Allow lookup of "dead" manifests so manifest links can be clicked in the tag history

Fixes https://jira.coreos.com/browse/QUAY-1021

data/model/tag.py

@@ -649,9 +649,9 @@ def delete_manifest_by_digest(namespace, repo_name, digest):
   return [tag_manifest.tag for tag_manifest in tag_manifests]
 
 
-def load_manifest_by_digest(namespace, repo_name, digest):
+def load_manifest_by_digest(namespace, repo_name, digest, allow_dead=False):
   try:
-    return (_load_repo_manifests(namespace, repo_name)
+    return (_load_repo_manifests(namespace, repo_name, allow_dead=allow_dead)
             .where(TagManifest.digest == digest)
             .get())
   except TagManifest.DoesNotExist:
@@ -659,15 +659,19 @@ def load_manifest_by_digest(namespace, repo_name, digest):
     raise InvalidManifestException(msg)
 
 
-def _load_repo_manifests(namespace, repo_name):
-  return _tag_alive(TagManifest
-                    .select(TagManifest, RepositoryTag)
-                    .join(RepositoryTag)
-                    .join(Image)
-                    .join(Repository)
-                    .join(Namespace, on=(Namespace.id == Repository.namespace_user))
-                    .where(Repository.name == repo_name, Namespace.username == namespace))
+def _load_repo_manifests(namespace, repo_name, allow_dead=False):
+  query = (TagManifest
+           .select(TagManifest, RepositoryTag)
+           .join(RepositoryTag)
+           .join(Image)
+           .join(Repository)
+           .join(Namespace, on=(Namespace.id == Repository.namespace_user))
+           .where(Repository.name == repo_name, Namespace.username == namespace))
 
+  if not allow_dead:
+    query = _tag_alive(query)
+
+  return query
 
 def change_repository_tag_expiration(namespace_name, repo_name, tag_name, expiration_date):
   """ Changes the expiration of the tag with the given name to the given expiration datetime. If

endpoints/api/manifest_models_pre_oci.py

@@ -41,7 +41,8 @@ class ManifestLabelPreOCI(ManifestLabelInterface):
   
   def get_repository_manifest(self, namespace_name, repository_name, digest):
     try:
-      tag_manifest = model.tag.load_manifest_by_digest(namespace_name, repository_name, digest)
+      tag_manifest = model.tag.load_manifest_by_digest(namespace_name, repository_name, digest,
+                                                       allow_dead=True)
     except model.DataModelException:
       return None