Drupal的错误和异常处理

Drupal在配置阶段的最开始就设置了自己的错误处理器和异常处理器：

function _drupal_bootstrap_configuration() {
  set_error_handler('_drupal_error_handler');
  set_exception_handler('_drupal_exception_handler');
  // ... ...
}

先来看看错误处理器_drupal_error_handler()是如何做的？

function _drupal_error_handler($error_level, $message, $filename, $line, $context) {
  require_once DRUPAL_ROOT . '/includes/errors.inc';
  _drupal_error_handler_real($error_level, $message, $filename, $line, $context);
}

调用了_drupal_error_handler_real()函数，该函数与当前error_reporting()比较，判断当前错误是否需要被处理：

function _drupal_error_handler_real($error_level, $message, $filename, $line, $context) {
  if ($error_level & error_reporting()) {
    $types = drupal_error_levels();
    list($severity_msg, $severity_level) = $types[$error_level];
    $caller = _drupal_get_last_caller(debug_backtrace());

    if (!function_exists('filter_xss_admin')) {
      require_once DRUPAL_ROOT . '/includes/common.inc';
    }

    // We treat recoverable errors as fatal.
    _drupal_log_error(array(
      '%type' => isset($types[$error_level]) ? $severity_msg : 'Unknown error',
      // The standard PHP error handler considers that the error messages
      // are HTML. We mimick this behavior here.
      '!message' => filter_xss_admin($message),
      '%function' => $caller['function'],
      '%file' => $caller['file'],
      '%line' => $caller['line'],
      'severity_level' => $severity_level,
    ), $error_level == E_RECOVERABLE_ERROR);
  }
}

重点是_drupal_log_error()函数，该函数具体地处理错误逻辑：

function _drupal_log_error($error, $fatal = FALSE) {
  // Initialize a maintenance theme if the bootstrap was not complete.
  // Do it early because drupal_set_message() triggers a drupal_theme_initialize().
  if ($fatal && (drupal_get_bootstrap_phase() != DRUPAL_BOOTSTRAP_FULL)) {
    unset($GLOBALS['theme']);
    if (!defined('MAINTENANCE_MODE')) {
      define('MAINTENANCE_MODE', 'error');
    }
    drupal_maintenance_theme();
  }

  // When running inside the testing framework, we relay the errors
  // to the tested site by the way of HTTP headers.
  $test_info = &$GLOBALS['drupal_test_info'];
  if (!empty($test_info['in_child_site']) && !headers_sent() && (!defined('SIMPLETEST_COLLECT_ERRORS') || SIMPLETEST_COLLECT_ERRORS)) {
    // $number does not use drupal_static as it should not be reset
    // as it uniquely identifies each PHP error.
    static $number = 0;
    $assertion = array(
      $error['!message'],
      $error['%type'],
      array(
        'function' => $error['%function'],
        'file' => $error['%file'],
        'line' => $error['%line'],
      ),
    );
    header('X-Drupal-Assertion-' . $number . ': ' . rawurlencode(serialize($assertion)));
    $number++;
  }

  watchdog('php', '%type: !message in %function (line %line of %file).', $error, $error['severity_level']);

  if ($fatal) {
    drupal_add_http_header('Status', '500 Service unavailable (with message)');
  }

  if (drupal_is_cli()) {
    if ($fatal) {
      // When called from CLI, simply output a plain text message.
      print html_entity_decode(strip_tags(t('%type: !message in %function (line %line of %file).', $error))). "\n";
      exit;
    }
  }

  if (isset($_SERVER['HTTP_X_REQUESTED_WITH']) && $_SERVER['HTTP_X_REQUESTED_WITH'] == 'XMLHttpRequest') {
    if ($fatal) {
      if (error_displayable($error)) {
        // When called from JavaScript, simply output the error message.
        print t('%type: !message in %function (line %line of %file).', $error);
      }
      exit;
    }
  }
  else {
    // Display the message if the current error reporting level allows this type
    // of message to be displayed, and unconditionnaly in update.php.
    if (error_displayable($error)) {
      $class = 'error';

      // If error type is 'User notice' then treat it as debug information
      // instead of an error message, see dd().
      if ($error['%type'] == 'User notice') {
        $error['%type'] = 'Debug';
        $class = 'status';
      }

      drupal_set_message(t('%type: !message in %function (line %line of %file).', $error), $class);
    }

    if ($fatal) {
      drupal_set_title(t('Error'));
      // We fallback to a maintenance page at this point, because the page generation
      // itself can generate errors.
      print theme('maintenance_page', array('content' => t('The website encountered an unexpected error. Please try again later.')));
      exit;
    }
  }
}

有两点需要先说一下：
1. 什么是严重错误$fatal？$error_level == E_RECOVERABLE_ERROR，还有异常也是严重错误。
2. 哪些错误可以显示error_displayable($error)==TRUE？这是在Drupal系统配置中设置的：

在_drupal_log_error()函数中，首先检查是否是在启动过程中发生的严重地不可恢复错误，如果是不可恢复的错误，则切换到维护显示主题:

if ($fatal && (drupal_get_bootstrap_phase() != DRUPAL_BOOTSTRAP_FULL)) {
  unset($GLOBALS['theme']);
  if (!defined('MAINTENANCE_MODE')) {
    define('MAINTENANCE_MODE', 'error');
  }
  drupal_maintenance_theme();
}

检查是否运行在SimpleTest框架，如果是，则需要为SimpleTest返回一个包含错误信息的HTTP头：

$test_info = &$GLOBALS['drupal_test_info'];
if (!empty($test_info['in_child_site']) && !headers_sent() && (!defined('SIMPLETEST_COLLECT_ERRORS') || SIMPLETEST_COLLECT_ERRORS)) {
  static $number = 0;
  $assertion = array(
    $error['!message'],
    $error['%type'],
    array(
      'function' => $error['%function'],
      'file' => $error['%file'],
      'line' => $error['%line'],
    ),
  );
  header('X-Drupal-Assertion-' . $number . ': ' . rawurlencode(serialize($assertion)));
  $number++;
}

然后调用watchdog()记录错误信息。watchdog()使用了钩子，这样允许开发人员自定义模块将错误信息记录到各种不同的地方。默认的Drupal使用dblog模块将错误信息保存到后台数据库watchdog表中。

watchdog('php', '%type: !message in %function (line %line of %file).', $error, $error['severity_level']);

function watchdog($type, $message, $variables = array(), $severity = WATCHDOG_NOTICE, $link = NULL) {
  global $user, $base_root;

  static $in_error_state = FALSE;

  // It is possible that the error handling will itself trigger an error. In that case, we could
  // end up in an infinite loop. To avoid that, we implement a simple static semaphore.
  if (!$in_error_state && function_exists('module_implements')) {
    $in_error_state = TRUE;

    // The user object may not exist in all conditions, so 0 is substituted if needed.
    $user_uid = isset($user->uid) ? $user->uid : 0;

    // Prepare the fields to be logged
    $log_entry = array(
      'type'        => $type,
      'message'     => $message,
      'variables'   => $variables,
      'severity'    => $severity,
      'link'        => $link,
      'user'        => $user,
      'uid'         => $user_uid,
      'request_uri' => $base_root . request_uri(),
      'referer'     => isset($_SERVER['HTTP_REFERER']) ? $_SERVER['HTTP_REFERER'] : '',
      'ip'          => ip_address(),
      // Request time isn't accurate for long processes, use time() instead.
      'timestamp'   => time(),
    );

    // Call the logging hooks to log/process the message
    foreach (module_implements('watchdog') as $module) {
      module_invoke($module, 'watchdog', $log_entry);
    }

    // It is critical that the semaphore is only cleared here, in the parent
    // watchdog() call (not outside the loop), to prevent recursive execution.
    $in_error_state = FALSE;
  }
}

// dblog模块
function dblog_watchdog(array $log_entry) {
  Database::getConnection('default', 'default')->insert('watchdog')
    ->fields(array(
      'uid' => $log_entry['uid'],
      'type' => substr($log_entry['type'], 0, 64),
      'message' => $log_entry['message'],
      'variables' => serialize($log_entry['variables']),
      'severity' => $log_entry['severity'],
      'link' => substr($log_entry['link'], 0, 255),
      'location' => $log_entry['request_uri'],
      'referer' => $log_entry['referer'],
      'hostname' => substr($log_entry['ip'], 0, 128),
      'timestamp' => $log_entry['timestamp'],
    ))
    ->execute();
}

如果是严重错误，Drupal会返回500状态码：

if ($fatal) {
  drupal_add_http_header('Status', '500 Service unavailable (with message)');
}

检查是否是AJAX请求。$_SERVER['HTTP_X_REQUESTED_WITH']等于XMLHttpRequest是AJAX请求。如果是AJAX请求，且错误允许显示（error_displayable($error)为TRUE），则显示错误信息，终止请求。

if (isset($_SERVER['HTTP_X_REQUESTED_WITH']) && $_SERVER['HTTP_X_REQUESTED_WITH'] == 'XMLHttpRequest') {
  if ($fatal) {
    if (error_displayable($error)) {
      // When called from JavaScript, simply output the error message.
      print t('%type: !message in %function (line %line of %file).', $error);
    }
    exit;
  }
}

如果不是AJAX请求，且错误允许显示，则加入一条FLASH信息，这条信息会显示在下一次请求的顶部。

if (error_displayable($error)) {
  $class = 'error';

  // If error type is 'User notice' then treat it as debug information
  // instead of an error message, see dd().
  if ($error['%type'] == 'User notice') {
    $error['%type'] = 'Debug';
    $class = 'status';
  }

  drupal_set_message(t('%type: !message in %function (line %line of %file).', $error), $class);
}

最后，对于非AJAX的严重错误，直接输出信息，终止请求。

if ($fatal) {
  drupal_set_title(t('Error'));
  // We fallback to a maintenance page at this point, because the page generation
  // itself can generate errors.
  print theme('maintenance_page', array('content' => t('The website encountered an unexpected error. Please try again later.')));
  exit;
}

异常处理器和错误处理器实质上是相同的，内部都是通过_drupal_log_error()来处理的：

function _drupal_exception_handler($exception) {
  require_once DRUPAL_ROOT . '/includes/errors.inc';

  try {
    // Log the message to the watchdog and return an error page to the user.
    _drupal_log_error(_drupal_decode_exception($exception), TRUE);
  }
  catch (Exception $exception2) {
    // Another uncaught exception was thrown while handling the first one.
    // If we are displaying errors, then do so with no possibility of a further uncaught exception being thrown.
    if (error_displayable()) {
      print '<h1>Additional uncaught exception thrown while handling exception.</h1>';
      print '<h2>Original</h2><p>' . _drupal_render_exception_safe($exception) . '</p>';
      print '<h2>Additional</h2><p>' . _drupal_render_exception_safe($exception2) . '</p><hr />';
    }
  }
}