[regression] drush 8.3.3 site-install: DROP USER drops all grants

[torotil]

Describe the bug

drush 8.3.3 introduced a change (#4261) that added a DROP USER command before recreating the DB user. Dropping the DB user implicitly removes all grants for the user. This is a problem if the same user is used for accessing DBs of multiple sites.

To Reproduce

# Install site A
drush site-install --db-url=mysql://user:pw@localhost/drupal_site_a --db-su=root --db-su-pw=super-secret minimal
# Site A works.

# Install site B
drush site-install --db-url=mysql://user:pw@localhost/drupal_site_b --db-su=root --db-su-pw=super-secret minimal
# Site A is broken because the user can’t access the first database anymore.

Expected behavior

Existing grants for the DB user should not be harmed by installing a new Drupal site.

Actual behavior

Existing grants are deleted when running drush si.

Workaround

Manage databases and grants outside of drush by not passing --db-su or downgrade to versions before 8.3.3

System Configuration

Q	A
Drush version?	8.3.3
Drupal version?	7.96
PHP version	7.4.33
OS?	Debian 12

Additional information

I was able to git bisect the problem to b559117576bea1f958cb5e8151af5161b3f6d3ce .

[greg-1-anderson]

Hm, yes, in hindsight, it does make a lot more sense to create the user if it does not exist rather than to drop it if it does exist. PRs welcome.

[torotil]

Was there a reason to not use CREATE USER IF NOT EXISTS back then? It seems both MySQL and MariaDB support it.

[greg-1-anderson]

I think that would have been the right solution.