fix: minor edge case in long_to_base64 utility function.

This was detected by mypy.

CHANGELOG.md

@@ -4,6 +4,8 @@ Note: Breaking changes between versions are indicated by "💥".
 
 ## Unreleased
 
+- [Bugfix] Fix minor edge case in `long_to_base64` utility function.
+
 ## v11.2.3 (2021-02-20)
 
 - [Bugfix] Make LMS celery workers actually process LMS tasks, and not CMS tasks.

tests/test_utils.py

@@ -1,3 +1,4 @@
+import base64
 import unittest
 
 from tutor import utils
@@ -30,3 +31,6 @@ class UtilsTests(unittest.TestCase):
         self.assertNotEqual(encrypted1, encrypted2)
         self.assertTrue(utils.verify_encrypted(encrypted1, password))
         self.assertTrue(utils.verify_encrypted(encrypted2, password))
+
+    def test_long_to_base64(self):
+        self.assertEqual(b"\x00", base64.urlsafe_b64decode(utils.long_to_base64(0) + "=="))

tutor/utils.py

@@ -112,7 +112,7 @@ def long_to_base64(n):
     bys = long2intarr(n)
     data = struct.pack("%sB" % len(bys), *bys)
     if not data:
-        data = "\x00"
+        data = b"\x00"
     s = base64.urlsafe_b64encode(data).rstrip(b"=")
     return s.decode("ascii")