Steps to reproduce the issue

Run the following code:

$installer = new JInstaller();
$installer->loadAllAdapters();

$result = $installer->discover_install($EXTENSION_ID_TO_BE_INSTALLED);

Expected result

Extension should be installed

Actual result

Installation failure + PHP Notice (+ a PHP warning down the road) following this execution path:

• JInstaller::loadAllAdapters caches all adapter types in $_adapters
• JInstaller::discover_install lazy loads the adapter here: https://github.com/joomla/joomla-cms/blob/3.4.0-beta1/libraries/cms/installer/installer.php#L510
• Since the adapter is already loaded the $params variable is never set in the adapter.
• JInstallerAdapterPlugin::prepareDiscoverInstall is called.
• It tries to find the client info by doing $this->extension->client_id. However since $this->extension has never been populated by the installer this value is null.
• Next line tries to access $client->path here but $client is an array. https://github.com/joomla/joomla-cms/blob/3.4.0-beta1/libraries/cms/installer/adapter/plugin.php#L305

A notice is generated and the object is in an undetermined state.

I think a proper fix here would be abstracting the code in JInstaller in a getAdapter method. This method would set the parameters in the returned object whether it was just constructed or not.