AttributeError OOPS in codebrowse (lazy_import not threadsafe)

As seen on OOPS-1839CBA1:

http://bazaar.launchpad.net/%7Evcs-imports/busybox/main/changes

   Traceback (most recent call last):
  Module launchpad_loggerhead.app, line 340, in wrapped_app
    app_iter = iter(app(environ, wrapped.start_response))
  Module launchpad_loggerhead.debug, line 106, in wrapped_application
    result = application(environ, response_hook)
  Module __main__, line 165, in wrapped
  Module launchpad_loggerhead.debug, line 45, in wrapped
    return app(environ, start_response)
  Module paste.translogger, line 68, in __call__
    return self.application(environ, replacement_start_response)
  Module paste.deploy.config, line 285, in __call__
    return self.app(environ, start_response)
  Module __main__, line 149, in wrapped
  Module paste.wsgilib, line 179, in catch_errors
    app_iter = application(environ, start_response)
  Module launchpad_loggerhead.session, line 53, in __call__
    return self.cookie_handler(environ, start_response)
  Module paste.auth.cookie, line 305, in __call__
    return self.application(environ, response_hook)
  Module launchpad_loggerhead.session, line 80, in _process
    return self.application(environ, response_hook)
  Module paste.httpexceptions, line 636, in __call__
    return self.application(environ, start_response)
  Module launchpad_loggerhead.app, line 219, in __call__
    lp_server.get_url().strip(':/'), branch_url)
  Module lp.codehosting.bzrutils, line 361, in safe_open
    makeURLChecker(allowed_scheme), url, possible_transports)
  Module lp.codehosting.bzrutils, line 333, in checked_open
    return Branch.open(url, possible_transports=possible_transports)
  Module bzrlib.branch, line 168, in open
    return control.open_branch(unsupported=_unsupported)
  Module bzrlib.bzrdir, line 1871, in open_branch
    _found=True, ignore_fallbacks=ignore_fallbacks)
  Module bzrlib.branch, line 2069, in open
    _repository=a_bzrdir.find_repository(),
  Module bzrlib.bzrdir, line 735, in find_repository
    found_repo = self._find_containing(usable_repository)
  Module bzrlib.bzrdir, line 701, in _find_containing
    result, stop = evaluate(found_bzrdir)
  Module bzrlib.bzrdir, line 725, in usable_repository
    repository = found_bzrdir.open_repository()
  Module bzrlib.bzrdir, line 1878, in open_repository
    return format.open(self, _found=True)
  Module bzrlib.repofmt.pack_repo, line 2584, in open
    _serializer=self._serializer)
  Module bzrlib.repofmt.pack_repo, line 2234, in __init__
    use_chk_index=self._format.supports_chks,
  Module bzrlib.repofmt.pack_repo, line 1399, in __init__
    self.revision_index = AggregateIndex(self.reload_pack_names, flush)
  Module bzrlib.repofmt.pack_repo, line 589, in __init__
    flush_func=flush_func)
  Module bzrlib.lazy_import, line 124, in __call__
    _cleanup()
  Module bzrlib.lazy_import, line 97, in _cleanup
    del self._factory
AttributeError: _factory