What happened?

#43792 describes this as a deliberate backward-compatibility break targeted at 6.0. The testing documented there covers extension installation, but apparently not the upgrade from Joomla 5, which is where this particular transition exposes the problem

Version

5.4

Expected result

The Joomla 5.4 to Joomla 6.1 update completes successfully when initiated through the CLI with no errors

Actual result

The update fails during the finalisation phase with:

Too few arguments to function Joomla\CMS\Installer\Installer::setAdapter(), 1 passed .../administrator/components/com_joomlaupdate/src/Model/UpdateModel.php on line 946 and exactly 2 expected

System Information

No response

Additional Comments

The Joomla 5.4 updater calls:

$installer->setAdapter($installer->extension->type);

but the Joomla 6 Installer class expects:

setAdapter($name, $adapter)

The old call is no longer required by the Joomla 6 updater, but it can still be executed by the Joomla 5.4 updater during the transition from Joomla 5 to Joomla 6.

The upgrade process therefore needs to account for the fact that the updater code executing the finalisation may be from Joomla 5.4 while the framework classes have already been replaced with their Joomla 6 versions.