Image Index support

[miker985]

As of 0.1.25 it doesn't appear to me this supports Image Indexes.

The lift looks small - maybe as little as adding a media type to defaults.py, a schemas.py definition, and minor work to support it in the Registry object

Sample code might be

class Registry(oras.provider.Registry):
    """
    Oras registry with support for image indexes.
    """

    @decorator.ensure_container
    def get_image_index(self, container, allowed_media_type=None):
        """
        Get an image index as a manifest.

        This is basically Registry.get_manifest with the following changes

        - different default allowed_media_type
        - no JSON schema validation
        """
        if not allowed_media_type:
            default_image_index_media_type = "application/vnd.oci.image.index.v1+json"  # TODO: need this in defaults.py
            allowed_media_type = [default_image_index_media_type]

        headers = {"Accept": ";".join(allowed_media_type)}

        manifest_url = f"{self.prefix}://{container.manifest_url()}"
        response = self.do_request(manifest_url, "GET", headers=headers)
        self._check_200_response(response)
        manifest = response.json()
        # TODO: jsonschema.validate(manifest, schema=...)
        return manifest

If there's interest please let me know and I can put in a PR. In that case I ask if a new method like get_image_index is desired or if something else would be preferred.

In any case, thank you for the project!

[vsoch]

Perhaps under examples https://github.com/oras-project/oras-py/tree/main/examples would suffice? Can you give me the use case of doing this?

[miker985]

My use case is downloading an image index, selecting a platform-compatible manifest, and then following that to ultimately download a platform-specific artifact.

[vsoch]

That would be a great (full) example script if you are up for it!