merge "ManifestDescriptor" and "Descriptor" types

[stevvooe]

Currently, we have ManifestDescriptor and Descriptor types. The only difference is the inclusion of the platform field. For making generic processors that filter based on platform type, this creates a number of complexities (see https://github.com/docker/containerd/pull/638 for an example of this pattern).

We'll simply need to merge these types in the specification and struct definitions.

[qianzhangxa]

@stevvooe @jbouzane Based on the description of this ticket:

We'll simply need to merge these types in the specification and struct definitions.

We need to update both spec and struct definition, however I think #620 only updates the struct definition, we need another PR to update spec (descriptor.md and image-index.md)?

[jonboulle]

@qianzhangxa oops, for some reason I thought that had already been done, sorry. Feel like putting up a PR? <edit: never mind, just saw #638>

[stevvooe]

We need to update both spec and struct definition, however I think #620 only updates the struct definition, we need another PR to update spec (descriptor.md and image-index.md)?

Actually, this was a conscious decision not to. The alternative fields can be ignored outside the context of processing a manifest index. We only merge the Go datatype to make usage easier in practice.

[stevvooe]

@jonboulle Can we close this or did you want me to add some clarification on the difference between the spec and datatype?

[stevvooe]

@jonboulle Ping.

[jonboulle]

I'd still like to see the tweak discussed in #638 - @qianzhangxa @q384566678 either of you feel like taking this on?

[qianzhangxa]

Sure. I have submitted #649 for it.

[zhouhao3]

I think it should be explained for the difference. #649 looks good to me.