Add backwards compatibility for the blog subdomain

Still send pings for the old `blog.opensource.org` Feed-URLs.

Author: pfefferle

.gitignore

@@ -18,6 +18,7 @@ mu-plugins/*
 !mu-plugins/mu-autoloader.php
 !mu-plugins/osi-event-list
 !mu-plugins/osi-sponsors-list
+!websub-compat.php
 
 themes/*
 !themes/osi

mu-plugins/websub-compat.php

@@ -0,0 +1,37 @@
+<?php
+/**
+ * Plugin Name: WebSub Backwards Compatibility
+ * Version: 1.0.0
+ * Author: WordPress.com Special Projects
+ * Author URI: https://wpspecialprojects.wordpress.com/
+ */
+
+if ( ! defined( 'ABSPATH' ) ) {
+	return;
+}
+
+/**
+ * Filter `pubsubhubbub_feed_urls` and `pubsubhubbub_comment_feed_urls`.
+ *
+ * Still send pings for the old `blog.opensource.org` Feed-URLs.
+ *
+ * @param array $links Array of Feed URLs.
+ *
+ * @return array $links Array of filtered Feed URLs.
+ */
+function websub_compat( $links ) {
+	if ( ! $links || ! is_array( $links ) ) {
+		return $links;
+	}
+
+	$host         = wp_parse_url( get_home_url(), PHP_URL_HOST );
+	$compat_links = array();
+
+	foreach ( $links as $link ) {
+		$compat_links[] = str_replace( $host, 'blog.' . $host, $link );
+	}
+
+	return array_unique( array_merge( $links, $compat_links ) );
+}
+add_filter( 'pubsubhubbub_feed_urls', 'websub_compat' );
+add_filter( 'pubsubhubbub_comment_feed_urls', 'websub_compat' );