REST API: Preloading silences deprecation errors

[dlh]

Loading a post in the block editor involves preloading data from the REST API via rest_preload_api_request(). This initializes the REST server, including rest_api_default_filters() on rest_api_init, which, in turn, invokes the REST deprecation handlers that covert the errors to headers rather than calling trigger_error().

To replicate, enable WP_DEBUG and add this hook:

<?php

add_action(
	'admin_footer',
	function () {
		_deprecated_function( 'oops', '1.2.3' );
	}
);

This code will trigger an error when /wp-admin/edit.php is loaded, but not /wp-admin/post-new.php.

The attached patch would address this inconsistency in a somewhat naive way by just checking for REST_REQUEST before adding the deprecation handlers (which would also address the problem for all internal uses of the API, not just preloading). I'm open to ideas that are a little leaner.

[TimothyBlynJacobs]

I think this is one of the times that checking for REST_REQUEST is the right way to go about it.

[whyisjake]

In 48150:

REST API: Ensure depracation errors are called while preloading data with the REST API.

Fixes #50318.

Props dlh, TimothyBlynJacobs.