Basic labels support

Adds basic labels support to the registry code (V2), and the API. Note that this does not yet add any UI related support.

data/database.py

@@ -423,8 +423,7 @@ class Repository(BaseModel):
     # These models don't need to use transitive deletes, because the referenced objects
     # are cleaned up directly
     skip_transitive_deletes = {RepositoryTag, RepositoryBuild, RepositoryBuildTrigger, BlobUpload,
-                               Image, TagManifest, DerivedStorageForImage}
-
+                               Image, TagManifest, TagManifestLabel, Label, DerivedStorageForImage}
     delete_instance_filtered(self, Repository, delete_nullable, skip_transitive_deletes)
 
 
@@ -920,5 +919,40 @@ class ServiceKey(BaseModel):
   approval = ForeignKeyField(ServiceKeyApproval, null=True)
 
 
+class MediaType(BaseModel):
+  """ MediaType is an enumeration of the possible formats of various objects in the data model. """
+  name = CharField(index=True, unique=True)
+
+
+class LabelSourceType(BaseModel):
+  """ LabelSourceType is an enumeration of the possible sources for a label. """
+  name = CharField(index=True, unique=True)
+  mutable = BooleanField(default=False)
+
+
+class Label(BaseModel):
+  """ Label represents user-facing metadata associated with another entry in the
+      database (e.g. a Manifest). """
+  uuid = CharField(default=uuid_generator, index=True, unique=True)
+  key = CharField(index=True)
+  value = TextField()
+  media_type = ForeignKeyField(MediaType)
+  source_type = ForeignKeyField(LabelSourceType)
+
+
+class TagManifestLabel(BaseModel):
+  """ Mapping from a tag manifest to a label. """
+  repository = ForeignKeyField(Repository, index=True)
+  annotated = ForeignKeyField(TagManifest, index=True)
+  label = ForeignKeyField(Label)
+
+  class Meta:
+    database = db
+    read_slaves = (read_slave,)
+    indexes = (
+      (('annotated', 'label'), True),
+    )
+
+
 is_model = lambda x: inspect.isclass(x) and issubclass(x, BaseModel) and x is not BaseModel
 all_models = [model[1] for model in inspect.getmembers(sys.modules[__name__], is_model)]