Implement the new OCI-based registry data model

Note that this change does *not* enable the new data model by default, but does allow it to be used when a special environment variable is specified.

test/registry/registry_tests.py

@@ -1,4 +1,5 @@
 # pylint: disable=W0401, W0621, W0613, W0614, R0913
+import os
 import hashlib
 import tarfile
 
@@ -51,7 +52,30 @@ def test_multi_layer_images_push_pull(pusher, puller, multi_layer_images, livese
               credentials=credentials)
 
 
+def test_overwrite_tag(pusher, puller, basic_images, different_images, liveserver_session,
+                       app_reloader):
+  """ Test: Basic push and pull of an image to a new repository, followed by a push to the same
+      tag with different images. """
+  credentials = ('devtable', 'password')
 
+  # Push a new repository.
+  pusher.push(liveserver_session, 'devtable', 'newrepo', 'latest', basic_images,
+              credentials=credentials)
+
+  # Pull the repository to verify.
+  puller.pull(liveserver_session, 'devtable', 'newrepo', 'latest', basic_images,
+              credentials=credentials)
+
+  # Push a new repository.
+  pusher.push(liveserver_session, 'devtable', 'newrepo', 'latest', different_images,
+              credentials=credentials)
+
+  # Pull the repository to verify.
+  puller.pull(liveserver_session, 'devtable', 'newrepo', 'latest', different_images,
+              credentials=credentials)
+
+
+@pytest.mark.skipif(os.getenv('OCI_DATA_MODEL') == 'true', reason="no backfill in new model")
 def test_no_tag_manifests(pusher, puller, basic_images, liveserver_session, app_reloader,
                           liveserver, registry_server_executor):
   """ Test: Basic pull without manifests. """