Move wp_oembed_add_discovery_links() up to print at wp_head priority 4

[westonruter]

This addresses an issue whereby if there is a lot of markup printed at wp_head (e.g. inline styles from Core-63018), WordPress sites looking for the oEmbed discovery links will fail to find them because they only read the first 150KB of the HTML response, by default:

wordpress-develop/src/wp-includes/class-wp-oembed.php

Line 444 in bafd51b

'limit_response_size' => 153600, // 150 KB

Note that scripts and styles are printed at priorities 8 and 9:

wordpress-develop/src/wp-includes/default-filters.php

Lines 355 to 356 in bafd51b

	add_action( 'wp_head', 'wp_print_styles', 8 );
	add_action( 'wp_head', 'wp_print_head_scripts', 9 );

In order to ensure the oEmbed links remain discoverable, it seems prudent to print them early, specifically right after other similar links are printed, namely feed_links() and feed_links_extra().

It is not so simple as changing the action priority from 10 to 4:

wordpress-develop/src/wp-includes/default-filters.php

Line 707 in bafd51b

add_action( 'wp_head', 'wp_oembed_add_discovery_links' );

This is because there are many plugins which remove the oEmbed discovery links via:

remove_action( 'wp_head', 'wp_oembed_add_discovery_links' );

See GitHub Search (1.7k files) and WPDirectory (145 plugins).

Aside: The better way to do this would be to do:

add_filter( 'oembed_discovery_links', '__return_empty_string' );

In order to maintain backwards-compatibility with the ecosystem which unhooks the action in this way, it is necessary to add a second instance of wp_oembed_add_discovery_links running at wp_head with the earlier priority of 4. Inside of wp_oembed_add_discovery_links(), when it runs first at priority 4, it can check if it has been unhooked at priority 10, and if so, short-circuit. Otherwise, it can continue with printing the oEmbed discovery links, but also preventing them from being duplicated by itself doing:

remove_action( 'wp_head', 'wp_oembed_add_discovery_links' );

In order to facilitate this, it is necessary to extend the has_action()/has_filter() functions with a new $priority parameter to be able to check whether a given callback is added at a specific priority. This is something I've missed in the past. It also "feels right" given that add_action()/remove_action()/add_filter()/remove_filter() all have an optional $priority parameter. See examples on GitHub and on WP Directory where ecosystem code assumed there was such a $priority parameter. This is important because the same callback can be added multiple times on the same hook with different priorities.

A similar approach to backwards-compatibility has been done recently, with wp_print_auto_sizes_contain_css_fix() in example r60910. The difference there is that a new function was introduced, whereas here the function remains the same.

TODO

• Add tests

Trac ticket: https://core.trac.wordpress.org/ticket/64178

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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[swissspidy]

If this is what it takes to maintain back compat... I just hope we never have to change the priority again :-)

[westonruter]

I've opened #10456 to introduce the has_filter()/has_action() change in isolation. Tests have been added there.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 61119
GitHub commit: 972d21c

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.