IntegrityError: NOT NULL constraint failed: tags.resource_id during instance_tag_set

[Sergey Nikitin]

Steps to reproduce:

 1. create instance
 2. set ['tag1', 'tag2'] to the instance
 3. set ['tag1'] to the instance

Expected result:
    tags were added.

Actual result:

    2015-08-18 16:26:36,601 ERROR [oslo_db.sqlalchemy.exc_filters] DBAPIError exception wrapped from (sqlite3.IntegrityError) NOT NULL constraint failed: tags.resource_id [SQL: u'INSERT INTO tags DEFAULT VALUES']
    Traceback (most recent call last):
      File "/opt/stack/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1139, in _execute_context
        context)
      File "/opt/stack/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 450, in do_execute
        cursor.execute(statement, parameters)
    IntegrityError: NOT NULL constraint failed: tags.resource_id

It happens because list 'to_add' in method 'instance_tag_set' is empty in second case. It's empty because on step 3 must delete one tag and create zero tags. So to fix bug we must check list to_add.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/214189

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/214189
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=8ffe9e9a8c41aafb46de3eac467b027bf3f4cf2b
Submitter: Jenkins
Branch: master

commit 8ffe9e9a8c41aafb46de3eac467b027bf3f4cf2b
Author: Sergey Nikitin <email address hidden>
Date: Tue Aug 18 16:39:31 2015 +0300

    Don't query database with an empty list of tags for creation

    It raises IntegrityError: NOT NULL constraint failed: tags.resource_id

    Closes-Bug: #1486041

    Change-Id: I9fe43e50ef38bda7b81d0ff201a623742a39b824