Update 7420 fails when webforms disabled

Steps to reproduce:

1. Download and enable webforms from 4.1.
2. Disable, but do not uninstall webforms.
3. Download webforms 4.2 but do not enable it still.
4. Attempt to run a database update.

What you should see is a SQL error:

SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'module' cannot be null [error]

After some digging of my own, I determined that it is intended behaviour for disabled modules to run update hooks (see http://www.drupal.org/node/194310) and probably a reason why they advise caution when relying on hooks in update hooks (see hook_update_N). This error happens because, in user_role_grant_permission(), the module of the permission is looked up using user_permission_get_modules() which only uses enabled modules to build the permission list.

The simplest solution I can think of is to verify that webform is enabled before running the update hook, but then if webform is enabled again later, it won't have the permission granted properly. An alternative would be to bypass user_role_grant_permission() and insert the permission directly into the database with the proper module. I can write a patch for either of these, just need some direction.

For those looking for a workaround, you can enable webform, do the update, and then disable it again.