Issue #29061: secrets.randbelow() would hang with a negative input

Lib/secrets.py

@@ -26,6 +26,8 @@ choice = _sysrand.choice
 
 def randbelow(exclusive_upper_bound):
     """Return a random int in the range [0, n)."""
+    if exclusive_upper_bound <= 0:
+        raise ValueError("Upper bound must be positive.")
     return _sysrand._randbelow(exclusive_upper_bound)
 
 DEFAULT_ENTROPY = 32  # number of bytes to return by default

Lib/test/test_secrets.py

@@ -70,6 +70,7 @@ class Random_Tests(unittest.TestCase):
         for i in range(2, 10):
             self.assertIn(secrets.randbelow(i), range(i))
         self.assertRaises(ValueError, secrets.randbelow, 0)
+        self.assertRaises(ValueError, secrets.randbelow, -1)
 
 
 class Token_Tests(unittest.TestCase):

Misc/ACKS

@@ -369,6 +369,7 @@ Daniel Dittmar
 Josip Djolonga
 Walter Dörwald
 Jaromir Dolecek
+Brendan Donegan
 Ismail Donmez
 Robert Donohue
 Marcos Donolo

Misc/NEWS

@@ -43,6 +43,9 @@ Library
 - Issue #29085: Allow random.Random.seed() to use high quality OS randomness
   rather than the pid and time.
 
+- Issue #29061: Fixed bug in secrets.randbelow() which would hang when given
+  a negative input.  Patch by Brendan Donegan.
+
 - Issue #29079: Prevent infinite loop in pathlib.resolve() on Windows
 
 - Issue #13051: Fixed recursion errors in large or resized