Add example for pulling and following index images (#109)

* Add examples/follow-image-index.py

Signed-off-by: Mike Richards <[email protected]>

Author: miker985

examples/README.md

@@ -6,6 +6,7 @@ The directory here has the following examples:
 
  - [simple](simple): simple examples for individual commands adopted from oras-py [before the client was removed](https://github.com/oras-project/oras-py/tree/3b4e6d74d49b8c6a5d8180e646d52fcc50b3508a).
  - [conda-mirror.py](conda-mirror.py): upload to a conda mirror with ORAS with a manifest and custom content types.
+ - [follow-image-index.py](follow-image-index.py): Download a homebrew image index and select a platform-specific image.
 
 ## In the Wild Examples
 

examples/follow-image-index.py

@@ -0,0 +1,89 @@
+"""
+Follow homebrew image index to get the 'hello' bottle specific to your platform
+"""
+import re
+
+import oras.client
+import oras.provider
+from oras import decorator
+
+
+class MyRegistry(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"
+            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()
+        # this would be a good point to validate the schema of the manifest
+        # jsonschema.validate(manifest, schema=...)
+        return manifest
+
+
+def get_uri_for_digest(uri, digest):
+    """
+    Given a URI for an image, return a URI for the related digest.
+
+    URI may be in any of the following forms:
+
+        ghcr.io/homebrew/core/hello
+        ghcr.io/homebrew/core/hello:2.10
+        ghcr.io/homebrew/core/hello@sha256:ff81...47a
+    """
+    base_uri = re.split(r"[@:]", uri, maxsplit=1)[0]
+    return f"{base_uri}@{digest}"
+
+
+def get_image_for_platform(client, uri, download_to, platform_details):
+    def matches_platform(manifest):
+        platform = manifest.get("platform", {})
+        return all(
+            platform.get(key) == requested_value
+            for key, requested_value in platform_details.items()
+        )
+
+    index_manifest = client.remote.get_image_index(container=uri)
+    # use first compatible manifest. YMMV and a tie-breaker may be more suitable
+    for manifest in index_manifest["manifests"]:
+        if matches_platform(manifest):
+            break
+    else:
+        raise RuntimeError(
+            f"No manifest definition matched platform {platform_details}"
+        )
+
+    platform_image_uri = get_uri_for_digest(uri, manifest["digest"])
+    client.pull(target=platform_image_uri, outdir=download_to)
+
+
+if __name__ == "__main__":
+    client = oras.client.OrasClient(registry=MyRegistry())
+    platform_details = {
+        "architecture": "amd64",
+        "os": "darwin",
+        "os.version": "macOS 10.14",
+    }
+    get_image_for_platform(
+        client,
+        "ghcr.io/homebrew/core/hello:2.10",
+        download_to="downloads",
+        platform_details=platform_details,
+    )