Filtered posts_request query can break found_posts query

[andy]

Suppose the posts_request query is built with SQL_CALC_FOUND_ROWS. The stage is set for WP_Query::set_found_posts issue SELECT FOUND_ROWS() because $limits is non-empty.

Now suppose a plugin filters posts_requests to the empty string because it gets results another way. WP_Query will still go ahead and issue SELECT FOUND_ROWS() erroneously.

Some plugins avoid this by filtering found_posts_query to the empty string. However, it seems like there is a better way to write the logic of set_found_posts so that it respects the filtered posts_request query and avoids the problematic statement: simply check the filtered query for SQL_CALC_FOUND_ROWS instead of looking at $limits.

Proposed fix:

 	private function set_found_posts( $q, $limits ) {
 		global $wpdb;
 		// Bail if posts is an empty array. Continue if posts is an empty string,
 		// null, or false to accommodate caching plugins that fill posts later.
 		if ( $q['no_found_rows'] || ( is_array( $this->posts ) && ! $this->posts ) )
 			return;
 
-		if ( ! empty( $limits ) ) {
+		if ( substr( $this->request, 0, 26 ) === 'SELECT SQL_CALC_FOUND_ROWS' ) ) {
 			/**
 			 * Filters the query to run for retrieving the found posts.
 			 *

[andy]

The proposed fix might actually break something for plugins that hook into the found_posts_query filter to clear that query.