Bug #1414218 is not fixed on the stable/juno branch

[Stephen Ma]

On the stable/juno branch, https://review.openstack.org/#/c/164329 (ChangeId: I3ad7864eeb2f959549ed356a1e34fa18804395cc, fixed bug https://bugs.launchpad.net/neutron/+bug/1414218) was merged on April 1st. Less than 1 hour before this merge, https://review.openstack.org/#/c/153181 was merged. Both patches modified the same function _output_hosts_file() in the same file (neutron/agent/linux/dhcp.py). https://review.openstack.org/#/c/164329 removed LOG.debug statements from the _output_hosts_file while https://review.openstack.org/#/c/153181 added LOG.debug statements. The end result is that the bad performance problem fixed by https://review.openstack.org/#/c/164329 is reverted by https://review.openstack.org/#/c/153181 unintentionally.

The https://review.openstack.org/#/c/164329 fixes bug https://bugs.launchpad.net/neutron/+bug/1414218. The root cause is the performance overhead due to the LOG.debug statements in the for-loop of the _output_hosts_file() function.

This problem is only found on the stable/juno branch of neutron.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/juno
Review: https://review.openstack.org/174596

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: master
Review: https://review.openstack.org/175061

[OpenStack Infra (hudson-openstack)]

Change abandoned by stephen-ma (<email address hidden>) on branch: master
Review: https://review.openstack.org/175061
Reason: The unit test has been added in Change-Id I5d43021932d6a994638c348eda277dd8337cf041.

[OpenStack Infra (hudson-openstack)]

Change abandoned by stephen-ma (<email address hidden>) on branch: stable/juno
Review: https://review.openstack.org/174596