Twisted web client does not support IPv6 address

When I configure the default MAAS URL to an IPv6 address (e.g. http://[fd0d:1727:6eb6:db5::1]/MAAS ) the cluster controller can no longer connect to the region controller, even though they are running on the same server.

The pserv.log shows errors like:

«
Unhandled Error
        Traceback (most recent call last):
          File "/usr/lib/python2.7/dist-packages/twisted/internet/task.py", line 218, in __call__
            d = defer.maybeDeferred(self.f, *self.a, **self.kw)
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 139, in maybeDeferred
            result = f(*args, **kw)
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1237, in unwindGenerator
            return _inlineCallbacks(None, gen, Deferred())
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1099, in _inlineCallbacks
            result = g.send(result)
        --- <exception caught here> ---
          File "/usr/lib/python2.7/dist-packages/provisioningserver/rpc/clusterservice.py", line 409, in update
            info = yield self._fetch_rpc_info(info_url)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/rpc/clusterservice.py", line 431, in _fetch_rpc_info
            return getPage(url).addCallback(json.loads)
          File "/usr/lib/python2.7/dist-packages/twisted/web/client.py", line 718, in getPage
            *args, **kwargs).deferred
          File "/usr/lib/python2.7/dist-packages/twisted/web/client.py", line 692, in _makeGetterFactory
            uri = _URI.fromBytes(url)
          File "/usr/lib/python2.7/dist-packages/twisted/web/client.py", line 608, in fromBytes
            host, port = host.split(b':')
        exceptions.ValueError: too many values to unpack

Logged OOPS id OOPS-199af31e45ee2462dd0c64c369e7b27f: ValueError: too many values to unpack
»

The failure happens when ClusterClientService tries to obtain the RPC information page from the region server, using the IPv6 URL. Twisted's _URI.fromBytes tries to parse a URL, but assumes that only a port number can introduce a colon into the URL's network location.