[core] only start collector/dogstatsd when needed

Before any call to `agent.py` or `dogstatsd.py` would actually create a
`Agent` or `Dogstatsd` instance, without doing anaything with it (and
diplaying a message about its pidfile).

This commit changes this behaviour and only create them when needed.

agent.py

@@ -77,6 +77,7 @@ def _handle_sigusr1(self, signum, frame):
         self._handle_sigterm(signum, frame)
         self._do_restart()
 
+    @classmethod
     def info(self, verbose=None):
         logging.getLogger().setLevel(logging.ERROR)
         return CollectorStatus.print_latest_status(verbose=verbose)
@@ -212,19 +213,24 @@ def main():
     autorestart = agentConfig.get('autorestart', False)
     hostname = get_hostname(agentConfig)
 
-    COMMANDS = [
+    COMMANDS_AGENT = [
         'start',
         'stop',
         'restart',
-        'foreground',
         'status',
+        'foreground',
+    ]
+
+    COMMANDS_NO_AGENT = [
         'info',
         'check',
         'configcheck',
         'jmx',
         'flare',
     ]
 
+    COMMANDS = COMMANDS_AGENT + COMMANDS_NO_AGENT
+
     if len(args) < 1:
         sys.stderr.write("Usage: %s %s\n" % (sys.argv[0], "|".join(COMMANDS)))
         return 2
@@ -240,12 +246,13 @@ def main():
         from utils.deprecations import deprecate_old_command_line_tools
         deprecate_old_command_line_tools()
 
-    pid_file = PidFile('dd-agent')
+    if command in COMMANDS_AGENT:
+        pid_file = PidFile('dd-agent')
 
-    if options.clean:
-        pid_file.clean()
+        if options.clean:
+            pid_file.clean()
 
-    agent = Agent(pid_file.get_path(), autorestart)
+        agent = Agent(pid_file.get_path(), autorestart)
 
     if command in START_COMMANDS:
         log.info('Agent version %s' % get_version())
@@ -266,7 +273,7 @@ def main():
         agent.status()
 
     elif 'info' == command:
-        return agent.info(verbose=options.verbose)
+        return Agent.info(verbose=options.verbose)
 
     elif 'foreground' == command:
         logging.info('Running in foreground')

daemon.py

@@ -227,7 +227,7 @@ def run(self):
         """
         raise NotImplementedError
 
-
+    @classmethod
     def info(self):
         """
         You should override this method when you subclass Daemon. It will be

dogstatsd.py

@@ -352,6 +352,7 @@ def run(self):
             if self.autorestart:
                 sys.exit(AgentSupervisor.RESTART_EXIT_STATUS)
 
+    @classmethod
     def info(self):
         logging.getLogger().setLevel(logging.ERROR)
         return DogstatsdStatus.print_latest_status()
@@ -424,16 +425,23 @@ def main(config_path=None):
     from utils.deprecations import deprecate_old_command_line_tools
     deprecate_old_command_line_tools()
 
+    COMMANDS_START_DOGSTATSD = [
+        'start',
+        'stop',
+        'restart',
+        'status'
+    ]
+
     parser = optparse.OptionParser("%prog [start|stop|restart|status]")
     parser.add_option('-u', '--use-local-forwarder', action='store_true',
                       dest="use_forwarder", default=False)
     opts, args = parser.parse_args()
 
-    reporter, server, cnf = init(config_path, use_watchdog=True,
-                                 use_forwarder=opts.use_forwarder, args=args)
-    pid_file = PidFile('dogstatsd')
-    daemon = Dogstatsd(pid_file.get_path(), server, reporter,
-                       cnf.get('autorestart', False))
+    if not args or args[0] in COMMANDS_START_DOGSTATSD:
+        reporter, server, cnf = init(config_path, use_watchdog=True, use_forwarder=opts.use_forwarder, args=args)
+        pid_file = PidFile('dogstatsd')
+        daemon = Dogstatsd(pid_file.get_path(), server, reporter,
+                           cnf.get('autorestart', False))
 
     # If no args were passed in, run the server in the foreground.
     if not args:
@@ -453,7 +461,7 @@ def main(config_path=None):
         elif command == 'status':
             daemon.status()
         elif command == 'info':
-            return daemon.info()
+            return Dogstatsd.info()
         else:
             sys.stderr.write("Unknown command: %s\n\n" % command)
             parser.print_help()