Remove transaction around OCI blobs

Fixes https://www.pivotaltracker.com/story/show/142341399

data/oci_model/blob.py

@@ -1,8 +1,11 @@
+import logging
+
 from peewee import IntegrityError
 
 from data.model import db_transaction
 from data.database import Blob, BlobPlacementLocation, BlobPlacement
 
+logger = logging.getLogger(__name__)
 
 def _ensure_sha256_header(digest):
   if digest.startswith('sha256:'):
@@ -17,21 +20,27 @@ def get_blob(digest):
 
 def get_or_create_blob(digest, size, media_type_name, locations):
   """ Try to find a blob by its digest or create it. """
-  with db_transaction():
-    try:
-      blob = get_blob(digest)
-    except Blob.DoesNotExist:
-      blob = Blob.create(digest=_ensure_sha256_header(digest),
-                         media_type_id=Blob.media_type.get_id(media_type_name),
-                         size=size)
-    for location_name in locations:
-      location_id = BlobPlacement.location.get_id(location_name)
-      try:
-        BlobPlacement.create(blob=blob, location=location_id)
-      except IntegrityError:
-        pass
 
-    return blob
+  # Get or create the blog entry for the digest.
+  try:
+    blob = get_blob(digest)
+    logger.debug('Retrieved blob with digest %s', digest)
+  except Blob.DoesNotExist:
+    blob = Blob.create(digest=_ensure_sha256_header(digest),
+                       media_type_id=Blob.media_type.get_id(media_type_name),
+                       size=size)
+    logger.debug('Created blob with digest %s', digest)
+
+  # Add the locations to the blob.
+  for location_name in locations:
+    location_id = BlobPlacement.location.get_id(location_name)
+    try:
+      BlobPlacement.create(blob=blob, location=location_id)
+    except IntegrityError:
+      logger.debug('Location %s already existing for blob %s', location_name, blob.id)
+      pass
+
+  return blob
 
 
 def get_blob_locations(digest):