[Bug 1604608] Re: ipv6 client causes errors in twisted.web

[LaMont Jones]

What is not addressed by this patch is that twisted assumes that if it is given a hostname, then the user clearly means IPv4.  The web client setup is not in the correct order, it should be:
1) resolve the hostname
2) iterate through the available IP addresses (probably preferring ipv6, like the rest of linux), trying each one until success, or end-of-list
2a) create the AF_INET/AF_INET6 socket based on the address family of the current trial IP.
2b) connect.

Today, it creates the socket based on the name it's given (ipv4 address,
ipv6 address, or "huh... hostname, let's try ipv4") and then resolves
it.

The other option would be to create an AF_INET6 socket, and if it turns
out that it's an IPv4 address, change it to the compatible ipv6 address
of "::ffff:192.168.1.1" or such.  Note that if the app then calls
getsockname() on the socket, it will see the ipv6-form, and may be
confused.

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Title:
  ipv6 client causes errors in twisted.web

Status in twisted package in Ubuntu:
  New

Bug description:
  If the client connecting to the twisted (16.0.0-1, still present in upstream 16.3.0), then twisted dies as follows:
  2016-07-18 15:40:21 [HTTPChannel,468,::ffff:127.0.0.1] Unhandled Error
          Traceback (most recent call last):
            File "/usr/lib/python3/dist-packages/twisted/web/http.py", line 1657, in lineReceived
              self.allContentReceived()
            File "/usr/lib/python3/dist-packages/twisted/web/http.py", line 1736, in allContentReceived
              req.requestReceived(command, path, version)
            File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 80, in requestReceived
              command, path, version)
            File "/usr/lib/python3/dist-packages/twisted/web/http.py", line 762, in requestReceived
              self.process()
          --- <exception caught here> ---
            File "/usr/lib/python3/dist-packages/twisted/web/server.py", line 183, in process
              self.render(resrc)
            File "/usr/lib/python3/dist-packages/twisted/web/server.py", line 234, in render
              body = resrc.render(self)
            File "/usr/lib/python3/dist-packages/twisted/web/wsgi.py", line 561, in render
              self._reactor, self._threadpool, self._application, request)
            File "/usr/lib/python3/dist-packages/twisted/web/wsgi.py", line 293, in __init__
              'REMOTE_ADDR': _wsgiString(request.getClientIP()),
            File "/usr/lib/python3/dist-packages/twisted/web/wsgi.py", line 85, in _wsgiString
              return string.decode("iso-8859-1")
          builtins.AttributeError: 'NoneType' object has no attribute 'decode'

  In the attached patch, the middle block addresses the above immediate
  issue, and the other two blocks are my best guess at what the code
  _should_ read for completeness.

  We'll want to get this SRUed into xenial, once it's fixed in wily, as
  it's required for MAAS 2.1 (which runs on "latest LTS release" ==
  xenial.)

  thanks

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