Merge pull request #1260 from WordPress/fix/server-timing-output-buffer-priority

Fix Server-Timing compatibility with other plugins that do output buffering

Author: westonruter

plugins/performance-lab/includes/server-timing/class-perflab-server-timing.php

@@ -225,6 +225,25 @@ public function use_output_buffer(): bool {
 		return (bool) apply_filters( 'perflab_server_timing_use_output_buffer', $enabled );
 	}
 
+	/**
+	 * Adds hooks to send the Server-Timing header.
+	 *
+	 * When output buffering is enabled, buffer as early as possible so that any other plugins that also do output
+	 * buffering will be able to register Server-Timing metrics. The first output buffer callback to be registered
+	 * is the last one to be called, so by starting the Server-Timing output buffer as soon as possible we can be
+	 * assured that other plugins' output buffer callbacks will run before the Server-Timing one that sends the
+	 * Server-Timing header.
+	 *
+	 * @since n.e.x.t
+	 */
+	public function add_hooks(): void {
+		if ( $this->use_output_buffer() ) {
+			add_action( 'template_redirect', array( $this, 'start_output_buffer' ), PHP_INT_MIN );
+		} else {
+			add_filter( 'template_include', array( $this, 'on_template_include' ), PHP_INT_MAX );
+		}
+	}
+
 	/**
 	 * Hook callback for the 'template_include' filter.
 	 *
@@ -238,18 +257,22 @@ public function use_output_buffer(): bool {
 	 * @return mixed Unmodified value of $passthrough.
 	 */
 	public function on_template_include( $passthrough = null ) {
-		if ( ! $this->use_output_buffer() ) {
-			$this->send_header();
-			return $passthrough;
-		}
+		$this->send_header();
+		return $passthrough;
+	}
 
+	/**
+	 * Starts output buffering to send the Server-Timing header right before returning the buffer.
+	 *
+	 * @since n.e.x.t
+	 */
+	public function start_output_buffer(): void {
 		ob_start(
 			function ( $output ) {
 				$this->send_header();
 				return $output;
 			}
 		);
-		return $passthrough;
 	}
 
 	/**

plugins/performance-lab/includes/server-timing/load.php

@@ -45,7 +45,7 @@ function perflab_server_timing(): Perflab_Server_Timing {
 			return $server_timing;
 		}
 
-		add_filter( 'template_include', array( $server_timing, 'on_template_include' ), PHP_INT_MAX );
+		$server_timing->add_hooks();
 	}
 
 	return $server_timing;

tests/plugins/performance-lab/includes/server-timing/test-load.php

@@ -16,12 +16,29 @@ public function test_perflab_server_timing(): void {
 		$this->assertTrue( has_filter( 'template_include' ) );
 
 		$server_timing = perflab_server_timing();
+		$this->assertFalse( $server_timing->use_output_buffer() );
 		$this->assertSame( PHP_INT_MAX, has_filter( 'template_include', array( $server_timing, 'on_template_include' ) ), 'template_include filter not added' );
+		$this->assertFalse( has_action( 'template_redirect', array( $server_timing, 'start_output_buffer' ) ), 'template_redirect action added' );
 
 		$server_timing2 = perflab_server_timing();
 		$this->assertSame( $server_timing, $server_timing2, 'Different instance returned' );
 	}
 
+	/**
+	 * @covers Perflab_Server_Timing::add_hooks
+	 */
+	public function test_perflab_server_timing_with_output_buffering(): void {
+		remove_all_actions( 'template_redirect' );
+		remove_all_filters( 'template_include' );
+
+		$server_timing = perflab_server_timing();
+		add_filter( 'perflab_server_timing_use_output_buffer', '__return_true' );
+		$this->assertTrue( $server_timing->use_output_buffer() );
+		$server_timing->add_hooks();
+		$this->assertFalse( has_filter( 'template_include', array( $server_timing, 'on_template_include' ) ), 'template_include filter added' );
+		$this->assertSame( PHP_INT_MIN, has_action( 'template_redirect', array( $server_timing, 'start_output_buffer' ) ), 'template_redirect action not added' );
+	}
+
 	public function test_perflab_server_timing_register_metric(): void {
 		$this->assertFalse( perflab_server_timing()->has_registered_metric( 'test-metric' ) );