Fix race condition by using lock on enable_radvd

For an HA Router, radvd is spawned only in the Master Node.
KeepalivedStateChangeHandler after receiving the state notification
on the unix domain socket, would either enable/disable the radvd
process depending on the state of the HA Router. In parallel, if
there is a request to add an interface to the router, router.process()
would try to enable radvd in parallel. So there is a possible race
condition as two threads try to enable radvd in parallel. This patch
uses appropriate locking mechanism to prevent this race condition.

Closes-Bug: #1466663
Change-Id: I40df78cc57fcde5512336d9dfef88b3b06a69bcb
(cherry picked from commit 27503dad33)

neutron/agent/l3/ha_router.py

@@ -364,6 +364,7 @@ class HaRouter(router.RouterInfo):
         if self.ha_port:
             self.enable_keepalived()
 
+    @common_utils.synchronized('enable_radvd')
     def enable_radvd(self, internal_ports=None):
         if (self.keepalived_manager.get_process().active and
                 self.ha_state == 'master'):