Race in instance_create when checking for duplicate hostname

[Alexey I. Froloff]

There's a race condition in instance_create, that allows creation several instances with duplicate hostname/display_name. Before storing instance record in database, there's a query for existing records with same hostname.

    with session.begin():
        if 'hostname' in values:
            _validate_unique_server_name(context, session, values['hostname'])
        instance_ref.security_groups = _get_sec_group_models(session,
                security_groups)
        session.add(instance_ref)

If another instance with same hostname is stored after _validate_unique_server_name, there will be duplicates.

We hit this bug when badly-written client resubmitted POST request after timeout.

[jichenjc (jichenjc)]

it should be valid ....

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/172693

[OpenStack Infra (hudson-openstack)]

Change abandoned by Zhengguang Ou (<email address hidden>) on branch: master
Review: https://review.openstack.org/172693
Reason: I can't find good solution