Manual:How to debug

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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.

PHP errors[edit | edit source]

To see PHP errors, add this to the very bottom of LocalSettings.php:

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

Or set it in php.ini:

error_reporting = E_ALL | E_STRICT
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())

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.

Turning display_startup_errors on[edit | edit source]

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[edit | edit source]

To display SQL errors in error messages instead of "(SQL query hidden)", add the following to LocalSettings.php:

$wgShowSQLErrors = true;
$wgDebugDumpSql  = true;

You can also enable backtrace on SQL error by setting $wgShowDBErrorBacktrace:

$wgShowDBErrorBacktrace = true;

In-depth debugging[edit | edit source]

Logging[edit | edit source]

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

Setting up a debug log file[edit | edit source]

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.

Database transaction lifecycle debugging can be enabled for some databases with $wgDebugDBTransactions.

Warning Warning: 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[edit | edit source]

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", '/tmp/my-custom-debug.log' );

Creating custom log groups[edit | edit source]

If you're debugging several different components, it may be useful to direct certain log groups to write to a separate file.

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

 * The debug log file should be not be publicly accessible if it is used, as it
 * may contain private data. But it must be in a directory to which PHP run
 * within your Web server can write.
 * It is not necessary to prefix these logs if you have multiple wikis as
 * individual log entries are prefixed with sufficient information to identify
 * the origin wiki (web server hostname and wiki-id).
// Groups from MediaWiki core
$wgDBerrorLog = '/var/log/mediawiki/dberror.log';
$wgRateLimitLog = '/var/log/mediawiki/ratelimit.log';
$wgDebugLogGroups = array(
	'resourceloader' => '/var/log/mediawiki/resourceloader.log',
	'exception' => '/var/log/mediawiki/exception.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.

Send debug data to an HTML comment in the output[edit | edit source]

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;

Working live with MediaWiki objects[edit | edit source]

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.

Profiling[edit | edit source]

To get more detail, you need to enable profiling. Profiling tracks code execution during a page action and reports back the percentage of total code execution that was spent in any specific function. The generated profile only includes functions that have specifically been marked to be profiled. Note that you have to set $wgDebugLogFile in LocalSettings; this is the file, to which your profiling data will be appended.

MediaWiki version: 1.18

If you are not using profiling, but have a StartProfiler.php file in the MediaWiki root folder, you may receive errors referring to /includes/Profiler.php. Deleting, or renaming, the StartProfiler.php file will resolve this error. The StartProfiler.sample file, also in the MediaWiki root folder, can serve as a template should you enable profiling in the future.

MediaWiki version: 1.8

To enable profiling, you need to modify the StartProfiler.php (see StartProfiler.sample in the MediaWiki root folder for an example). By default the file includes a ProfilerStub which just dumps profiling information. To instead direct this information to a file, edit StartProfiler.php so that it looks like this:

$wgProfiler['class'] = 'Profiler';

Then you can customize profiling options in LocalSettings.php (not StartProfiler.php; be sure to edit beneath the requirement of DefaultSettings.php).

Common configuration (both <1.7 and >1.8):

// Only record profiling info for pages that took longer than this
$wgProfileLimit = 0.0;
// Don't put non-profiling info into log file
$wgProfileOnly = false;
// Log sums from profiling into "profiling" table in db
$wgProfileToDatabase = false;
// If true, print a raw call tree instead of per-function report
$wgProfileCallTree = false;
// Should application server host be put into profiling table
$wgProfilePerHost = false;
// Settings for UDP profiler
$wgUDPProfilerHost = '';
$wgUDPProfilerPort = '3811';
// Detects non-matching wfProfileIn/wfProfileOut calls
$wgDebugProfiling = false;
// Output debug message on every wfProfileIn/wfProfileOut
$wgDebugFunctionEntry = 0;
// Lots of debugging output from SquidUpdate.php
$wgDebugSquid = false;

MediaWiki version: 1.7

In MediaWiki 1.7 and earlier, instead of editing StartProfiler.php, you have to set $wgProfiling to true. This will generate basic page timing information in the file defined by $wgDebugLogFile.

In addition to the settings list above, these additional settings are available:

// Enable for more detailed by-function times in debug log
$wgProfiling  = true;
// Only profile every n requests when profiling is turned on
$wgProfileSampleRate = 1;
// If not empty, specifies profiler type to load
$wgProfilerType = '';

Advanced profiling[edit | edit source]

Once you have enabled profiling, you can trace code execution through any function that you want to investigate as a bottleneck by wrapping the function with the following code:

function doSomething() {
    wfProfileIn( __METHOD__ ); # You can replace __METHOD__ with any string. This will appear in the profile.
    # The actual function

    wfProfileOut( __METHOD__ );

After you've added this information, browse to a page in the wiki. This will generate profiling info in the log file you defined above. Change $wgProfileCallTree in LocalSettings.php to true or false for different display formats.

Logging to Database[edit | edit source]

To log profiling information to a database, set $wgProfileToDatabase = true; in LocalSettings.php. Then either run update.php (since 1.21) and the profiling table will be added or manually apply the file maintenance/archives/patch-profiling.sql (the recommended way to do this is php maintenance/patchSql.php profiling).

$wgProfileCallTree must be set to false.

Viewing Profile Info[edit | edit source]

If you log your profiling information to the database, you can view the information in a webpage by browsing to profileinfo.php. You must also set $wgEnableProfileInfo = true; in AdminSettings.php. Then, after gathering data by browsing wiki pages, visit profileinfo.php to see how much time your profiled code is using and how many times it's being called.

To view profiling information as HTML comments appended to the bottom of a page, just add ?forceprofile=true to the URL. This feature is not in the standard product, you can enable it by adding this to StartProfiler.php:

if ( array_key_exists( 'forceprofile', $_REQUEST ) ) {
    $wgProfiler['class'] = 'ProfilerSimpleText';
} elseif ( array_key_exists( 'forcetrace', $_REQUEST ) ) {
    $wgProfiler['class'] = 'ProfilerSimpleTrace';

See also[edit | edit source]

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