Sorry, _MARKER is not overkill, so it have to be added again.

Well, _MARKER is used to backward compatibility and to raise an error
which if the networkcache server is not consistenty.

Without _MARKER there is not way to make these conditions.

slapos/libnetworkcache.py

@@ -24,6 +24,9 @@ import urlparse
 import M2Crypto
 
 
+_MARKER = ([], [''], None, '')
+
+
 class NetworkcacheClient(object):
   '''
     NetworkcacheClient is a wrapper for httplib.
@@ -172,11 +175,14 @@ class NetworkcacheClient(object):
 
     # Filtering...
     data_list = json.loads(data)
-    if len(data_list) > 1 and self.signature_certificate_file_list in _MARKER:
+    if len(data_list) > 1 and not \
+         (self.signature_certificate_file_list in _MARKER or \
+          self.signature_certificate_url_list in _MARKER):
       raise DirectoryNotFound('Too many entries for a given directory. ' \
                'Directory: %s. Entries: %s.' % (directory_key, str(data_list)))
 
-    if self.signature_certificate_file_list not in _MARKER:
+    if self.signature_certificate_file_list not in _MARKER or \
+         self.signature_certificate_url_list not in _MARKER:
       method = self._verifySignatureInCertificateList
       data_list = filter(lambda x: method(x[1]), data_list)
       if len(data_list) > 1:
@@ -194,7 +200,7 @@ class NetworkcacheClient(object):
     """
       Return the signature based on certification file.
     """
-    if self.signature_private_key_file is None:
+    if self.signature_private_key_file in _MARKER:
       return ''
 
     SignEVP = M2Crypto.EVP.load_key(self.signature_private_key_file)