[Pulp-dev] ContentViewset assumes that POST is needed for any content

[Tatiana Tereshchenko]

Current implementation of ContentViewset
<https://github.com/pulp/pulpcore/blob/master/pulpcore/app/viewsets/content.py#L98-L102>
includes
mixins for create (POST) and retrieve/list (GET).
In case a plugin doesn't need to support POST for a content endpoint, a
plugin writer compiles a viewset from the mixins they need, e.g. distribution
trees and custom metadata
<https://github.com/pulp/pulp_rpm/blob/master/pulp_rpm/app/viewsets.py#L233-L258>
(same
use case is expected for modularity endpoints)
This leads to the inconsistent REST API.

# ContentViewset is used
/pulp/api/v3/content/rpm/advisories/

# custom plugin content viewset
/pulp/api/v3/distribution_trees/rpm/distribution_trees/

Possible solutions:
1. Make ContentViewset more generic (no mixins, or only GET ones?) and let
plugins include any mixins they need.
This option might be painful to switch to for plugin writers, because every
plugin will be affected and will need to make this change.
At the same time probably not many plugins support upload for every content
type, so in many cases the POST is broken/not used anyway.

2. Disable POST at the plugin level in some other way.
I'm not sure if there is any native option to disable it.
Hacky way is to override `create` method which will return appropriate HTTP
error that POST is not supported.

3. Make plugin writers manually define a proper endpoint name.
Apart from not being reliable, I'm not sure how to do it because of how we
tweak endpoint generation.
Notice the distribution trees example ^, "distribution_trees" is used twice
in the endpoint.

4. Any other solutions? Easy ones which I missed?

Thank you,
Tanya
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