Installer corrupts cache when database connection is in settings.php

If you reinstall a site by deleting its tables, but leaving the settings.php in place, then Drupal detects the valid database connection and skips the DB settings. Unfortunately, it also connects to the real DB cache prematurely, rather than using cache-install.inc. When it does this, it primes the cache (theme and block, probably) badly, which leads to the site having no visible blocks until the cache is deleted. This has to be done in MySQL, of course, since the site is broken.

This is a rare bug, as it requires these conditions to apply:

1. settings.php from a former installation exists and has a valid database.
2. Installer runs non-interactively (when run in the browser, the cache appears to be reset properly).

I'm not sure how to fix this. Either do a cache_reset on the final installation task, or always use cache-install.inc regardless of whether the DB connection is set up.