Enable starting executors in paused mode

In some cases, especially on systems under heavy load, it is
helpful to start executors in paused mode. Preventing them
to start accepting new jobs right away until such executors
are unpaused manually allows to test new features, configuration
or with analysing production problems.

Change-Id: I64c39e3b58c802577201280c855fdf7f13cc7538

doc/source/admin/components.rst

@@ -751,6 +751,12 @@ The following sections of ``zuul.conf`` are used by the executor:
       where it cannot determine its hostname correctly this can be overridden
       here.
 
+   .. attr:: paused_on_start
+      :default: false
+
+      Whether the executor should start in a paused mode. Such executor will not
+      accept tasks until it is unpaused.
+
    .. attr:: zone
       :default: None
 

zuul/executor/sensors/pause.py

@@ -20,8 +20,8 @@ from zuul.executor.sensors import SensorInterface
 class PauseSensor(SensorInterface):
     log = logging.getLogger("zuul.executor.sensor.pause")
 
-    def __init__(self):
-        self.pause = False
+    def __init__(self, pause=False):
+        self.pause = pause
 
     def isOk(self):
         if self.pause:

zuul/executor/server.py

@@ -2401,7 +2401,10 @@ class ExecutorServer(object):
                                               self.stopJobDiskFull,
                                               self.merge_root)
 
-        self.pause_sensor = PauseSensor()
+        self.pause_sensor = PauseSensor(get_default(self.config, 'executor',
+                                                    'paused_on_start', False))
+        self.log.info("Starting executor (hostname: %s) in %spaused mode" % (
+            self.hostname, "" if self.pause_sensor.pause else "un"))
         cpu_sensor = CPUSensor(config)
         self.sensors = [
             cpu_sensor,