use_index.patch

diff --git a/swift/common/ring/builder.py b/swift/common/ring/builder.py
index 742e020..5fe9b2c 100644
--- a/swift/common/ring/builder.py
+++ b/swift/common/ring/builder.py
@@ -334,18 +334,10 @@ class RingBuilder(object):
                   users may depend on it)
         """
         if 'id' not in dev:
-            dev['id'] = 0
-            if self.devs:
-                dev_id_found = False
-                # look for existing slots that we can reuse
-                for i in range(len(self.devs)):
-                    if self.devs[i] is None:
-                        dev['id'] = i
-                        dev_id_found = True
-                        break
-                if not dev_id_found:
-                    # no slots available for reuse, grab a brand new one
-                    dev['id'] = len(self.devs)
+            try:
+                dev['id'] = self.devs.index(None)
+            except ValueError:
+                dev['id'] = len(self.devs)
         if dev['id'] < len(self.devs) and self.devs[dev['id']] is not None:
             raise exceptions.DuplicateDeviceError(
                 'Duplicate device id: %d' % dev['id'])