Fix bug which allowed for implicit library namespace access via the V1 registry protocol when the feature flag was off

Now we raise a 400 as expected

util/names.py

@@ -14,6 +14,13 @@ TAG_REGEX = re.compile(FULL_TAG_PATTERN)
 TAG_ERROR = ('Invalid tag: must match [A-Za-z0-9_.-], NOT start with "." or "-", '
              'and can contain 1-128 characters')
 
+
+class ImplicitLibraryNamespaceNotAllowed(Exception):
+  """ Exception raised if the implicit library namespace was specified but is
+      not allowed. """
+  pass
+
+
 def escape_tag(tag, default='latest'):
   """ Escapes a Docker tag, ensuring it matches the tag regular expression. """
   if not tag:
@@ -24,13 +31,16 @@ def escape_tag(tag, default='latest'):
   return tag[0:127]
 
 
-def parse_namespace_repository(repository, library_namespace, include_tag=False):
+def parse_namespace_repository(repository, library_namespace, include_tag=False,
+                               allow_library=True):
   repository = repository.encode('unidecode', 'ignore')
 
   parts = repository.rstrip('/').split('/', 1)
   if len(parts) < 2:
     namespace = library_namespace
     repository = parts[0]
+    if not allow_library:
+      raise ImplicitLibraryNamespaceNotAllowed()
   else:
     (namespace, repository) = parts