Query monitor tweaks: Implement a new Header outputter for PHP Errors. (

#5040)

* Implement a new Header outputter for PHP Errors. The main purpose of it is to reduce the chances of requests breaking due to too many errors presented as headers for REST and redirect contexts

* Pad values some more

qm-plugins/qm-limited-header-php-errors/output/headers/php-errors.php

@@ -0,0 +1,118 @@
+<?php
+/**
+ * PHP error output for HTTP headers.
+ *
+ * @package query-monitor
+ */
+
+declare(strict_types=1);
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+class QM_Output_Headers_Limited_PHP_Errors extends QM_Output_Headers {
+
+	/**
+	 * Collector instance.
+	 *
+	 * @var QM_Collector_PHP_Errors Collector.
+	 */
+	protected $collector;
+
+	/**
+	 * @return array<string, mixed>
+	 */
+	public function get_output() {
+		/** @var QM_Data_PHP_Errors $data */
+		$data    = $this->collector->get_data();
+		$headers = array();
+
+		if ( empty( $data->errors ) ) {
+			return array();
+		}
+
+		$count = 0;
+
+		foreach ( $data->errors as $type => $errors ) {
+
+			foreach ( $errors as $error_key => $error ) {
+
+				// phpcs:ignore Universal.Operators.DisallowStandalonePostIncrementDecrement.PostIncrementFound
+				$count++;
+
+				$stack = array();
+
+				if ( ! empty( $error['filtered_trace'] ) ) {
+					$stack = array_column( $error['filtered_trace'], 'display' );
+				}
+
+				$output_error = array(
+					'key'       => $error_key,
+					'type'      => $error['type'],
+					'message'   => $error['message'],
+					'file'      => QM_Util::standard_dir( $error['file'], '' ),
+					'line'      => $error['line'],
+					'stack'     => $stack,
+					'component' => $error['component']->name,
+				);
+
+				$key             = sprintf( 'error-%d', $count );
+				$headers[ $key ] = json_encode( $output_error ); // phpcs:ignore WordPress.WP.AlternativeFunctions.json_encode_json_encode
+
+			}
+		}
+
+		// VIP: hack to avoid failed requests due to headers being too large
+		// these are subject to change at any time
+		// All of the values are padded to make sure failsafe is triggered earlier
+		$max_header_length       = 10 * 1024;  // padded
+		$max_total_header_length = 30 * 1024;  // padded
+		$max_total_headers       = 50;        // padded
+
+		$current_headers_length = strlen( join( "\n", headers_list() ) );
+		$current_headers_count  = count( headers_list() );
+
+		// Any single header too long, truncate it
+		foreach ( $headers as $key => $value ) {
+			if ( strlen( $value ) + strlen( $key ) > $max_header_length ) {
+				$headers[ $key ] = substr( $value, 0, $max_header_length );
+			}
+		}
+
+		// Too many headers, slice the array
+		if ( $current_headers_count + $count > $max_total_headers ) {
+			$headers = array_slice( $headers, 0, $max_total_headers - $current_headers_count );
+		}
+
+		// Wholesale remove QM error headers if we're still over the limit
+		$max_qm_headers_length = $max_total_header_length - $current_headers_length;
+		if ( strlen( join( "\n", $headers ) ) > $max_qm_headers_length ) {
+			$headers = [ 'errors-truncated' => 'Too many errors, check your Application performance monitor for details.' ];
+		}
+		// End VIP Hack
+
+		return array_merge(
+			array(
+				'error-count' => $count,
+			),
+			$headers
+		);
+	}
+}
+
+/**
+ * @param array<string, QM_Output> $output
+ * @param QM_Collectors $collectors
+ * @return array<string, QM_Output>
+ * phpcs:disable Generic.CodeAnalysis.UnusedFunctionParameter.FoundAfterLastUsed,Universal.Files.SeparateFunctionsFromOO.Mixed
+ */
+function vip_register_qm_output_headers_php_errors( array $output, QM_Collectors $collectors ) {
+	$collector = QM_Collectors::get( 'php_errors' );
+	if ( $collector ) {
+		$output['php_errors'] = new QM_Output_Headers_Limited_PHP_Errors( $collector );
+	}
+	return $output;
+}
+
+add_filter( 'qm/outputter/headers', 'vip_register_qm_output_headers_php_errors', 111, 2 );

query-monitor.php

@@ -107,6 +107,10 @@ function wpcom_vip_qm_require() {
 	if ( file_exists( __DIR__ . '/qm-plugins/qm-db-connections/qm-db-connections.php' ) ) {
 		require_once __DIR__ . '/qm-plugins/qm-db-connections/qm-db-connections.php';
 	}
+
+	if ( file_exists( __DIR__ . '/qm-plugins/qm-limited-header-php-errors/output/headers/php-errors.php' ) ) {
+		require_once __DIR__ . '/qm-plugins/qm-limited-header-php-errors/output/headers/php-errors.php';
+	}
 }
 add_action( 'plugins_loaded', 'wpcom_vip_qm_require', 1 );