Ringbuilder: Forbid writing empty rings

Swift definitely can't make any use of empty rings, so it should
not be allowed to write them.

Replace warning with an error message & error exit.

Change-Id: I3a1b86368d363e67d1f91d7d8af4b391a0a53fff
Closes-Bug: #1396841

swift/cli/ringbuilder.py

@@ -1085,14 +1085,16 @@ swift-ring-builder <builder_file> write_ring
     'set_info' calls when no rebalance is needed but you want to send out the
     new device information.
         """
+        if not builder.devs:
+            print('Unable to write empty ring.')
+            exit(EXIT_ERROR)
+
         ring_data = builder.get_ring()
         if not ring_data._replica2part2dev_id:
             if ring_data.devs:
                 print('Warning: Writing a ring with no partition '
                       'assignments but with devices; did you forget to run '
                       '"rebalance"?')
-            else:
-                print('Warning: Writing an empty ring')
         ring_data.save(
             pathjoin(backup_dir, '%d.' % time() + basename(ring_file)))
         ring_data.save(ring_file)

test/unit/cli/test_ringbuilder.py

@@ -46,7 +46,6 @@ class RunSwiftRingBuilderMixin(object):
 
         if 'exp_results' in kwargs:
             exp_results = kwargs['exp_results']
-            argv = argv[:-1]
         else:
             exp_results = None
 
@@ -2078,6 +2077,13 @@ class TestCommands(unittest.TestCase, RunSwiftRingBuilderMixin):
         argv = ["", self.tmpfile, "write_ring"]
         self.assertSystemExit(EXIT_SUCCESS, ringbuilder.main, argv)
 
+    def test_write_empty_ring(self):
+        ring = RingBuilder(6, 3, 1)
+        ring.save(self.tmpfile)
+        exp_results = {'valid_exit_codes': [2]}
+        out, err = self.run_srb("write_ring", exp_results=exp_results)
+        self.assertEqual('Unable to write empty ring.\n', out)
+
     def test_write_builder(self):
         # Test builder file already exists
         self.create_sample_ring()