[FIX] bus: compatibility with Odoo's PAAS external long polling handler

Only start the bus events dispatcher when needed (lazy starting).
On Odoo PAAS it's never started because the platform has its own event
dispatcher.

addons/bus/models/bus.py

@@ -108,6 +108,7 @@ class ImBus(models.Model):
 class ImDispatch(object):
     def __init__(self):
         self.channels = {}
+        self.started = False
 
     def poll(self, dbname, channels, last, options=None, timeout=TIMEOUT):
         if options is None:
@@ -129,6 +130,10 @@ class ImDispatch(object):
             notifications = env['bus.bus'].poll(channels, last, options)
         # or wait for future ones
         if not notifications:
+            if not self.started:
+                # Lazy start of events listener
+                self.start()
+
             event = self.Event()
             for channel in channels:
                 self.channels.setdefault(hashable(channel), []).append(event)
@@ -178,15 +183,16 @@ class ImDispatch(object):
             import gevent
             self.Event = gevent.event.Event
             gevent.spawn(self.run)
-        elif odoo.multi_process:
-            # disabled in prefork mode
-            return
         else:
             # threaded mode
             self.Event = threading.Event
             t = threading.Thread(name="%s.Bus" % __name__, target=self.run)
             t.daemon = True
             t.start()
+        self.started = True
         return self
 
-dispatch = ImDispatch().start()
+dispatch = None
+if not odoo.multi_process or odoo.evented:
+    # We only use the event dispatcher in threaded and gevent mode
+    dispatch = ImDispatch()