wp_get_global_stylesheet: substitute 1-minute transient by non-persistent object cache

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

@@ -83,18 +83,34 @@ function wp_get_global_styles( $path = array(), $context = array() ) {
  * @return string Stylesheet.
  */
 function wp_get_global_stylesheet( $types = array() ) {
-	// Return cached value if it can be used and exists.
-	// It's cached by theme to make sure that theme switching clears the cache.
-	$can_use_cached = (
-		( empty( $types ) ) &&
-		( ! defined( 'WP_DEBUG' ) || ! WP_DEBUG ) &&
-		( ! defined( 'SCRIPT_DEBUG' ) || ! SCRIPT_DEBUG ) &&
-		( ! defined( 'REST_REQUEST' ) || ! REST_REQUEST ) &&
-		! is_admin()
-	);
-	$transient_name = 'global_styles_' . get_stylesheet();
+	/*
+	 * Ignore cache when `WP_DEBUG` is enabled, so it doesn't interfere with the theme
+	 * developer's workflow.
+	 *
+	 * @todo Replace `WP_DEBUG` once an "in development mode" check is available in Core.
+	 */
+	$can_use_cached = empty( $types ) && ! WP_DEBUG;
+
+	/*
+	 * 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.).
+	 * See 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_get_global_stylesheet';
 	if ( $can_use_cached ) {
-		$cached = get_transient( $transient_name );
+		$cached = wp_cache_get( $cache_key, $cache_group );
 		if ( $cached ) {
 			return $cached;
 		}
@@ -150,11 +166,8 @@ function wp_get_global_stylesheet( $types = array() ) {
 	}
 
 	$stylesheet = $styles_variables . $styles_rest;
-
 	if ( $can_use_cached ) {
-		// Cache for a minute.
-		// This cache doesn't need to be any longer, we only want to avoid spikes on high-traffic sites.
-		set_transient( $transient_name, $stylesheet, MINUTE_IN_SECONDS );
+		wp_cache_set( $cache_key, $stylesheet, $cache_group );
 	}
 
 	return $stylesheet;

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' ) );
 	}
 
 	/**

tests/phpunit/tests/theme/wpGetGlobalStylesheet.php

@@ -270,4 +270,18 @@ public function test_should_enqueue_stored_styles() {
 			'Registered styles with handle of "wp-style-engine-my-styles" do not match expected value from the Style Engine store.'
 		);
 	}
+	/**
+	 * Tests that switching themes recalculates the stylesheet.
+	 *
+	 * @ticket 56970
+	 */
+	public function test_switching_themes_should_recalculate_stylesheet() {
+		$stylesheet_for_default_theme = wp_get_global_stylesheet();
+		switch_theme( 'block-theme' );
+		$stylesheet_for_block_theme = wp_get_global_stylesheet();
+		switch_theme( WP_DEFAULT_THEME );
+
+		$this->assertStringNotContainsString( '--wp--preset--font-size--custom: 100px;', $stylesheet_for_default_theme, 'custom font size (100px) not present for default theme' );
+		$this->assertStringContainsString( '--wp--preset--font-size--custom: 100px;', $stylesheet_for_block_theme, 'custom font size (100px) is present for block theme' );
+	}
 }