Support pulling of schema2 manifests directly via a manifest list tag

This change ensures that if a manifest list is requested with an accepts header for a *schema 2* manifest, the legacy manifest (if any) is returned as schema 2 if it was pushed as a schema 2 manifest (rather than being auto-converted to schema 1)

data/registry_model/interface.py

@@ -322,3 +322,10 @@ class RegistryDataInterface(object):
     """ Returns a cached set of ISO country codes blacklisted for pulls for the namespace
         or None if the list could not be loaded.
     """
+
+  @abstractmethod
+  def convert_manifest(self, manifest, namespace_name, repo_name, tag_name, allowed_mediatypes,
+                       storage):
+    """ Attempts to convert the specified into a parsed manifest with a media type
+        in the allowed_mediatypes set. If not possible, or an error occurs, returns None.
+    """

data/registry_model/registry_oci_model.py

@@ -482,6 +482,22 @@ class OCIModel(SharedModel, RegistryDataInterface):
     retriever = RepositoryContentRetriever(manifest_row.repository_id, storage)
     return parsed.get_schema1_manifest(namespace_name, repo_name, tag_name, retriever)
 
+  def convert_manifest(self, manifest, namespace_name, repo_name, tag_name, allowed_mediatypes,
+                       storage):
+    try:
+      parsed = manifest.get_parsed_manifest()
+    except ManifestException:
+      return None
+
+    try:
+      manifest_row = database.Manifest.get(id=manifest._db_id)
+    except database.Manifest.DoesNotExist:
+      return None
+
+    retriever = RepositoryContentRetriever(manifest_row.repository_id, storage)
+    return parsed.convert_manifest(allowed_mediatypes, namespace_name, repo_name, tag_name,
+                                   retriever)
+
   def create_manifest_with_temp_tag(self, repository_ref, manifest_interface_instance,
                                     expiration_sec, storage):
     """ Creates a manifest under the repository and sets a temporary tag to point to it.

data/registry_model/registry_pre_oci_model.py

@@ -549,6 +549,18 @@ class PreOCIModel(SharedModel, RegistryDataInterface):
     except ManifestException:
       return None
 
+  def convert_manifest(self, manifest, namespace_name, repo_name, tag_name, allowed_mediatypes,
+                       storage):
+    try:
+      parsed = manifest.get_parsed_manifest()
+    except ManifestException:
+      return None
+
+    try:
+      return parsed.convert_manifest(allowed_mediatypes, namespace_name, repo_name, tag_name, None)
+    except ManifestException:
+      return None
+
   def create_manifest_with_temp_tag(self, repository_ref, manifest_interface_instance,
                                     expiration_sec, storage):
     """ Creates a manifest under the repository and sets a temporary tag to point to it.

data/registry_model/test/test_interface.py

@@ -23,7 +23,7 @@ from data.registry_model.datatypes import RepositoryReference
 from data.registry_model.blobuploader import upload_blob, BlobUploadSettings
 from data.registry_model.modelsplitter import SplitModel
 from image.docker.types import ManifestImageLayer
-from image.docker.schema1 import DockerSchema1ManifestBuilder
+from image.docker.schema1 import DockerSchema1ManifestBuilder, DOCKER_SCHEMA1_CONTENT_TYPES
 from image.docker.schema2.manifest import DockerSchema2ManifestBuilder
 
 from test.fixtures import *
@@ -780,6 +780,18 @@ def test_get_schema1_parsed_manifest(registry_model):
   assert registry_model.get_schema1_parsed_manifest(manifest, '', '', '', storage)
 
 
+def test_convert_manifest(registry_model):
+  repository_ref = registry_model.lookup_repository('devtable', 'simple')
+  latest_tag = registry_model.get_repo_tag(repository_ref, 'latest', include_legacy_image=True)
+  manifest = registry_model.get_manifest_for_tag(latest_tag)
+
+  mediatypes = DOCKER_SCHEMA1_CONTENT_TYPES
+  assert registry_model.convert_manifest(manifest, '', '', '', mediatypes, storage)
+
+  mediatypes = []
+  assert registry_model.convert_manifest(manifest, '', '', '', mediatypes, storage) is None
+
+
 def test_create_manifest_and_retarget_tag_with_labels(registry_model):
   repository_ref = registry_model.lookup_repository('devtable', 'simple')
   latest_tag = registry_model.get_repo_tag(repository_ref, 'latest', include_legacy_image=True)