image-layout: make index.json OPTIONAL

[AkihiroSuda]

The index.json is OPTIONAL, according to the description in
image-index.md and media-types.dot.

Signed-off-by: Akihiro Suda suda.akihiro@lab.ntt.co.jp

discussed in #716

[jonboulle]

index support within a layout is required, otherwise a layout doesn't make much sense.
I agree it's a little awkward. This is consistent with the image index being optional in media-types.dot

Maybe you could add a line here saying that index support is necessary for implementations working with layouts. Or we just strike that line. the provider/consumer distinction is unclear.

[wking]

More previous discussion here. Note that even though index.json is currently required, you don't have to list any references inside it.

[stevvooe]

What does this achieve? While refs are one function of the index, without an index, it is fairly unsafe to be inferring types. Is there something that cannot be with a required index?

[stevvooe]

I think I misread this initially.

The current wording is correct. index.json is required.

Remember that layouts are not the primary transport of OCI by any means, so make sure to interpret the statements under the assumption that there is a blob access model. To be clear, https://github.com/opencontainers/image-spec/blame/master/image-index.md#L4 says that images don't have to be packed into indexes, but they should be able to support consumption of indexes.

Moving for close here.

[philips]

I think I agree this is something to close.

[stevvooe]

@AkihiroSuda I'm going to go ahead and close this.

If there is something I've missed here, please reach out!