neutron migrations are not idempotent or invariant to enabled services

Neutron is currently failing during grenade upgrade testing. The symptom is that meteringlabels table doesn't exist.

The root cause is that neutron migrations are not idempotent or invariant to enabled services (which must be true for migrations for the system to work as expected). As can be seen here - https://github.com/openstack/neutron/blob/master/neutron/db/migration/alembic_migrations/versions/569e98a8132b_metering.py#L50-L51

Migration content might be skipped based on configuration conditionals. This means it's possible for 2 different installations to report as db schema 569e98a8132b but have different schemas. The logic of db migrations assumes that 569e98a8132b can only result in a single schema, so future migrations depend on the content all being the same here.

The solution here is to probably build a set of "healing migrations" that undo all the past conditional skips to get neutron back in line with a migration id means exactly a single schema. Anything other than that will be largely impossible to support in the field.

(previous description below)
Hello,
I just got a neutron-db-manage failure in the Gate : http://logs.openstack.org/77/83777/1/check/check-tempest-dsvm-neutron-heat-slow/a7ad8c0/

Relevant stack trace is

"
2014-04-13 23:53:15.281 | INFO [alembic.migration] Running upgrade b65aa907aec -> 33c3db036fe4, set_length_of_description_field_metering
2014-04-13 23:53:15.296 | Traceback (most recent call last):
2014-04-13 23:53:15.296 | File "/usr/local/bin/neutron-db-manage", line 10, in <module>
.....................
2014-04-13 23:53:15.340 | sqlalchemy.exc.ProgrammingError: (ProgrammingError) (1146, "Table 'neutron_ml2.meteringlabels' doesn't exist") 'ALTER TABLE meteringlabels CHANGE description description VARCHAR(1024) NULL' ()"
"