Failed to detach volume cause of root device attribute

[Eric Xie]

Description
===========
At first use one volume as bootable device for the instance,
then use another attached bootable volume as the instance's
bootable device.
But when detaching the first volume from the instance,
got error.

Steps to reproduce
==================
* Boot instance vm01 from volume vol01
* Create another volume vol02 from image, update 'bootable' is true
* Shutoff vm01
* Attach vol02 to vm01
* Update 'bootable' of vol01 to false
* Start vm01
* Detach vol01 from vm01

Expected result
===============
Detach successfully

Actual result
=============
Failed to detached

Environment
===========
1. Exact version of OpenStack you are running.
# dpkg -l | grep nova
ii nova-api 2:16.1.0-1~u16.04+mcp134 all OpenStack Compute - compute API frontend
ii nova-common 2:16.1.0-1~u16.04+mcp134 all OpenStack Compute - common files
ii nova-conductor 2:16.1.0-1~u16.04+mcp134 all OpenStack Compute - conductor service
ii nova-consoleauth 2:16.1.0-1~u16.04+mcp134 all OpenStack Compute - Console Authenticator
ii nova-consoleproxy 2:16.1.0-1~u16.04+mcp134 all OpenStack Compute - NoVNC proxy
ii nova-doc 2:16.1.0-1~u16.04+mcp134 all OpenStack Compute - documentation
ii nova-placement-api 2:16.1.0-1~u16.04+mcp134 all OpenStack Compute - placement API frontend
ii nova-scheduler 2:16.1.0-1~u16.04+mcp134 all OpenStack Compute - virtual machine scheduler
ii python-nova 2:16.1.0-1~u16.04+mcp134 all OpenStack Compute - libraries
ii python-novaclient 2:9.1.1-1~u16.04+mcp6 all client library for OpenStack Compute API - Python 2.7

2. Which hypervisor did you use?
   Libvirt + KVM

2. Which storage type did you use?
   Ceph

3. Which networking type did you use?
   Neutron with OpenVSwitch

Logs & Configs
==============
Forbidden: Can't detach root device volume (HTTP 403) (Request-ID: req-c6957e61-c375-4110-939e-6ebd6bf37077)

[Matt Riedemann (mriedem)]

This isn't a bug. Detaching the root volume/device from a volume-backed server is not supported at this time. That feature is being proposed for the Stein release though:

https://review.openstack.org/#/c/600628/