Optimize lookup of shared global blobs

Currently, we only have one (the shared empty layer), but this should make the blob lookups for repositories significantly faster, as we won't need to do the massive join.

data/registry_model/registry_pre_oci_model.py

@@ -120,7 +120,7 @@ class PreOCIModel(SharedModel, RegistryDataInterface):
 
     # Ensure all the blobs in the manifest exist.
     digests = manifest_interface_instance.checksums
-    query = model.storage.lookup_repo_storages_by_content_checksum(repository_ref._db_id, digests)
+    query = self._lookup_repo_storages_by_content_checksum(repository_ref._db_id, digests)
     blob_map = {s.content_checksum: s for s in query}
     for layer in manifest_interface_instance.layers:
       digest_str = str(layer.digest)
@@ -481,9 +481,7 @@ class PreOCIModel(SharedModel, RegistryDataInterface):
     if manifest is None:
       return None
 
-    blob_query = model.storage.lookup_repo_storages_by_content_checksum(repo,
-                                                                        manifest.checksums)
-
+    blob_query = self._lookup_repo_storages_by_content_checksum(repo, manifest.checksums)
     storage_map = {blob.content_checksum: blob.id for blob in blob_query}
     try:
       tag_manifest, _ = model.tag.associate_generated_tag_manifest_with_tag(tag_obj, manifest,
@@ -585,10 +583,12 @@ class PreOCIModel(SharedModel, RegistryDataInterface):
     there may be multiple records in the same repository for the same blob digest, so the return
     value of this function may change.
     """
-    try:
-      image_storage = model.blob.get_repository_blob_by_digest(repository_ref._db_id, blob_digest)
-    except model.BlobDoesNotExist:
-      return None
+    image_storage = self._get_shared_storage(blob_digest)
+    if image_storage is None:
+      try:
+        image_storage = model.blob.get_repository_blob_by_digest(repository_ref._db_id, blob_digest)
+      except model.BlobDoesNotExist:
+        return None
 
     assert image_storage.cas_path is not None