prevent startup when layout.yaml has invalid syntax

Similar issue, layout.yaml had no 'jobs' section despite it being marked as optional in the documentation.

Stack trace from zuul dev branch:

2012-09-27 11:17:22,859 ERROR zuul.Scheduler: Exception in run handler:
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/zuul-1.0-py2.7.egg/zuul/scheduler.py", line 330, in run
    self._doPauseEvent()
  File "/usr/local/lib/python2.7/dist-packages/zuul-1.0-py2.7.egg/zuul/scheduler.py", line 294, in _doPauseEvent
    self._parseConfig(self.config.get('zuul', 'layout_config'))
  File "/usr/local/lib/python2.7/dist-packages/zuul-1.0-py2.7.egg/zuul/scheduler.py", line 105, in _parseConfig
    for config_job in data['jobs']:
TypeError: 'NoneType' object is not iterable

Same happens when key was named 'project' instead of 'projects' (aka singular vs plural form).

At startup zuul should probably run scheduler._parseConfig() and abort whenever an exception is raised.

Agreed. We should also consider what to do when there's an error during a reconfiguration while zuul is running. We could either remember the old state and keep using it, or exit. Remembering the old state will take a good chunk of extra code, but would be best. Exiting is slightly better than what we have now (keep running while broken). So I think either of those are acceptable solutions, with the first being the preferable long-term goal.

You could add a new option to zuul-server that will just do the config check just like apachectl. Then the init script could use that before 'reload' and gracefully fail if anything is wrong in the configuration.