Add cache domain to site/network query

[ocean90]

A cache domain allows to define a custom cache key to ensure unique results when one of the query filters, like sites_clauses, is used to customize the default query. This is necessary because the default cache key is only computed from the default query variables.

Previously: [18128] for terms and [38117] for comments.

[ocean90]

On second thought, you can use the parse_site_query to register a custom query var like so:

<?php
function add_custom_query_var_to_default_query_vars( $query ) {
        if ( ! empty( $query->query_vars['custom_query_var' ] ) ) {
                $query->query_var_defaults = array_merge( $query->query_var_defaults, [ 'custom_query_var' => '' ] );
        }
}
add_action( 'parse_site_query', __NAMESPACE__ . '\add_custom_query_var_to_default_query_vars' );

[flixos90]

Per Slack: ​https://wordpress.slack.com/archives/C03BVB47S/p1494269010909400

Let's get back to this ticket as necessary. @spacedmonkey expressed interest in working on this.

[ocean90]

@spacedmonkey Do you still have an use case for this? Otherwise I think we can close this as wontfix.
Worth to mention that when an argument is added any existing cache will be invalidated due to the new cache key unless we add some special handling for cache_domain = 'core'.