Wait that Nagios creates its FIFO file

Change-Id: I1a78db73be23d65bf021a561b7b330f893202f2d
Closes-bug: #1595975

deployment_scripts/puppet/modules/nagios/files/process-service-checks.wsgi

@@ -17,6 +17,7 @@
 # command PROCESS_SERVICE_CHECK_RESULT
 #
 
+import os
 import time
 from urlparse import parse_qs
 
@@ -25,6 +26,10 @@ PROCESS_CHECK_CMD = 'PROCESS_SERVICE_CHECK_RESULT'
 VALID_STATES = range(0, 4)
 
 
+class NagiosNotReady(Exception):
+    pass
+
+
 def http_response(response, status_string, output=None):
     if output is None:
         output = ''
@@ -79,6 +84,9 @@ def validate_params(params):
 
 
 def write_command(cmd_file, p, timestamp):
+    if not os.path.exists(cmd_file):
+        raise NagiosNotReady()
+
     cmd = "[{timestamp}] {cmd};{host};{service};{state};{output}".format(
         timestamp=timestamp,
         cmd=PROCESS_CHECK_CMD,
@@ -112,7 +120,9 @@ def application(environ, response):
     cmd_file = environ.get('NAGIOS_CMD_FILE', '/var/lib/nagios3/rw/nagios.cmd')
     try:
         write_command(cmd_file, params, timestamp)
-    except e:
+    except NagiosNotReady:
+        return http_response(response, '503 Service Unavailable')
+    except Exception as e:
         return http_response(response, '500 Internal Server Error', str(e))
 
     return http_response(response, '204 No Content')