60926]*/
 {
+    NULLABLE(path);
     FILE *fp = Py_fopen(path, mode);
     if (fp == NULL) {
         return NULL;