MU plugin for priming the attachment cache for WordPress

wpp-prime-attachment-cache.php

<?php
/**
 * Plugin Name: WPP Prime Attachment Cache
 * Description: MU plugin for priming the attachment cache.
 * Version: 1.0
 */

/**
 * Primes attachment into the cache with a single database query.
 *
 * @since 6.7.0
 *
 * @param string $content The HTML content.
 * @param string $context Optional. Additional context to pass to the filters.
 *                        Defaults to `current_filter()` when not set.
 * @return string The HTML content.
 */
function wp_prime_attachment_caches( $content, $context = null ) {
	
	if ( null === $context ) {
		$context = current_filter();
	}

	if ( ! preg_match_all( '/<img\s[^>]+>/', $content, $matches, PREG_SET_ORDER ) ) {
		return $content;
	}

	// List of the unique `img` tags found in $content.
	$images = array();

	foreach ( $matches as $match ) {
		list( $tag_name ) = $match;

		if ( preg_match( '/wp-image-([0-9]+)/i', $tag_name, $class_id ) ) {
			$attachment_id = absint( $class_id[1] );

			if ( $attachment_id ) {
				/*
					* If exactly the same image tag is used more than once, overwrite it.
					* All identical tags will be replaced later with 'str_replace()'.
					*/
				$images[ $tag_name ] = $attachment_id;
			}
		}
		$images[ $tag_name ] = 0;
	}

	// Reduce the array to unique attachment IDs.
	$attachment_ids = array_unique( array_filter( array_values( $images ) ) );

	if ( count( $attachment_ids ) > 1 ) {
		/*
		 * Warm the object cache with post and meta information for all found
		 * images to avoid making individual database calls.
		 */
		_prime_post_caches( $attachment_ids, false, true );
	}

	return $content;
}
add_filter( 'the_content', 'wp_prime_attachment_caches', 6 );