Fix inefficient rewrite flushing

composer.json

@@ -15,6 +15,7 @@
     "phpcompatibility/phpcompatibility-wp": "2.1.4",
     "wp-cli/wp-cli": "^2.9",
     "wp-coding-standards/wpcs": "3.0.1",
+    "wp-phpunit/wp-phpunit": "^6.4",
     "yoast/wp-test-utils": "1.2.0"
   },
   "minimum-stability": "dev",
@@ -60,4 +61,4 @@
       "npm run lint:staged"
     ]
   }
-}
+}

wp-includes/class-wp.php

@@ -12,21 +12,11 @@
  * @global WP_Rewrite $wp_rewrite WordPress rewrite component.
  */
 function pwa_add_rewrite_rules() {
-	global $wp_rewrite;
-	$rewrite_rule_regex = '^wp\.serviceworker$';
-	$rules              = $wp_rewrite->wp_rewrite_rules();
-
-	if ( ! isset( $rules[ $rewrite_rule_regex ] ) ) {
-		// Note: This logic will not be required as part of core merge since rewrite rules are flushed upon DB upgrade (as long as the DB version is bumped).
-		add_action(
-			'admin_init',
-			function () {
-				flush_rewrite_rules( false ); // phpcs:ignore WordPressVIPMinimum.Functions.RestrictedFunctions.flush_rewrite_rules_flush_rewrite_rules -- Not theme code.
-			}
-		);
-	}
-	add_rewrite_rule( $rewrite_rule_regex, 'index.php?' . WP_Service_Workers::QUERY_VAR . '=' . WP_Service_Workers::SCOPE_FRONT, 'top' );
-
+	add_rewrite_rule(
+		'^wp\.serviceworker$',
+		'index.php?' . WP_Service_Workers::QUERY_VAR . '=' . WP_Service_Workers::SCOPE_FRONT,
+		'top'
+	);
 	add_rewrite_tag( '%' . WP_Service_Workers::QUERY_VAR . '%', '([^?]+)' );
 }
 

wp-includes/service-workers.php

@@ -212,7 +212,7 @@ function wp_service_worker_loaded( WP_Query $query ) {
 		return;
 	}
 
-	// Handle case where rewrite rules have not yet been flushed.
+	// Do not require rewrite rules to have been flushed.
 	if ( 'wp.serviceworker' === $wp->request ) {
 		$query->set( WP_Service_Workers::QUERY_VAR, 1 );
 	}