Handle invalid tar layers and add tests

util/tarlayerformat.py

@@ -1,6 +1,11 @@
 import os
 import tarfile
 
+class TarLayerReadException(Exception):
+  """ Exception raised when reading a layer has failed. """
+  pass
+  
+
 class TarLayerFormat(object):
   """ Class which creates a generator of the combined TAR data. """
   def __init__(self, tar_iterator):
@@ -10,9 +15,14 @@ class TarLayerFormat(object):
     for current_tar in self.tar_iterator():
       # Read the current TAR. If it is empty, we just continue
       # to the next one.
+      tar_file = None
       try:
         tar_file = tarfile.open(mode='r|*', fileobj=current_tar)
       except tarfile.ReadError as re:
+        if re.message != 'empty file':
+          raise TarLayerReadException('Could not read layer')
+
+      if not tar_file:
         continue
 
       # For each of the tar entries, yield them IF and ONLY IF we have not