ring-builder: require part_power>=0, give better error messages

Closes-Bug: #1697860
Change-Id: I500a86de390b24b9d08a478d695a7d62c447e779

swift/cli/ringbuilder.py

@@ -544,7 +544,11 @@ swift-ring-builder <builder_file> create <part_power> <replicas>
         if len(argv) < 6:
             print(Commands.create.__doc__.strip())
             exit(EXIT_ERROR)
-        builder = RingBuilder(int(argv[3]), float(argv[4]), int(argv[5]))
+        try:
+            builder = RingBuilder(int(argv[3]), float(argv[4]), int(argv[5]))
+        except ValueError as e:
+            print(e)
+            exit(EXIT_ERROR)
         backup_dir = pathjoin(dirname(builder_file), 'backups')
         try:
             mkdir(backup_dir)

swift/common/ring/builder.py

@@ -87,6 +87,9 @@ class RingBuilder(object):
         if part_power > 32:
             raise ValueError("part_power must be at most 32 (was %d)"
                              % (part_power,))
+        if part_power < 0:
+            raise ValueError("part_power must be at least 0 (was %d)"
+                             % (part_power,))
         if replicas < 1:
             raise ValueError("replicas must be at least 1 (was %.6f)"
                              % (replicas,))

test/unit/common/ring/test_builder.py

@@ -85,6 +85,12 @@ class TestRingBuilder(unittest.TestCase):
             ring.RingBuilder(33, 3, 1)
         self.assertEqual(str(ctx.exception), expected_msg)
 
+    def test_oversmall_part_powers(self):
+        expected_msg = 'part_power must be at least 0 (was -1)'
+        with self.assertRaises(ValueError) as ctx:
+            ring.RingBuilder(-1, 3, 1)
+        self.assertEqual(str(ctx.exception), expected_msg)
+
     def test_insufficient_replicas(self):
         expected_msg = 'replicas must be at least 1 (was 0.999000)'
         with self.assertRaises(ValueError) as ctx: