Fix lookup of manifests referenced solely by a manifest list

We need to ensure we can find them if there is an active tag pointing to the parent list

data/model/oci/manifest.py

@@ -2,7 +2,7 @@ import logging
 
 from collections import namedtuple
 
-from peewee import IntegrityError
+from peewee import IntegrityError, JOIN
 
 from data.database import (Tag, Manifest, ManifestBlob, ManifestLegacyImage, ManifestChild,
                            db_transaction)
@@ -32,8 +32,24 @@ def lookup_manifest(repository_id, manifest_digest, allow_dead=False):
            .where(Manifest.repository == repository_id)
            .where(Manifest.digest == manifest_digest))
 
-  if not allow_dead:
-    query = filter_to_alive_tags(query.join(Tag)).group_by(Manifest.id)
+  if allow_dead:
+    try:
+      return query.get()
+    except Manifest.DoesNotExist:
+      return None
+
+  # Try first to filter to those manifests referenced by an alive tag,
+  try:
+    return filter_to_alive_tags(query.join(Tag)).get()
+  except Manifest.DoesNotExist:
+    pass
+
+  # Try referenced as the child of a manifest that has an alive tag.
+  query = (query
+           .join(ManifestChild, on=(ManifestChild.child_manifest == Manifest.id))
+           .join(Tag, on=(Tag.manifest == ManifestChild.manifest)))
+
+  query = filter_to_alive_tags(query)
 
   try:
     return query.get()