Fixed #24115 -- Allowed bcrypt hashers to upgrade passwords on rounds change.

Thanks Florian Apolloner for the review.

tests/auth_tests/test_hashers.py

@@ -177,6 +177,38 @@ class TestUtilsHashPass(SimpleTestCase):
         self.assertTrue(check_password('', blank_encoded))
         self.assertFalse(check_password(' ', blank_encoded))
 
+    @skipUnless(bcrypt, "bcrypt not installed")
+    def test_bcrypt_upgrade(self):
+        hasher = get_hasher('bcrypt')
+        self.assertEqual('bcrypt', hasher.algorithm)
+        self.assertNotEqual(hasher.rounds, 4)
+
+        old_rounds = hasher.rounds
+        try:
+            # Generate a password with 4 rounds.
+            hasher.rounds = 4
+            encoded = make_password('letmein', hasher='bcrypt')
+            rounds = hasher.safe_summary(encoded)['work factor']
+            self.assertEqual(rounds, '04')
+
+            state = {'upgraded': False}
+
+            def setter(password):
+                state['upgraded'] = True
+
+            # Check that no upgrade is triggered.
+            self.assertTrue(check_password('letmein', encoded, setter, 'bcrypt'))
+            self.assertFalse(state['upgraded'])
+
+            # Revert to the old rounds count and ...
+            hasher.rounds = old_rounds
+
+            # ... check if the password would get updated to the new count.
+            self.assertTrue(check_password('letmein', encoded, setter, 'bcrypt'))
+            self.assertTrue(state['upgraded'])
+        finally:
+            hasher.rounds = old_rounds
+
     def test_unusable(self):
         encoded = make_password(None)
         self.assertEqual(len(encoded), len(UNUSABLE_PASSWORD_PREFIX) + UNUSABLE_PASSWORD_SUFFIX_LENGTH)