Start on a basic registry_model interface and change a single module to use it. This will allow us to completely abstract out how we deal with registry-related tables and ensure that transitioning to the new OCI-like model will be easier to do.

data/registry_model/__init__.py

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+from data.registry_model.registry_pre_oci_model import pre_oci_model
+
+registry_model = pre_oci_model

data/registry_model/datatypes.py

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+from collections import namedtuple
+
+class RepositoryReference(object):
+  """ RepositoryReference is a reference to a repository, passed to registry interface methods. """
+  def __init__(self, repo_id):
+    self.repo_id = repo_id
+
+  @classmethod
+  def for_repo_obj(cls, repo_obj):
+    return RepositoryReference(repo_obj.id)
+
+
+class Tag(namedtuple('Tag', ['id', 'name'])):
+  """ Tag represents a tag in a repository, which points to a manifest or image. """
+  @classmethod
+  def for_repository_tag(cls, repository_tag):
+    if repository_tag is None:
+      return None
+
+    return Tag(id=repository_tag.id, name=repository_tag.name)

data/registry_model/interface.py

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+from abc import ABCMeta, abstractmethod
+from six import add_metaclass
+
+@add_metaclass(ABCMeta)
+class RegistryDataInterface(object):
+  """ Interface for code to work with the registry data model. The registry data model consists
+      of all tables that store registry-specific information, such as Manifests, Blobs, Images,
+      and Labels.
+  """
+
+  @abstractmethod
+  def find_matching_tag(self, repository_ref, tag_names):
+    """ Finds an alive tag in the repository matching one of the given tag names and returns it
+        or None if none.
+    """
+
+  @abstractmethod
+  def get_most_recent_tag(self, repository_ref):
+    """ Returns the most recently pushed alive tag in the repository, if any. If none, returns
+        None.
+    """

data/registry_model/registry_pre_oci_model.py

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+from data import model
+from data.registry_model.interface import RegistryDataInterface
+from data.registry_model.datatypes import Tag
+
+
+class PreOCIModel(RegistryDataInterface):
+  """
+  PreOCIModel implements the data model for the registry API using a database schema
+  before it was changed to support the OCI specification.
+  """
+
+  def find_matching_tag(self, repository_ref, tag_names):
+    """ Finds an alive tag in the repository matching one of the given tag names and returns it
+        or None if none.
+    """
+    found_tag = model.tag.find_matching_tag(repository_ref.repo_id, tag_names)
+    return Tag.for_repository_tag(found_tag)
+
+  def get_most_recent_tag(self, repository_ref):
+    """ Returns the most recently pushed alive tag in the repository, if any. If none, returns
+        None.
+    """
+    found_tag = model.tag.get_most_recent_tag(repository_ref.repo_id)
+    return Tag.for_repository_tag(found_tag)
+
+
+pre_oci_model = PreOCIModel()

data/registry_model/test/test_pre_oci_model.py

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+import pytest
+
+from data import model
+from data.registry_model.registry_pre_oci_model import PreOCIModel
+from data.registry_model.datatypes import RepositoryReference
+from test.fixtures import *
+
+@pytest.fixture()
+def pre_oci_model(initialized_db):
+  return PreOCIModel()
+
+
+@pytest.mark.parametrize('names, expected', [
+  (['unknown'], None),
+  (['latest'], 'latest'),
+  (['latest', 'prod'], 'latest'),
+  (['foo', 'prod'], 'prod'),
+])
+def test_find_matching_tag(names, expected, pre_oci_model):
+  repo = model.repository.get_repository('devtable', 'simple')
+  repository_ref = RepositoryReference.for_repo_obj(repo)
+  found = pre_oci_model.find_matching_tag(repository_ref, names)
+  if expected is None:
+    assert found is None
+  else:
+    assert found.name == expected
+
+
+@pytest.mark.parametrize('repo_namespace, repo_name, expected', [
+  ('devtable', 'simple', 'latest'),
+  ('buynlarge', 'orgrepo', 'latest'),
+])
+def test_get_most_recent_tag(repo_namespace, repo_name, expected, pre_oci_model):
+  repo = model.repository.get_repository(repo_namespace, repo_name)
+  repository_ref = RepositoryReference.for_repo_obj(repo)
+  found = pre_oci_model.get_most_recent_tag(repository_ref)
+  if expected is None:
+    assert found is None
+  else:
+    assert found.name == expected