false positives verifying database credentials in validation of DRUSH_BOOTSTRAP_DRUPAL_DATABASE

drush_valid_db_credentials() is used to validate the bootstrap to DRUSH_BOOTSTRAP_DRUPAL_DATABASE. If it does return TRUE the bootstrap can happen. There's a problem in some circumstances where it return TRUE and it will not prevent further errors.

drush_valid_db_credentials() assumes yes and return TRUE:
* if PDO is not available
* if PDO is available but mysql driver is not available

if it does return TRUE the bootstrap validation success and so does the bootstrap itself, that is just printing a not-always valid message: "Successfully connected to the Drupal database.".

I've made some tests with drush cache clear reproducing #567062: command "drush status" returning HTML-output and error "Drush command could not be completed." and those are the facts I've seen:

• If no PDO-mysql and no php-mysql this is the drush/drupal output:
  

  PDO support not available. Could not pre-validate database credentials. Assuming success
  Drush bootstrap phase : _drush_bootstrap_drupal_database()
  Successfully connected to the Drupal database.
  {drupal dumps a maintenance page because it is not able to connect to the database}.
  

• If PDO-mysql but no php-mysql driver: drush is able to connect and verify db credentials but drupal is not able to connect to the database. Same messages as above but the first one, as PDO-mysql is available.
• If no PDO available and php-mysql driver available: drush will not be able to verify credentials (it will "assume yes") and drupal is able to connect and do the work.
  

  PDO support not available. Could not pre-validate database credentials. Assuming success
  Drush bootstrap phase : _drush_bootstrap_drupal_database() 
  Successfully connected to the Drupal database.            
  

Drupal 7 will come with PDO support so I figure out we will get rid of this asymmetry but in drupal 5,6 using PDO in drush and php-mysql will need to handle special cases and extra checks. For sure we must check php-mysql is available in php-cli. We can assume yes if PDO is not available but we also need to check that php-mysql is available and can't assume yes on that.

I wonder what's the need for PDO currently.. if it is not used anywhere else in drush (nor aegir) but to validate the database bootstrap phase.

Do we need to simply add a extra check for php-mysql if drupal version in (5,6) or refactor that function to check credentials by using php-mysql or PDO based on the drupal version?