Allow pecan debug middleware to be turned off

The Pecan DebugMiddleware is turned on or off based on the
ceilometer.conf-wide debug setting. This is not useful for situations
where running ceilometer-api under mod_wsgi with multiple processes
is desired.

This change adds a pecan_debug setting to the api section of config,
defaulting to the DEFAULT debug value. mod_wsgi install docs updated
accordingly.

Configuration docs have not yet been udpated. They will be pending a
fix to bug 1370030

Closes-Bug: #1370009

Change-Id: Ib4a472e3c8d1f34f64a5f3ab993c1211dee8af9a

ceilometer/api/app.py

@@ -40,8 +40,17 @@
                ),
 ]
 
+api_opts = [
+    cfg.BoolOpt('pecan_debug',
+                default='$debug',
+                help='Toggle Pecan Debug Middleware. '
+                'Defaults to global debug value.'
+                ),
+]
+
 CONF = cfg.CONF
 CONF.register_opts(auth_opts)
+CONF.register_opts(api_opts, group='api')
 
 
 def get_pecan_config():
@@ -70,7 +79,7 @@ def setup_app(pecan_config=None, extra_hooks=None):
         pecan_config.app.root,
         static_root=pecan_config.app.static_root,
         template_path=pecan_config.app.template_path,
-        debug=CONF.debug,
+        debug=CONF.api.pecan_debug,
         force_canonical=getattr(pecan_config.app, 'force_canonical', True),
         hooks=app_hooks,
         wrap_app=middleware.ParsableErrorMiddleware,
@@ -83,7 +92,7 @@ def setup_app(pecan_config=None, extra_hooks=None):
 class VersionSelectorApplication(object):
     def __init__(self):
         pc = get_pecan_config()
-        pc.app.debug = CONF.debug
+        pc.app.debug = CONF.api.pecan_debug
 
         def not_found(environ, start_response):
             start_response('404 Not Found', [])

doc/source/install/mod_wsgi.rst

@@ -60,10 +60,6 @@ Limitation
 
 As Ceilometer is using Pecan and Pecan's DebugMiddleware doesn't support
 multiple processes, there is no way to set debug mode in the multiprocessing
-case. So user will run into HTTP 500 error if the mod_wsgi's multiprocessing
-is enabled and the Ceilometer debug mode is enabled at the same time. There
-is no good way to make both of them work, since Pecan is sharing the debug
-mode with Ceilometer, see[1]. If you really need to enable both, a possible
-workaround is hacking that line and hardcode the debug configration to False.
-
-[1] https://github.com/openstack/ceilometer/blob/master/ceilometer/api/app.py
+case. To allow multiple processes the DebugMiddleware may be turned off by
+setting ``pecan_debug`` to ``False`` in the ``api`` section of
+``ceilometer.conf``.