Problem: the update of custom models/fields is not fully transactional, and may
potentially lead to an inconsistent database. An other problem is creating two
custom fields by writing on a model: if the second one fails, the first one has
been committed without notice. Retrying the request will give an unexpected
error (duplicate field name).
Solution: never commit in the middle of a request. If the changes have an
impact on the registry, then mark it as invalid (with a new flag), and signal
registry invalidation after everything has been committed. If the request
fails, reset the registry. Both registry and cache invalidation are handled
the same way.
Move the post-install tests execution outside `Registry.new`, and add a flag on
class `HttpCase` to enable/disable the registry "test mode".
This allows a test to run actual transactions that will reload the registry,
which may be used to test the creation of `ir.model` instances, etc.
Olivier Dony