Allow disabling Server-Timing entirely using PERFLAB_DISABLE_SERVER_TIMING constant

admin/server-timing.php

@@ -9,6 +9,11 @@
 	exit; // Exit if accessed directly.
 }
 
+// Do not add any of the hooks if Server-Timing is disabled.
+if ( defined( 'PERFLAB_DISABLE_SERVER_TIMING' ) && PERFLAB_DISABLE_SERVER_TIMING ) {
+	return;
+}
+
 /**
  * Adds the Server-Timing page to the Tools menu.
  *

load.php

@@ -316,8 +316,9 @@ function perflab_load_active_and_valid_modules() {
  * This only runs in WP Admin to not have any potential performance impact on
  * the frontend.
  *
- * This function will short-circuit if the constant
- * 'PERFLAB_DISABLE_OBJECT_CACHE_DROPIN' is set as true.
+ * This function will short-circuit if at least one of the constants
+ * 'PERFLAB_DISABLE_SERVER_TIMING' or 'PERFLAB_DISABLE_OBJECT_CACHE_DROPIN' is
+ * set as true.
  *
  * @since 1.8.0
  * @since 2.1.0 No longer attempts to use two of the drop-ins together.
@@ -327,6 +328,11 @@ function perflab_load_active_and_valid_modules() {
 function perflab_maybe_set_object_cache_dropin() {
 	global $wp_filesystem;
 
+	// Bail if Server-Timing is disabled entirely.
+	if ( defined( 'PERFLAB_DISABLE_SERVER_TIMING' ) && PERFLAB_DISABLE_SERVER_TIMING ) {
+		return;
+	}
+
 	// Bail if disabled via constant.
 	if ( defined( 'PERFLAB_DISABLE_OBJECT_CACHE_DROPIN' ) && PERFLAB_DISABLE_OBJECT_CACHE_DROPIN ) {
 		return;

server-timing/defaults.php

@@ -10,6 +10,11 @@
 	exit; // Exit if accessed directly.
 }
 
+// Do not add any of the hooks if Server-Timing is disabled.
+if ( defined( 'PERFLAB_DISABLE_SERVER_TIMING' ) && PERFLAB_DISABLE_SERVER_TIMING ) {
+	return;
+}
+
 /**
  * Registers the default Server-Timing metrics for before rendering the template.
  *

server-timing/load.php

@@ -10,6 +10,11 @@
 	exit; // Exit if accessed directly.
 }
 
+// Do not add any of the hooks if Server-Timing is disabled.
+if ( defined( 'PERFLAB_DISABLE_SERVER_TIMING' ) && PERFLAB_DISABLE_SERVER_TIMING ) {
+	return;
+}
+
 define( 'PERFLAB_SERVER_TIMING_SETTING', 'perflab_server_timing_settings' );
 define( 'PERFLAB_SERVER_TIMING_SCREEN', 'perflab-server-timing' );
 
@@ -27,6 +32,17 @@ function perflab_server_timing() {
 
 	if ( null === $server_timing ) {
 		$server_timing = new Perflab_Server_Timing();
+
+		/*
+		 * Do not add the hook for Server-Timing header output if it is entirely disabled.
+		 * While the constant checks on top of the file prevent this from happening by default, external code could
+		 * still call the `perflab_server_timing()` function. It needs to be ensured that such calls do not result in
+		 * fatal errors, but they should at least not lead to the header being output.
+		 */
+		if ( defined( 'PERFLAB_DISABLE_SERVER_TIMING' ) && PERFLAB_DISABLE_SERVER_TIMING ) {
+			return $server_timing;
+		}
+
 		add_filter( 'template_include', array( $server_timing, 'on_template_include' ), PHP_INT_MAX );
 	}
 

server-timing/object-cache.copy.php

@@ -41,12 +41,17 @@
 	/**
 	 * Loads the Performance Lab Server-Timing API if available.
 	 *
-	 * This function will short-circuit if the constant
+	 * This function will short-circuit if at least one of the constants
+	 * 'PERFLAB_DISABLE_SERVER_TIMING' or
 	 * 'PERFLAB_DISABLE_OBJECT_CACHE_DROPIN' is set as true.
 	 *
 	 * @since 1.8.0
 	 */
 	function perflab_load_server_timing_api_from_dropin() {
+		if ( defined( 'PERFLAB_DISABLE_SERVER_TIMING' ) && PERFLAB_DISABLE_SERVER_TIMING ) {
+			return;
+		}
+
 		if ( defined( 'PERFLAB_DISABLE_OBJECT_CACHE_DROPIN' ) && PERFLAB_DISABLE_OBJECT_CACHE_DROPIN ) {
 			return;
 		}