There are third party database drivers (at least mssql, oracle and mongodb_dbtng) and Drupal hardcodes the database drivers inside 'core'. Given that the motivation behind 'core' was to allow for deletion of the whole dir on upgrade, it seems rather necessary to allow database drivers outside of core.
Allow database drivers in a directory (proposed name
drivers/lib/Drupal/drivers/Database) from the Drupal root (much like modules and themes). Do not pre-create this because there will be so few people needing this.
A followup is necessary for the original issue: update_status should recognize these projects.
User interface changes
Existing installs need a new
custom => TRUE key-value pair in their
$database array in
settings.php. Given the extremely small number and very high experience level of sites running any of these I do not see the need for an actual update function.
Original report by Damien Tournoud
One of the promises of the new database layer was that third-party database drivers could be shipped as contrib modules. This is not the case anymore, because the new database autoloader hardcodes
'includes/database/[driver_name]' as the path.
One of the major implications of this is that we will not be able to track the usage of those third-party db drivers.