Move validation background tasks behind feature flag filter until Site Scanning is implemented

[westonruter]

I've been thinking about the new cron jobs for checking site validation, specifically these introduced in #5515 for #1756:

• The amp_validate_urls cron which checks a sample set of URLs.
• The amp_single_post_validate cron which checks a URL after a post is published and the user doesn't have DevTools enabled.

At the moment, these two cron jobs are responsible for generating validation data which we are not yet using. We do not have the aforementioned notification center to communicate AMP Site Health and we haven't yet implemented Site Scanning as part of the onboarding wizard. Validation data is being generated even when DevTools is disabled now. This causes a problem also because we don't have any garbage collection of validation data (#4779). Therefore, since we aren't using the output of these additional validation checks, I propose that we put them behind a feature flag (internal filter) and turn them off for 2.1. This will allow us to punt a couple more issues to 2.2 to coincide with Site Scanning which is what they are the foundation for. Specifically #5750 and #4779.

[codecov]

Codecov Report

Merging #5892 (850e6de) into develop (d12bd98) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##             develop    #5892      +/-   ##
=============================================
- Coverage      75.16%   75.12%   -0.04%     
- Complexity      5665     5667       +2     
=============================================
  Files            210      210              
  Lines          17019    17023       +4     
=============================================
- Hits           12792    12789       -3     
- Misses          4227     4234       +7     

Flag	Coverage Δ	Complexity Δ
javascript	75.13% <ø> (ø)	0.00 <ø> (ø)
php	75.12% <100.00%> (-0.04%)	0.00 <2.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
src/Validation/SavePostValidationEvent.php	91.66% <100.00%> (-2.46%)	18.00 <1.00> (+1.00)	⬇️
src/Validation/URLValidationCron.php	73.68% <100.00%> (-20.44%)	8.00 <1.00> (+1.00)	⬇️
...c/BackgroundTask/SingleScheduledBackgroundTask.php	83.33% <0.00%> (-16.67%)	5.00% <0.00%> (ø%)

[github-actions]

Plugin builds for f12c5d5 are ready 🛎️!

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[pierlon]

On second thought, I think the related tests should be updated accordingly.

[pierlon]

Updated tests in f12c5d5.

[pierlon]

QA Passed

Getting a list of scheduled cron events with wp cron event list does not include the amp_validate_urls nor amp_single_post_validate cron hooks.

After activating a plugin with the following filter:

add_filter( 'amp_temp_validation_cron_tasks_enabled', '__return_true' );

I verified that the amp_validate_urls hook was scheduled by running wp cron event list:

+------------------------------------+---------------------+-----------------------+------------+
| hook                               | next_run_gmt        | next_run_relative     | recurrence |
+------------------------------------+---------------------+-----------------------+------------+
| amp_validate_urls                  | 2021-04-27 17:58:59 | now                   | 1 day      |
+------------------------------------+---------------------+-----------------------+------------+

To verify that the amp_single_post_validate cron event is being scheduled, I added a filter to capture cron events before they're scheduled:

add_filter( 'pre_schedule_event', function ( $pre, $event ) {
	error_log( 'Scheduling: ' . $event->hook );
	return $pre;
}, 10, 2 );

When Developer Tools is disabled and I go to publish a post, I can see that the line Scheduling: amp_single_post_validate was logged, confirming that the cron event was scheduled.