Confusing behavior of default `image_cache_manager_interval` and `instance_delete_interval`

[Riccardo Murri]

In all periodic tasks, an "interval" setting of 0 means that the task is executed at every run of the scheduler.
Well, all except `image_cache_manager_interval`, where a setting of 0 means "disabled"!

Excerpt from Folsom's `nova/compute/manager.py` (lines 2926--2932):

    @manager.periodic_task(
        ticks_between_runs=FLAGS.image_cache_manager_interval)
    def _run_image_cache_manager_pass(self, context):
        """Run a single pass of the image cache manager."""

        if FLAGS.image_cache_manager_interval == 0:
            return
        # ...

What is even more confusing is that the periodic command scheduler will log "Running periodic task ComputeManager._run_image_cache_manager_pass" *before* entering the `_run_image_cache_manager_pass` method, so, according to the logs, the task is executing normally...

Please make `image_cache_manager_interval` behave like the other intervals, or -at least- log a line to the extent that the task is disabled.

Running Folsom's python-nova on Ubuntu 12.04:

# dpkg -l python-nova
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Description
+++-=================================-=================================-==================================================================================
ii python-nova 2012.2.1+stable-20121212-a99a802e OpenStack Compute Python libraries

[Matt Fischer (mfisch)]

This also applies to the delete task periodic job:

if CONF.instance_delete_interval == 0:
        return

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/119585
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=c1298f13e10c9d6b4d7c11968ee453c64c556006
Submitter: Jenkins
Branch: master

commit c1298f13e10c9d6b4d7c11968ee453c64c556006
Author: Davanum Srinivas <email address hidden>
Date: Sat Sep 6 22:28:21 2014 -0400

    Add warning to periodic_task with interval 0

    In Ia227f4c4e69ecf361ab02d1d17a3010303650104, warnings were
    added for several options but instance_delete_interval was
    left out by mistake.

    Closes-Bug: #1188226
    Change-Id: I20b396d664e40252743cac4f2a49f4f34882b32f