In https://gerrit.wikimedia.org/r/c/mediawiki/extensions/GrowthExperiments/+/1017256, @Sgs accidentally did not declare JSON schema defaults for the newly-added Help panel schema (T360472). This resulted in an ugly error:
MediaWiki internal error.
Original exception: [7a2bd9f7260ee9b7668ef1ce] /wiki/Special:CommunityConfiguration/HelpPanel MediaWiki\Config\ConfigException: Key GEHelpPanelExcludedNamespaces was not found.
Backtrace:
from /var/www/html/w/extensions/CommunityConfiguration/src/Provider/WikiPageConfigProvider.php(38)
#0 /var/www/html/w/extensions/CommunityConfiguration/src/Access/WikiPageConfigReader.php(128): MediaWiki\Extension\CommunityConfiguration\Provider\WikiPageConfigProvider->get()
#1 /var/www/html/w/extensions/GrowthExperiments/includes/HelpPanel.php(131): MediaWiki\Extension\CommunityConfiguration\Access\WikiPageConfigReader->get()
#2 /var/www/html/w/extensions/GrowthExperiments/includes/HelpPanelHooks.php(159): GrowthExperiments\HelpPanel::shouldShowHelpPanel()
#3 /var/www/html/w/includes/HookContainer/HookContainer.php(159): GrowthExperiments\HelpPanelHooks->onBeforePageDisplay()
#4 /var/www/html/w/includes/HookContainer/HookRunner.php(945): MediaWiki\HookContainer\HookContainer->run()
#5 /var/www/html/w/includes/Output/OutputPage.php(2998): MediaWiki\HookContainer\HookRunner->onBeforePageDisplay()
#6 /var/www/html/w/includes/actions/ActionEntryPoint.php(162): MediaWiki\Output\OutputPage->output()
#7 /var/www/html/w/includes/MediaWikiEntryPoint.php(199): MediaWiki\Actions\ActionEntryPoint->execute()
#8 /var/www/html/w/index.php(58): MediaWiki\MediaWikiEntryPoint->run()
#9 {main}This error was even uglier, as it only appeared following clearing wiki's internal caches (without that, it appeared to work correctly). Such a error should be definitely caught by (some) testcase – it is questionable whether it should belong to GrowthExperiments or MediaWiki-extensions-CommunityConfiguration (tagging with both momentarily), but the issue should not rely on manual detection, as it is the equivalent of taking the site fully down. Without providing such test for this, this has the potential of biting us in the future.