Re-add caching to wp_theme_has_theme_json(), but only when not in a WP Admin request

src/wp-includes/global-styles-and-settings.php

@@ -264,6 +264,48 @@ function ( $item ) {
  * @return bool Returns true if theme or its parent has a theme.json file, false otherwise.
  */
 function wp_theme_has_theme_json() {
+	/*
+	 * Ignore cache when `WP_DEBUG` is enabled, so it doesn't interfere with the theme
+	 * developer's workflow. Also ignore it when in a WP Admin request, to prevent
+	 * errors when used in the limited load-styles.php request.
+	 *
+	 * @todo Replace `WP_DEBUG` once an "in development mode" check is available in Core.
+	 */
+	$can_use_cache = ! WP_DEBUG && ! is_admin();
+
+	if ( $can_use_cache ) {
+		/*
+		 * By using the 'theme_json' group, this data is marked to be non-persistent across requests.
+		 * @see `wp_cache_add_non_persistent_groups()`.
+		 *
+		 * The rationale for this is to make sure derived data from theme.json
+		 * is always fresh from the potential modifications done via hooks
+		 * that can use dynamic data (modify the stylesheet depending on some option,
+		 * settings depending on user permissions, etc.).
+		 * For some of the existing hooks to modify theme.json behavior:
+		 * @see https://make.wordpress.org/core/2022/10/10/filters-for-theme-json-data/
+		 *
+		 * A different alternative considered was to invalidate the cache upon certain
+		 * events such as options add/update/delete, user meta, etc.
+		 * It was judged not enough, hence this approach.
+		 * @see https://github.com/WordPress/gutenberg/pull/45372
+		 */
+		$cache_group       = 'theme_json';
+		$cache_key         = 'wp_theme_has_theme_json';
+		$theme_has_support = wp_cache_get( $cache_key, $cache_group );
+
+		/*
+		 * $theme_has_support is stored as an int in the cache.
+		 *
+		 * The reason not to store it as a boolean is to avoid working
+		 * with the $found parameter which apparently had some issues in some implementations
+		 * @see https://developer.wordpress.org/reference/functions/wp_cache_get/
+		 */
+		if ( is_int( $theme_has_support ) ) {
+			return (bool) $theme_has_support;
+		}
+	}
+
 	// Does the theme have its own theme.json?
 	$theme_has_support = is_readable( get_stylesheet_directory() . '/theme.json' );
 
@@ -272,7 +314,12 @@ function wp_theme_has_theme_json() {
 		$theme_has_support = is_readable( get_template_directory() . '/theme.json' );
 	}
 
-	return $theme_has_support;
+	if ( $can_use_cache ) {
+		$theme_has_support = $theme_has_support ? 1 : 0;
+		wp_cache_set( $cache_key, $theme_has_support, $cache_group );
+	}
+
+	return (bool) $theme_has_support;
 }
 
 /**
@@ -281,5 +328,6 @@ function wp_theme_has_theme_json() {
  * @since 6.2.0
  */
 function wp_clean_theme_json_cache() {
+	wp_cache_delete( 'wp_theme_has_theme_json', 'theme_json' );
 	WP_Theme_JSON_Resolver::clean_cached_data();
 }

src/wp-includes/load.php

@@ -753,7 +753,7 @@ function wp_start_object_cache() {
 			)
 		);
 
-		wp_cache_add_non_persistent_groups( array( 'counts', 'plugins' ) );
+		wp_cache_add_non_persistent_groups( array( 'counts', 'plugins', 'theme_json' ) );
 	}
 
 	$first_init = false;

src/wp-includes/ms-blogs.php

@@ -575,7 +575,7 @@ function switch_to_blog( $new_blog_id, $deprecated = null ) {
 				);
 			}
 
-			wp_cache_add_non_persistent_groups( array( 'counts', 'plugins' ) );
+			wp_cache_add_non_persistent_groups( array( 'counts', 'plugins', 'theme_json' ) );
 		}
 	}
 
@@ -666,7 +666,7 @@ function restore_current_blog() {
 				);
 			}
 
-			wp_cache_add_non_persistent_groups( array( 'counts', 'plugins' ) );
+			wp_cache_add_non_persistent_groups( array( 'counts', 'plugins', 'theme_json' ) );
 		}
 	}
 

tests/phpunit/includes/abstract-testcase.php

@@ -401,7 +401,7 @@ public static function flush_cache() {
 			)
 		);
 
-		wp_cache_add_non_persistent_groups( array( 'counts', 'plugins' ) );
+		wp_cache_add_non_persistent_groups( array( 'counts', 'plugins', 'theme_json' ) );
 	}
 
 	/**