boot-from-volume broken by foreign key constraints

[Eoghan Glynn]

Recently we've had a foreign key constraint added for the block_device_mapping table[1], introduced in this commit[2].

The problem is that it causes boot-from-volume to break[3], as the current code-path[4] populates the block_device_mapping
*before* the entry in the instances table entry has been created.

[1] mysql> show create table block_device_mapping \G
...
Create Table: CREATE TABLE `block_device_mapping` (
...
  CONSTRAINT `block_device_mapping_instance_uuid_fkey` FOREIGN KEY (`instance_uuid`) REFERENCES `instances` (`uuid`)
...

[2] https://github.com/openstack/nova/commit/25c50ca4b3ed8523b7d1c522ad5d05481fce7300#L4R34

[3] File "/opt/stack/nova/nova/compute/api.py", line 579, in _update_block_device_mapping
...
     raise DBError(e)
 DBError: (IntegrityError) (1452, 'Cannot add or update a child row: a foreign key constraint fails (`nova`.`block_device_mapping`, CONSTRAINT `block_device_mapping_instance_uuid_fkey` FOREIGN KEY (`instance_uuid`) REFERENCES `instances` (`uuid`))') 'INSERT INTO block_device_mapping ...

[4] https://github.com/openstack/nova/blob/master/nova/compute/api.py#L665

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/9177

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/9177
Committed: http://github.com/openstack/nova/commit/ea711b229789d9db54b7e10fe4c67ee1bf1db8c6
Submitter: Jenkins
Branch: master

commit ea711b229789d9db54b7e10fe4c67ee1bf1db8c6
Author: Eoghan Glynn <email address hidden>
Date: Fri Jun 29 17:30:52 2012 +0000

    Create instance in DB before block device mapping

    Fixes lp 1019334

    A recently added foreign key constraint on the the block_device_mapping
    table causes boot-from-volume to fail, as the bdm is populated before the
    instance DB entry is created.

    This fix simply transposes the ordering to avoid breaking the constraint.

    Change-Id: Ic62ea3ba000a2cdf58d2bd5bb17dde2b8cf340e4