binascii.c: Modify exception behavior on invalid encoded length

When decoding invalid length (1, 3 or 6 mod 8) + no padding,
mention the invalid length instead of the improper padding in
the exception message to match what the base64 decoder does.

Additionally, move the logic for setting the exception message
(back) outside the "slow path" loop; if we do end up checking
canonicity of decoder input, it will feel (subjectively) better
to have several checks grouped together after the loop.

Author: kangtastic

Lib/test/test_binascii.py

@@ -782,12 +782,9 @@ def assertInvalidLength(*args):
         assertLeadingPadding(b"========BEEFCAKE", b"\t\x08Q\x01D")
         assertLeadingPadding(b"=========BEEFCAKE", b"\t\x08Q\x01D")
 
-        assertIncorrectPadding(b"A")
         assertIncorrectPadding(b"AB")
-        assertIncorrectPadding(b"ABC")
         assertIncorrectPadding(b"ABCD")
         assertIncorrectPadding(b"ABCDE")
-        assertIncorrectPadding(b"ABCDEF")
         assertIncorrectPadding(b"ABCDEFG")
 
         assertIncorrectPadding(b"BE=", b"\t")
@@ -806,6 +803,10 @@ def assertInvalidLength(*args):
         assertDiscontinuousPadding(b"BEEF=C==", b"\t\x08Q")
         assertDiscontinuousPadding(b"BEEFC=AK", b"\t\x08Q\x01")
 
+        assertInvalidLength(b"A")
+        assertInvalidLength(b"ABC")
+        assertInvalidLength(b"ABCDEF")
+
         assertInvalidLength(b"A=")
         assertInvalidLength(b"A==")
         assertInvalidLength(b"A===")

Modules/binascii.c

@@ -1562,24 +1562,16 @@ binascii_a2b_base32_impl(PyObject *module, Py_buffer *data,
             {
                 continue;
             }
-
+            if (octa_pos == 1 || octa_pos == 3 || octa_pos == 6) {
+                /* Set an error below. */
+                break;
+            }
             state = get_binascii_state(module);
             if (state) {
-                if (octa_pos == 1 || octa_pos == 3 || octa_pos == 6) {
-                    const unsigned char *ascii_data_start = data->buf;
-                    PyErr_Format(state->Error,
-                                 "Invalid base32-encoded string: "
-                                 "number of data characters (%zd) "
-                                 "cannot be 1, 3, or 6 more "
-                                 "than a multiple of 8",
-                                 ascii_data - ascii_data_start);
-                }
-                else {
-                    PyErr_SetString(state->Error,
-                                    (octa_pos == 0 && ascii_data == data->buf)
-                                    ? "Leading padding not allowed"
-                                    : "Excess padding not allowed");
-                }
+                PyErr_SetString(state->Error,
+                                (octa_pos == 0 && ascii_data == data->buf)
+                                ? "Leading padding not allowed"
+                                : "Excess padding not allowed");
             }
             goto error;
         }
@@ -1644,6 +1636,19 @@ binascii_a2b_base32_impl(PyObject *module, Py_buffer *data,
         }
     }
 
+    if (octa_pos == 1 || octa_pos == 3 || octa_pos == 6) {
+        state = get_binascii_state(module);
+        if (state) {
+            const unsigned char *ascii_data_start = data->buf;
+            PyErr_Format(state->Error,
+                         "Invalid base32-encoded string: "
+                         "number of data characters (%zd) "
+                         "cannot be 1, 3, or 6 more than a multiple of 8",
+                         ascii_data - ascii_data_start);
+        }
+        goto error;
+    }
+
     if ((octa_pos != 0 && octa_pos + pads != 8)
         || (octa_pos == 0 && pads != 0))
     {