Manual:How to debug

This page gives a basic introduction to debugging MediaWiki software.

One of the first things you will notice is that "echo" generally does not work; this is part of the general design.

There are several configuration options to aid debugging. The following are all false by default. Enable them by setting them to true in your LocalSettings.php :

$wgShowExceptionDetails Enable more details (like a stack trace) to be shown on the "Fatal error" page.
$wgDebugToolbar Shows a toolbar on the page with profiling, log messages and more.
$wgShowDebug Adds the "log messages" part of wgDebugToolbar as a raw list to the page.
$wgDevelopmentWarnings MediaWiki will throw notices for some possible error conditions and for deprecated functions.

Example line to be added in your LocalSettings.php :

$wgShowExceptionDetails = true;

PHP errors

To see PHP errors, add this to the second line from the top (right below the <?php) of LocalSettings.php :

error_reporting( -1 );
ini_set( 'display_errors', 1 );

Or set it in php.ini :

error_reporting = E_ALL
display_errors = On

Or set in .htaccess:

php_value error_reporting -1
php_flag display_errors On

This will cause PHP errors to be shown on-page. This might make it easier for attackers to find a way into your server, so disable it again when you have found the problem.

Note that fatal PHP errors may happen before the lines above are ever executed, or may prevent them from being shown. Fatal PHP errors are usually logged to Apache's error log – check the error_log setting in php.ini (or use phpinfo()).

Turning display_startup_errors on

Some providers turn display_startup_errors off, which hides the errors even if you raise the error_reporting level. Turning it on within the program is too late! Instead you'll have to create a wrapper file around your file. In the case of MediaWiki you can just add this on top of mediawiki/index.php:

--- index.php
    error_reporting( -1 );
    ini_set( 'display_startup_errors', 1 );
    ini_set( 'display_errors', 1 );

In other environments:

--- myTestFile.php
    error_reporting( -1 );
    ini_set( 'display_startup_errors', 1 );
    ini_set( 'display_errors', 1 );
    require 'your_file.php';

SQL errors

To log all SQL queries, rather than just the one that raised the exception, set $wgDebugDumpSql in LocalSettings.php:

$wgDebugDumpSql = true;

<td class="mw-version-versionbox" title="<translate nowrap> MediaWiki <tvar name=1>1.31</tvar> is unsupported version</translate>">

1.16 – 1.31

<translate> MediaWiki versions:</translate>

Prior to MediaWiki 1.32, you needed to set $wgShowSQLErrors and $wgShowDBErrorBacktrace to see details of database exceptions in the HTML output:

$wgShowSQLErrors = true;
$wgShowDBErrorBacktrace = true;

In-depth debugging

Debugger

You can debug your code step-by-step with XDebug . For some common setups, see:

MediaWiki-Vagrant has built in settings for this. If you're not using MediaWiki-Vagrant, but your setup is similar, you can reuse those values. In some cases (e.g. due to a firewall), you may have to use the IDE on the same machine as the web server. In this case, simply set:

xdebug.remote_enable = 1
xdebug.remote_host = 'localhost'

See the XDebug documentation for more information.

To debug a command-line script (e.g. PHPUnit, or a maintenance script) on MediaWiki-Vagrant, use:

xdebug_on; php /vagrant/mediawiki/path/to/script.php --wiki=wiki ; xdebug_off

Adjust the script, parameters, and remote host (it should be the IP of the computer where your IP is, 10.0.2.2 should work for MediaWiki-Vagrant) as needed.

Logging

For much greater detail, you need to profile and log errors.

The instructions below are only valid for the default configuration. If you change $wgMWLoggerDefaultSpi , for example to enable the psr3 role on a vagrant box, these settings will probably be ignored. In this case, see the documentation of your logger, for example, Manual:MonologSpi.

Setting up a debug log file

To save errors and debugging information to a log, add $wgDebugLogFile to the LocalSettings.php file. Change the value to a text file where you want to save the debug trace output.

The MediaWiki software must have permissions from your operating system to create and write to this file, for example in a default Ubuntu install it runs as user & group www-data:www-data. Here's a sample setting:

/**
 * The debug log file must never be publicly accessible because it contains private data.
 * But ensure that the directory is writeable by the PHP script running within your Web server.
 * The filename is with the database name of the wiki.
 */
$wgDebugLogFile = "/var/log/mediawiki/debug-{$wgDBname}.log";

This file will contain much debug information from MediaWiki core and extensions. Some subsystems write to custom logs, see #Creating a custom log file to capture their output.

File:OOjs UI icon notice-destructive.svg <translate> Warning:</translate>

The debug log file can contain private information such as login credentials, session cookies, and values of submitted forms. If this information is publicly accessible, attackers can use it to hack and compromise your machine and user account. If you need to share a debug log for diagnostic purposes, access the wiki without being logged in, and remove from the debug log any COOKIE lines, and don't capture any login attempt.

Creating a custom log file

<td class="mw-version-versionbox" title="<translate nowrap> MediaWiki <tvar name=1>1.31</tvar> is unsupported version</translate>">

<translate> ≤</translate> 1.31

<translate> MediaWiki version:</translate>

Prior to MediaWiki 1.32, to create a custom log file that only holds your specific debug statements, use the wfErrorLog() function. This function takes two arguments, the text string to log and the path to the log file:

wfErrorLog( "An error occurred.\n", '/var/log/mediawiki/my-custom-debug.log' );

Creating custom log groups

If you're debugging several different components, it may be useful to direct certain log groups to write to a separate file. See $wgDebugLogGroups for more information.

To set up custom log groups, use the following to LocalSettings.php:

/**
 * The debug log file should not be publicly accessible if it is used, as it
 * may contain private data. However, it must be in a directory to which PHP run
 * within your web server can write.
 *
 * Contrary to wgDebugLogFile, it is not necessary to include a wiki-id in these log file names
 * if you have multiple wikis. These log entries are prefixed with sufficient information to
 * identify the relevant wiki (web server hostname and wiki-id).
 */

// Groups from MediaWiki core
$wgDBerrorLog = '/var/log/mediawiki/dberror.log';
$wgDebugLogGroups = array(
	'exception' => '/var/log/mediawiki/exception.log',
	'resourceloader' => '/var/log/mediawiki/resourceloader.log',
	'ratelimit' => '/var/log/mediawiki/ratelimit.log',

	// Extra log groups from your extension
	#'myextension' => '/var/log/mediawiki/myextension.log',
	#'somegroup' => '/var/log/mediawiki/somegroup.log',
);

To log to one of these groups, call wfDebugLog like this:

if ( $module->hasFailed ) {
    wfDebugLog( 'myextension', "Something is not right, module {$module->name} failed." );
}

If you have carefully followed the instructions above but nothing gets written to your logging file(s), and if your system is using SELinux, have a look at the logging section on the SELinux page to get around this SELinux context issue.

Writing log files to the /tmp directory may not generate any log file at all, even if the /tmp directory is supposed to be writable by anyone. This could happen if your system is using one of the systemd features that create a virtual /tmp directory for that process. If that's the case, configure your log file to be written into a different directory, like /var/log/mediawiki

Structured logging

MediaWiki version:

<translate> ≥</translate> 1.25

Structured logging allows you to include fields in your log records. See Structured logging for more information.

You will need to configure a better logger to collect the extra fields, for example Monolog.

JavaScript error logging

<td class="mw-version-versionbox" title="<translate nowrap> The latest stable version is <tvar name=1>1.41</tvar></translate>">

<translate> ≥</translate> 1.36

<translate> MediaWiki version:</translate>

See the documentation of the mediawiki.errorLogger ResourceLoader module.

Statistics

Advanced client-side logging can be performed with Extension:EventLogging , which requires a complex setup and careful inspection of privacy issues.

Simple counting of certain kind of events is possible (since MediaWiki 1.25) using StatsD. StatsD offers meters, gauges, counters, and timing metrics.

Usage example:

$stats = $context->getStats();
$stats->increment( 'resourceloader.cache.hits' );
$stats->timing( 'resourceloader.cache.rtt', $rtt );

The metrics can be sent to a StatsD server, which may be specified via the wgStatsdServer configuration variable. (If not set, the metrics are discarded.) You can work with StatsD locally (without needing a Graphite server) by starting a StatsD server and configuring it with the "backends/console" backend, which will output metrics to the console.

As of MediaWiki 1.25, wfIncrStats() is a shortcut for the increment() method on the main RequestContext::getStats() instance.

Send debug data to an HTML comment in the output

This may occasionally be useful when supporting a non-technical end-user. It's more secure than exposing the debug log file to the web, since the output only contains private data for the current user. But it's not ideal for development use since data is lost on fatal errors and redirects. Use on production sites is not recommended. Debug comments reveal information in page views which could potentially expose security risks.

$wgDebugComments = true;

$wgDebugComments

Working live with MediaWiki objects

eval.php is an interactive script to evaluate and interact with MediaWiki objects and functions in a fully initialized environment.

 $ php maintenance/eval.php
 > print wfMessage("Recentchanges")->plain();
 Recent changes

The MediaWiki-Vagrant portable virtual machine integrates the interactive PHP shell phpsh (when using Zend).

Callable updates

Code embedded in the DeferredUpdates::addCallableUpdate() function, such as $rc->save() in RecentChange.php, is not executed during the web request, so no error message will be displayed if it fails. For debugging, it may be helpful to temporarily remove the code from within the function so that it is executed live.

Interactive shell

You can use shell.php as a PHP REPL with full access to MediaWiki internals.

Client side debugging (JavaScript)

Wikipedia offers a rich set of tools for debugging client side JavaScript. In addition to the MediaWiki tools, other techniques are available to assist with diagnosing client interactions.

Tools:

ResourceLoader offers a means to ensure JavaScript is easily viewable by client-side tools.
Open your browser's console. Many client side mediawiki scripts log error messages to the console using ResourceLoader, which provides a safety oriented way to log to the client console. Beyond the native JavaScript logging function, it provides a check to ensure that a console is available and that logging does not produce its own error. ResourceLoader/Architecture#Debug_mode also describes this feature.
Browser tools may provide native functionality to debug client side script.
Network tracers, like Wireshark can provide insight into the script that is being provided by a page.
You can add ?debug=true to your URL as in https://www.mediawiki.org/wiki/MediaWiki?debug=true to get more detailed information for debugging via your browser's console