Add method for retrieving a V1-compatible manifest for any manifest or manifest list

This is used to serve older clients that don't support the V2 schema format

image/docker/schema2/test/test_list.py

@@ -50,6 +50,22 @@ MANIFESTLIST_BYTES = json.dumps({
   ]
 })
 
+NO_AMD_MANIFESTLIST_BYTES = json.dumps({
+  "schemaVersion": 2,
+  "mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
+  "manifests": [
+    {
+      "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
+      "size": 983,
+      "digest": "sha256:e6",
+      "platform": {
+        "architecture": "ppc64le",
+        "os": "linux",
+      }
+    },
+  ]
+})
+
 def test_valid_manifestlist():
   def _get_manifest(digest):
     if digest == 'sha256:e6':
@@ -75,13 +91,33 @@ def test_valid_manifestlist():
       assert isinstance(manifest.manifest_obj, DockerSchema1Manifest)
       assert manifest.manifest_obj.schema_version == 1
 
-  assert manifestlist.get_v1_compatible_manifest(_get_manifest).manifest_obj.schema_version == 1
+  compatible_manifest = manifestlist.get_v1_compatible_manifest('foo', 'bar', 'baz', _get_manifest)
+  assert compatible_manifest.schema_version == 1
 
   assert manifestlist.layers is None
   assert manifestlist.leaf_layer_v1_image_id is None
   assert manifestlist.legacy_image_ids is None
 
 
+def test_get_v1_compatible_manifest_no_matching_list():
+  def _get_manifest(digest):
+    if digest == 'sha256:e6':
+      return v22_bytes
+    else:
+      return v21_bytes
+
+  manifestlist = DockerSchema2ManifestList(NO_AMD_MANIFESTLIST_BYTES)
+  assert len(manifestlist.manifests(_get_manifest)) == 1
+  assert (manifestlist.digest ==
+          'sha256:50150251101420a020ab4a3e77e9d167a18b09bd4eeb0cc65e0eafab95cf79cf')
+
+  assert manifestlist.media_type == 'application/vnd.docker.distribution.manifest.list.v2+json'
+  assert manifestlist.bytes == NO_AMD_MANIFESTLIST_BYTES
+
+  compatible_manifest = manifestlist.get_v1_compatible_manifest('foo', 'bar', 'baz', _get_manifest)
+  assert compatible_manifest is None
+
+
 def test_builder():
   def _get_manifest(digest):
     if digest == 'sha256:e6':