cache-downloads-w-org.php

<?php
/**
 * Plugin Name: Redirect WordPress.org downloads through a local caching proxy.
 */

/**
 * PLEASE NOTE:
 * - Please only cache downloads for a short period of time, ZIPs may change even though the URL doesn't.
 * - This will cause download counts to not be counted on WordPress.org.
 * - downloads.wordpress.org serves: Plugins, Themes, Translations, and Core Update packages, amongst other things.
 */

function redirect_downloads_to_local_cache( $result, $args, $url ) {
	// If something has already filtered this request, respect it.
	if ( $result ) {
		return $result;
	}

	// Only cache certain hostnames, specifically the downloads hostnames.
	// NOTE: downloads.wordpress.org is the normal hostname, downloads.w.org may be used occasionally though.
	// Add any other restrictions as appropriate.
	$hostname = parse_url( $url, PHP_URL_HOST );
	if ( ! in_array( $hostname, [ 'downloads.wordpress.org', 'downloads.w.org' ] ) ) {
		return $result;
	}

	// Request the file from the cache proxy instead:
	$path      = parse_url( $url, PHP_URL_PATH );
	$cache_url = 'https://cache-downloads.example.com' . $path;
	$cached    = wp_remote_request( $cache_url, $args );
	if ( 200 === wp_remote_retrieve_response_code( $cached ) ) {
		// Only return the cached response if it's not an error.
		// Note: This uses a 200 response code instead of WP_Error checks, this means 404's will not be cached, but also cache server failures will not cause issues.
		return $cached;
	}

	// Return the original result in the event that the cache didn't, this will cause the original request to `https://downloads.wordpress.org/` to continue.
	return $result;
}
add_filter( 'pre_http_request', 'redirect_downloads_to_local_cache', 10, 3 );

// Example:
var_dump( wp_remote_get( 'https://downloads.wordpress.org/plugin-checksums/hello-dolly/1.7.2.json' ) );

// The request will first hit https://cache-downloads.example.com/plugin-checksums/hello-dolly/1.7.2.json and get a NXDOMAIN
// The request will then continue on to the original URL.
// If cache-downloads.example.com returned a 200 response though, that'd have been used instead of falling back to downloads.wordpress.org