Short-circuit validate_url() when attempting validation requests to other sites (which will always fail)

[westonruter]

Summary

This fixes an issue I noticed when running wp amp validation run on a site which is configured to redirect permalinks for a custom post type to other sites. As a quick way to reproduce this issue, add the following plugin code:

add_filter(
	'page_link',
	function ( $url ) {
		if ( defined( 'WP_CLI' ) ) {
			$url = 'https://example.com/';
		}
		return $url;
	}
);

The output is as follows:

$ wp amp validation run --limit=1 --include=is_singular
Crawling the site for AMP validity.
Validating 3 URLs...  33 % [=================================================================>                                                                                                                                   ] 0:00 / 0:00
Warning: Validate URL error (response_not_json):  URL: https://example.com/
Validating 3 URLs...  100% [=====================================================================================================================================================================================================] 0:02 / 0:02
Success: 0 crawled URLs have invalid markup kept out of 0 total with AMP validation issue(s); 2 URLs were crawled.
+--------------------------+-----------+---------------+
| Template or content type | URL Count | Validity Rate |
+--------------------------+-----------+---------------+
| post                     | 1         | 100%          |
| attachment               | 1         | 100%          |
+--------------------------+-----------+---------------+
For more details, please see: https://wordpressdev.lndo.site/core-dev/src/wp-admin/edit.php?post_type=amp_validated_url

The output is a bit messy, but notice this line in particular:

Warning: Validate URL error (response_not_json): URL: https://example.com/

This means it's trying to fetch that other domain as if it is one of the URLs on the site. This obviously won't work. So we should short-circuit such requests from happening in the first place. This is addressed by this PR, so running the above command instead produces the following output:

$ wp amp validation run --limit=1 --include=is_singular
Crawling the site for AMP validity.
Validating 3 URLs...  33 % [=================================================================>                                                                                                                                   ] 0:00 / 0:00
Warning: Validate URL error (http_request_failed): Unable to validate a URL on another site. Attempted to validate: https://example.com/ URL: https://example.com/
Validating 3 URLs...  100% [=====================================================================================================================================================================================================] 0:02 / 0:03
Success: 0 crawled URLs have invalid markup kept out of 0 total with AMP validation issue(s); 2 URLs were crawled.
+--------------------------+-----------+---------------+
| Template or content type | URL Count | Validity Rate |
+--------------------------+-----------+---------------+
| post                     | 1         | 100%          |
| attachment               | 1         | 100%          |
+--------------------------+-----------+---------------+
For more details, please see: https://wordpressdev.lndo.site/core-dev/src/wp-admin/edit.php?post_type=amp_validated_url

Notice the message here instead:

Warning: Validate URL error (http_request_failed): Unable to validate a URL on another site. Attempted to validate: https://example.com/ URL: https://example.com/

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[github-actions]

Plugin builds for 73059f9 are ready 🛎️!

• Download development build
• Download production build

[pierlon]

LGTM.

[pierlon]

QA Passed

With a plugin active with the following code:

add_filter(
	'page_link',
	function ( $url ) {
		if ( defined( 'WP_CLI' ) ) {
			$url = 'https://example.com/';
		}
		return $url;
	}
);

Running the WP CLI command wp amp validation run --limit=1 --include=is_singular results in the output:

Crawling the site for AMP validity.
Validating 3 URLs...  33 % [=====================>                                           ] 0:00 / 0:00
Warning: Validate URL error (http_request_failed): Unable to validate a URL on another site. Attempted to validate: https://example.com/?amp=1 URL: https://example.com/
Validating 3 URLs...  100% [=================================================================] 0:01 / 0:01
Success: 0 crawled URLs have invalid markup kept out of 2 total with AMP validation issue(s); 2 URLs were crawled.
+--------------------------+-----------+---------------+
| Template or content type | URL Count | Validity Rate |
+--------------------------+-----------+---------------+
| post                     | 1         | 100%          |
| attachment               | 1         | 100%          |
+--------------------------+-----------+---------------+
For more details, please see: https://amp-qa.test/wp-admin/edit.php?post_type=amp_validated_url

Most notably the external URL being validated now results in a http_request_failed error and not a response_not_json.