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It could happen that when multiple instances are starting at the same time on a KVM host the Agent spawns multiple instances of security_group.py which both try to modify iptables/ebtables rules. This fails with on of the two processes failing. The instance is still started, but it doesn't have any IP connectivity due to the failed programming of the security groups. This modification lets the script aqcuire a exclusive lock on a file so that only one instance of the scripts talks to iptables/ebtables at once. Other instances of the script which start will poll every 500ms if they can obtain the lock and otherwise execute anyway after 15 seconds. The lock will be released as soon as the script exists, which is usually within a few hundred ms.