Factor PusherPool._start_pusher out of _start_pushers

... and use it from start_pusher_by_id. This mostly simplifies
start_pusher_by_id.

synapse/push/pusherpool.py

@@ -192,6 +192,9 @@ class PusherPool:
     @defer.inlineCallbacks
     def start_pusher_by_id(self, app_id, pushkey, user_id):
         """Look up the details for the given pusher, and start it"""
+        if not self._start_pushers:
+            return
+
         resultlist = yield self.store.get_pushers_by_app_id_and_pushkey(
             app_id, pushkey
         )
@@ -202,8 +205,7 @@ class PusherPool:
                 p = r
 
         if p:
-
-            self._start_pushers([p])
+            self._start_pusher(p)
 
     def _start_pushers(self, pushers):
         if not self.start_pushers:
@@ -211,12 +213,27 @@ class PusherPool:
             return
         logger.info("Starting %d pushers", len(pushers))
         for pusherdict in pushers:
+            self._start_pusher(pusherdict)
+        logger.info("Started pushers")
+
+    def _start_pusher(self, pusherdict):
+        """Start the given pusher
+
+        Args:
+            pusherdict (dict):
+
+        Returns:
+            None
+        """
         try:
             p = self.pusher_factory.create_pusher(pusherdict)
         except Exception:
             logger.exception("Couldn't start a pusher: caught Exception")
-                continue
-            if p:
+            return
+
+        if not p:
+            return
+
         appid_pushkey = "%s:%s" % (
             pusherdict['app_id'],
             pusherdict['pushkey'],
@@ -228,8 +245,6 @@ class PusherPool:
         byuser[appid_pushkey] = p
         run_in_background(p.on_started)
 
-        logger.info("Started pushers")
-
     @defer.inlineCallbacks
     def remove_pusher(self, app_id, pushkey, user_id):
         appid_pushkey = "%s:%s" % (app_id, pushkey)