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Unfortunately the device tag support in the API was broken in the old
Microversion https://bugs.launchpad.net/nova/+bug/1658571, which thanks to
Kevin Zheng to find out that.

Actually there are two bugs, just all of them are about device tag. The
first one [0] is a mistake in the initial introduce of device tag. The new
schema only available for the version 2.32, when the request version >
2.32, the schema fallback to the old one.

The second one [1] is that when we bump the API to 2.37, the network device
tag was removed accidentally which also added in 2.32 [2].

So the current API behavior is as below:

2.32: BDM tag and network device tag added.
2.33 - 2.36: 'tag' in the BDM disappeared. The network device tag still
works.
2.37: The network device tag disappeared also.

There are few questions we should think about:

1. Should we fix that by Microversion?
    Thanks to Chris Dent point that out in the review. I also think we need
to bump Microversion, which follow the rule of Microversion.

2. If we need Microversion, is that something we can do before release?
    We are very close to the feature freeze. And in normal, we need spec
for microversion. Maybe we only can do that in Pike. For now we can update
the API-ref, and microversion history to notice that, maybe a reno also.

2. How can we prevent that happened again?
   Both of those patches were reviewed multiple cycles. But we still miss
that. It is worth to think about how to prevent that happened again.

   Talk with Sean. He suggests stop passing plain string version to the
schema extension point. We should always pass APIVersionRequest object
instead of plain string. Due to "version == APIVersionRequest('2.32')" is
always wrong, we should remove the '__eq__'. The developer should always
use the 'APIVersionRequest.matches' [3] method.

   That can prevent the first mistake we made. But nothing help for second
mistake. Currently we only run the test on the specific Microversion for
the specific interesting point. In the before, the new tests always
inherits from the previous microversion tests, just like [4]. That can test
the old API behavior won't be changed in the new microversion. But now, we
said that is waste, we didn't do that again just like [5]. Should we change
that back?

Thanks
Alex

[0]
https://review.openstack.org/#/c/304510/64/nova/api/openstack/compute/block_device_mapping.py
[1]
https://review.openstack.org/#/c/316398/37/nova/api/openstack/compute/schemas/servers.py@88
[2]
https://review.openstack.org/#/c/316398/37/nova/api/openstack/compute/schemas/servers.py@79
[3]
https://github.com/openstack/nova/blob/master/nova/api/openstack/api_version_request.py#L219
[4]
https://github.com/openstack/nova/blob/master/nova/tests/unit/api/openstack/compute/test_evacuate.py#L415
[5]
https://github.com/openstack/nova/blob/master/nova/tests/unit/api/openstack/compute/test_serversV21.py#L3584
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