Add kill switch for cron.

[tollmanz]

When initially deploying this plugin, hundreds or thousands of cron event can be created to generate site maps over time. While this may be desirable for some, others may want to generate the sitemaps manually with WP CLI.

In order to do this without the interference of cron, there needs to be a way to shut down the cron events temporarily. This PR does just that.

wp msm-sitemaps cron --disable is introduced to set a option variable. When this option variable is set, the cron file is not loaded and thus the actions tied to the cron events won't execute. Additionally, the command removes all of the msm_cron_generate_sitemap_for_year_month_day events from the cron event list. This is more or less for cleaning up that list for sanity.

Once a developer wants to enable cron events again, she can do so with wp msm-sitemaps cron --enable. This removes the option that blocks the cron file from loading.

While I've explained this PR in the context of a specific use case, this cron disabling feature is also useful if a developer deploys this plugin and needs a quick way to kill the cron actions (perhaps the sitemap generation has caused too much load).

Note that this does not affect the msm_cron_update_sitemap which is part of the main plugin file. This is because it is not part of the cron file. Additionally, this cron action is harmless without the accompanying cron file.

[mjangda]

hundreds or thousands of cron event can be created to generate site maps over time. While this may be desirable for some

I don't know if this is desirable for anyone, actually, other than small sites (which are probably not a good target for this plugin).

Maybe we should have cron disabled by default? (We don't even actually use the cron generator on wp.com; we funnel all the sitemap generation to async jobs.)

[tollmanz]

I don't know if this is desirable for anyone, actually, other than small sites (which are probably not a good target for this plugin).

Probably true. My biggest concern is back compat. Ideally, the plugin wouldn't do anything initially. You would have to take an action to start building the sitemaps. Additionally, there would be easy ways to stop it.

In my testing, the msm_cron_update_sitemap will kick off the flurry of cron events (I was seeing ~300), spaced out by 60 seconds, as soon as I activated the plugin. This happened without invoking any generate action.

Maybe we should have cron disabled by default? (We don't even actually use the cron generator on wp.com; we funnel all the sitemap generation to async jobs.)

I think that this might be an interesting idea. Perhaps it is disabled initially and as soon as the full sitemap is generated, crons can be registered. This assumes that people will have an alternative way to generating the initial site map. For larger sites, I don't think this would be an issue. For smaller sites, it definitely would be.

[joshbetz]

For larger sites, I don't think this would be an issue. For smaller sites, it definitely would be.

Mo alluded to this, and I agree, that it should be fine to target big sites. The background for this plugin is to provide compressive sitemaps for (large) sites where other plugins otherwise can't because of the size.

[tollmanz]

Mo alluded to this, and I agree, that it should be fine to target big sites.

WFM

[pkevan]

+1 for removing the initial cron by default.

When this was initially developed, the cli wasn't present but can't really think of any reason to keep that option given the context of targeting large sites.

We'd just need to make sure it was clear that to generate the initial sitemap requires additional actions, whether in activation or readme docs (prob both).