Selectively import RPC backend retry config

Fixes bug 1244698

When setting the RPC publisher policies, selectively import
the retry config native to the configured RPC backend.

This avoid ImportErrors when all the dependencies of an unused
RPC backend are not installed locally.

Change-Id: If05a5a389a1b417875e0426fd83fb6d73d570487

ceilometer/publisher/rpc.py

@@ -57,8 +57,37 @@ def register_opts(config):
 
 register_opts(cfg.CONF)
 
-cfg.CONF.import_opt('rabbit_max_retries',
-                    'ceilometer.openstack.common.rpc.impl_kombu')
+
+def import_backend_retry_config():
+    """Import the retry config option native to the configured
+       rpc backend (if such a native config option exists).
+    """
+    cfg.CONF.import_opt('rpc_backend',
+                        'ceilometer.openstack.common.rpc')
+    kombu = 'ceilometer.openstack.common.rpc.impl_kombu'
+    if cfg.CONF.rpc_backend == kombu:
+        try:
+            cfg.CONF.import_opt('rabbit_max_retries', kombu)
+        except ImportError:
+            pass
+
+
+import_backend_retry_config()
+
+
+def override_backend_retry_config(value):
+    """Override the retry config option native to the configured
+       rpc backend (if such a native config option exists).
+
+       :param value: the value to override
+    """
+    # TODO(eglynn): ultimately we should add to olso a more generic concept
+    # of retry config (i.e. not specific to an individual AMQP provider)
+    # see: https://bugs.launchpad.net/ceilometer/+bug/1244698
+    kombu = 'ceilometer.openstack.common.rpc.impl_kombu'
+    if cfg.CONF.rpc_backend == kombu:
+        if 'rabbit_max_retries' in cfg.CONF:
+            cfg.CONF.set_override('rabbit_max_retries', value)
 
 
 def compute_signature(message, secret):
@@ -126,8 +155,8 @@ def __init__(self, parsed_url):
 
         if self.policy in ['queue', 'drop']:
             LOG.info('Publishing policy set to %s, \
-                     override rabbit_max_retries to 1' % self.policy)
-            cfg.CONF.set_override("rabbit_max_retries", 1)
+                     override backend retry config to 1' % self.policy)
+            override_backend_retry_config(1)
 
         elif self.policy == 'default':
             LOG.info('Publishing policy set to %s' % self.policy)