A cell cannot be deleted once a host is added to the cell

[Takashi Natsume]

Description
===========

In cell v2 environment, once a host is added to a cell,
the cell cannot be deleted even though the cell does not have any hosts.

Steps to reproduce
==================

In cell v2 environment, a cell to delete has only one compute host.

1. Delete the nova-compute service

> nova service-delete db8e01f5-021d-4c87-a4eb-f3e5089e6105

The records are soft deleted in the 'services' table and the 'compute_nodes' table of the cell database.
But the record in 'host_mappings' table of api database is not deleted.

2. Delete the cell

> nova-manage cell_v2 delete_cell --cell_uuid 0fcd92ba-b257-4fbc-94d5-9ac6a77ccf81
There are existing hosts mapped to cell with uuid 0fcd92ba-b257-4fbc-94d5-9ac6a77ccf81.

The command fails with the message above.
It is caused because the host record remains in the 'host_mappings' table of api database.

But there is no ways to delete the host record in the 'host_mappings' table.

Expected result
===============

The cell can be deleted if the cell does not have any hosts.
Or we should have the way to delete the host from the cell.

Actual result
=============

The cell cannot be deleted even though the cell does not have any hosts.
(We don't have the way to delete the host from the cell.)

Environment
===========

OS: Ubuntu 16.04.2 LTS
nova master(commit 8ca24bf1ff80f39b14726aca22b5cf52603ea5a0)

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/510324

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/510324
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=f4f17b364e28389f22bf9c6017dd7aef918c7104
Submitter: Zuul
Branch: master

commit f4f17b364e28389f22bf9c6017dd7aef918c7104
Author: Takashi NATSUME <email address hidden>
Date: Sun Oct 8 00:55:50 2017 +0900

    Add 'delete_host' command in 'nova-manage cell_v2'

    Add 'delete_host' command in 'nova-manage cell_v2'.
    Add an optional 'force' option in 'nova-manage cell_v2 delete_cell'.
    If specifying the 'force' option, a cell can be deleted
    even if the cell has hosts.

    Change-Id: I8cd7826c2c03687c6b85731519778f09d542b236
    Closes-Bug: #1721179

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/pike
Review: https://review.openstack.org/513717

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/ocata
Review: https://review.openstack.org/513721

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/nova 17.0.0.0b1 development milestone.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/513717
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=868ef987625d6867dc0e315af6245776fa2da18a
Submitter: Zuul
Branch: stable/pike

commit 868ef987625d6867dc0e315af6245776fa2da18a
Author: Takashi NATSUME <email address hidden>
Date: Sun Oct 8 00:55:50 2017 +0900

    Add 'delete_host' command in 'nova-manage cell_v2'

    Add 'delete_host' command in 'nova-manage cell_v2'.
    Add an optional 'force' option in 'nova-manage cell_v2 delete_cell'.
    If specifying the 'force' option, a cell can be deleted
    even if the cell has hosts.

    The 'features' description is changed to the 'fixes'
    in the release note.

    Conflicts:
            doc/source/cli/nova-manage.rst

    Change-Id: I8cd7826c2c03687c6b85731519778f09d542b236
    Closes-Bug: #1721179
    (cherry-picked from commit f4f17b364e28389f22bf9c6017dd7aef918c7104)

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/nova 16.1.0 release.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/513721
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=d34222286b2a215744d225b24d1548f93bc8515a
Submitter: Zuul
Branch: stable/ocata

commit d34222286b2a215744d225b24d1548f93bc8515a
Author: Takashi NATSUME <email address hidden>
Date: Sun Oct 8 00:55:50 2017 +0900

    Add 'delete_host' command in 'nova-manage cell_v2'

    Add 'delete_host' command in 'nova-manage cell_v2'.
    Add an optional 'force' option in 'nova-manage cell_v2 delete_cell'.
    If specifying the 'force' option, a cell can be deleted
    even if the cell has hosts.

    The 'features' description is changed to the 'fixes'
    in the release note.

    Conflicts:
            doc/source/cli/nova-manage.rst

    Change-Id: I8cd7826c2c03687c6b85731519778f09d542b236
    Closes-Bug: #1721179
    (cherry-picked from commit f4f17b364e28389f22bf9c6017dd7aef918c7104)
    (cherry-picked from commit 868ef987625d6867dc0e315af6245776fa2da18a)

[John Smith (wang-zengzhi)]

The commit provide a command to delete host in "host_mapping" table. And does not seem to solve the problem completely.
For first case:

1. Delete the nova-compute service

> nova service-delete db8e01f5-021d-4c87-a4eb-f3e5089e6105

The record in 'host_mappings' table still exists. Why not delete this record after service-delete a nova-compute service？

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/nova 15.1.1 release.