gh-148016: Improve "Leading padding not allowed" error in Base32 and Base64 decoders (GH-148017)

It is now raised instead of "Excess padding not allowed" if all characters
preceding "=" were ignored.

Author: serhiy-storchaka

Lib/test/test_binascii.py

@@ -171,8 +171,8 @@ def _assertRegexTemplate(assert_regex, data,
             self.assertEqual(binascii.a2b_base64(data),
                              non_strict_mode_expected_result)
 
-        def assertLeadingPadding(*args):
-            _assertRegexTemplate(r'(?i)Leading padding', *args)
+        def assertLeadingPadding(*args, **kwargs):
+            _assertRegexTemplate(r'(?i)Leading padding', *args, **kwargs)
 
         def assertDiscontinuousPadding(*args):
             _assertRegexTemplate(r'(?i)Discontinuous padding', *args)
@@ -203,6 +203,7 @@ def assertInvalidLength(data, *args, length=None, **kwargs):
         assertLeadingPadding(b'===abcd', b'i\xb7\x1d')
         assertLeadingPadding(b'====abcd', b'i\xb7\x1d')
         assertLeadingPadding(b'=====abcd', b'i\xb7\x1d')
+        assertLeadingPadding(b' =abcd', b'i\xb7\x1d', ignorechars=b' ')
 
         assertInvalidLength(b'a=b==', b'i')
         assertInvalidLength(b'a=bc=', b'i\xb7')
@@ -785,8 +786,8 @@ def assertExcessData(*args):
         def assertExcessPadding(*args):
             _assertRegexTemplate(r"(?i)Excess padding", *args)
 
-        def assertLeadingPadding(*args):
-            _assertRegexTemplate(r"(?i)Leading padding", *args)
+        def assertLeadingPadding(*args, **kwargs):
+            _assertRegexTemplate(r"(?i)Leading padding", *args, **kwargs)
 
         def assertIncorrectPadding(*args):
             _assertRegexTemplate(r"(?i)Incorrect padding", *args)
@@ -853,6 +854,7 @@ def assertInvalidLength(data, *args, length=None, **kwargs):
         assertLeadingPadding(b"=======BEEFCAKE", b"\t\x08Q\x01D")
         assertLeadingPadding(b"========BEEFCAKE", b"\t\x08Q\x01D")
         assertLeadingPadding(b"=========BEEFCAKE", b"\t\x08Q\x01D")
+        assertLeadingPadding(b" =BEEFCAKE", ignorechars=b' ')
 
         assertIncorrectPadding(b"AB")
         assertIncorrectPadding(b"ABCD")

Modules/binascii.c

@@ -816,8 +816,9 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode,
             }
             state = get_binascii_state(module);
             if (state) {
+                unsigned char *bin_data_start = PyBytesWriter_GetData(writer);
                 PyErr_SetString(state->Error,
-                                (quad_pos == 0 && ascii_data == data->buf)
+                                (quad_pos == 0 && bin_data == bin_data_start)
                                 ? "Leading padding not allowed"
                                 : "Excess padding not allowed");
             }
@@ -1601,8 +1602,9 @@ binascii_a2b_base32_impl(PyObject *module, Py_buffer *data,
             }
             state = get_binascii_state(module);
             if (state) {
+                unsigned char *bin_data_start = PyBytesWriter_GetData(writer);
                 PyErr_SetString(state->Error,
-                                (octa_pos == 0 && ascii_data == data->buf)
+                                (octa_pos == 0 && bin_data == bin_data_start)
                                 ? "Leading padding not allowed"
                                 : "Excess padding not allowed");
             }