Prevent infinite loop filtering fonts directory. Props @costdev.

src/wp-includes/default-filters.php

@@ -751,6 +751,7 @@
 add_action( 'deleted_post', '_wp_after_delete_font_family', 10, 2 );
 add_action( 'before_delete_post', '_wp_before_delete_font_face', 10, 2 );
 add_action( 'init', '_wp_register_default_font_collections' );
+add_filter( 'font_dir', 'wp_filter_font_dir' );
 
 // It might be nice to use a filter instead of an action, but the `WP_REST_Templates_Controller` doesn't
 // provide one (unlike e.g. `WP_REST_Posts_Controller`, which has `rest_pre_insert_{$this->post_type}`).

src/wp-includes/fonts.php

@@ -96,8 +96,11 @@ function wp_unregister_font_collection( string $slug ) {
  *
  * @since 6.5.0
  *
- * @return array $defaults {
- *     Array of information about the upload directory.
+ * @param array|null $font_dir Optional. Array of unfiltered uploads directory.
+ *                             This is used when the function is called from
+ *                             the upload_dir filter. Default null.
+ * @return array {
+ *     Array of information about the font upload directory.
  *
  *     @type string       $path    Base directory and subdirectory or full path to the fonts upload directory.
  *     @type string       $url     Base URL and subdirectory or absolute URL to the fonts upload directory.
@@ -107,13 +110,48 @@ function wp_unregister_font_collection( string $slug ) {
  *     @type string|false $error   False or error message.
  * }
  */
-function wp_get_font_dir() {
+function wp_get_font_dir( $font_dir = null ) {
+	if ( doing_filter( 'font_dir' ) ) {
+		/*
+		 * The font_dir filter is being run, avoid an infinite loop.
+		 *
+		 * This indicates that a plugin is calling wp_upload_dir() while filtering
+		 * the font directory and avoids an infinite loop.
+		 */
+		return $font_dir;
+	}
+
+	/**
+	 * Filters the fonts directory data.
+	 *
+	 * This filter allows developers to modify the fonts directory data.
+	 *
+	 * @since 6.5.0
+	 *
+	 * @param array $font_dir The original fonts directory data.
+	 */
+	return apply_filters( 'font_dir', $font_dir );
+}
+
+/**
+ * Filters the fonts directory data.
+ *
+ * Modifies the default fonts directory data, runs on @see 'font_dir' filter.
+ *
+ * @since 6.5.0
+ * @access private
+ * @see wp_get_font_dir()
+ *
+ * @param array $font_dir The original fonts directory data.
+ * @return array The filtered fonts directory data.
+ */
+function wp_filter_font_dir( $font_dir ) {
 	$site_path = '';
 	if ( is_multisite() && ! ( is_main_network() && is_main_site() ) ) {
 		$site_path = '/sites/' . get_current_blog_id();
 	}
 
-	$defaults = array(
+	$font_dir = array(
 		'path'    => path_join( WP_CONTENT_DIR, 'fonts' ) . $site_path,
 		'url'     => untrailingslashit( content_url( 'fonts' ) ) . $site_path,
 		'subdir'  => '',
@@ -122,16 +160,7 @@ function wp_get_font_dir() {
 		'error'   => false,
 	);
 
-	/**
-	 * Filters the fonts directory data.
-	 *
-	 * This filter allows developers to modify the fonts directory data.
-	 *
-	 * @since 6.5.0
-	 *
-	 * @param array $defaults The original fonts directory data.
-	 */
-	return apply_filters( 'font_dir', $defaults );
+	return $font_dir;
 }
 
 /**