swift-ring-builder rebalance goes into an infinite loop

[James Troup]

On Ubuntu 16.04 with a Mitaka cloud the attached ring files cause
swift-ring-builder to go into an infinite loop when asked to
rebalance.

The problem seems to be the following line in place_parts():

            while parts:

Which seems to assume that parts can never be << 0, but with these
ring files that assumption breaks.

I added the following debug statements:

  http://paste.ubuntu.com/23419920/

To get the following debug output:

  http://paste.ubuntu.com/23419916/

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Corey Bryant (corey.bryant)]

This looks to be fixed on more recent versions of swift via: https://bugs.launchpad.net/swift/+bug/1642538

[Corey Bryant (corey.bryant)]

Let's track this fix over in that bug since that's where it was fixed upstream. Marking this as a dup.