Steps to reproduce the issue

• Install two extensions, one with a script file (Extension A) and one without (Extension B)
• Go to System, Manage, Extensions and uninstall both extensions

Expected result

The script file runs once, for Extension A.

Actual result

The script file runs twice, for extensions A and B. Even though it's Extension A's file and has nothing to do with Extension B.

Bonus fun: if the extensions are of a different type (e.g. one Package and one Plugin) and the script file has its parameters type-hinted you get a lovely error message:

0 Pkg_blahgblahInstallerScript::postflight(): Argument #2 ($parent) must be of type Joomla\CMS\Installer\Adapter\PackageAdapter, Joomla\CMS\Installer\Adapter\PluginAdapter given

If you uninstall the extensions separately, however, no error occurs. Which proves that the problem is with Joomla!. And that's how I got dragged into debugging this, kicking and screaming…

System information (as much as possible)

Joomla 4.3 latest dev

Additional comments

BTW, this also applies to extension updates. We had observed that a few months ago on several client sites. The third person on the same day to report it told us he was indeed updating multiple extensions at once, and updating each extension separately "fixed" the issue for him. Then we never heard about it again and I had forgotten about it. Until today. But I digress.

The problem is in \Joomla\CMS\Installer\InstallerAdapter::setupScriptfile(), namely here:

        // When no script file, do nothing
        if (!$manifestScript) {
            return;
        }

A quick fix for that is:

        if (!$manifestScript) {
            $this->parent->manifestClass = null;

            return;
        }

The obvious reason is that the Installer instance ($this->parent) is reused whenever you loop through extensions (update, uninstallation). Therefore, $this->parent->manifestclass may be set and non-empty.

A better solution would be NOT reusing Joomla\CMS\Installer\Installer and Joomla\CMS\Updater\Updater objects. I do not understand why they are Singletons. The Singleton pattern would make sense if loadAllAdapters() was called —that's the only method with a performance impact which would benefit from a Singleton— but this is done literally nowhere in the CMS.

Therefore I would propose the band-aid patch for Joomla 4.x series with deprecation of getInstance() in the aforementioned objects. Joomla 4.4 / 5.0 should stop using getInstance for these objects, instead creating them afresh (just like the PackageAdapter does, presumably because of similar issues). Probably something for @laoneo and @HLeithner to think about.

BTW, this could very easily explain the issue I have reported under “Files are not copied on update” in #32592 and which @SniperSister has observed with non-Akeeba extensions. The now obvious explanation is that an installer script loaded earlier could return false from preflight / update, or otherwise cock up the files of an extension it was not meant to handle (because the developer very reasonably tests their own script with their own extension, not any random extension under the sun!). It also explains why adding installer scripts seems to alleviate the problem, a workaround I have applied a while ago to my Joomla 4 extensions by sheer dumb luck (explaining why I stopped seeing this issue on Joomla 4 without the installer code having changed in any significant way).