Properly handle the empty layer when pushing schema 2 manifests

Docker doesn't send us the contents of this layer, so we are forced to synthesize it ourselves

data/model/blob.py

@@ -1,3 +1,5 @@
+import logging
+
 from datetime import datetime
 from uuid import uuid4
 
@@ -7,6 +9,9 @@ from data.database import (Repository, Namespace, ImageStorage, Image, ImageStor
                            BlobUpload, ImageStorageLocation, db_random_func)
 
 
+logger = logging.getLogger(__name__)
+
+
 def get_repository_blob_by_digest(repository, blob_digest):
   """ Find the content-addressable blob linked to the specified repository.
   """
@@ -157,3 +162,31 @@ def initiate_upload(namespace, repo_name, uuid, location_name, storage_metadata)
   location = storage_model.get_image_location_for_name(location_name)
   return BlobUpload.create(repository=repo, location=location.id, uuid=uuid,
                            storage_metadata=storage_metadata)
+
+
+def get_or_create_shared_blob(digest, byte_data, storage):
+  """ Returns the ImageStorage blob with the given digest or, if not present,
+      adds a row and writes the given byte data to the storage engine.
+      This method is *only* to be used for shared blobs that are globally
+      accessible, such as the special empty gzipped tar layer that Docker
+      no longer pushes to us.
+  """
+  try:
+    return ImageStorage.get(content_checksum=digest, uploading=False)
+  except ImageStorage.DoesNotExist:
+    record = ImageStorage.create(image_size=len(byte_data), content_checksum=digest,
+                                 cas_path=True, uploading=True)
+    preferred = storage.preferred_locations[0]
+    location_obj = ImageStorageLocation.get(name=preferred)
+    try:
+      storage.put_content([preferred], storage_model.get_layer_path(record), byte_data)
+      ImageStoragePlacement.create(storage=record, location=location_obj)
+
+      record.uploading = False
+      record.save()
+    except:
+      logger.exception('Exception when trying to write special layer %s', digest)
+      record.delete_instance()
+      raise
+
+    return record