nlmanager: nlpacket: throw an error if the user of nlmanager tries to encode an attribute that we haven't added an encode() method for

+ adding one byte attribute class for protodown operations.

Ticket: CM-11581
Reviewed By: CCR-4721
Testing Done: Smoke + custom interface file with clag bond that requires protodown.

- nlmanager will now throw an exception if a user is trying to use the default
attribute class when in fact he should use a more specific attribute class.

- The protodown implementation needed to use a one byte attribute to set the protodown state

Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>

nlmanager/nlpacket.py

@@ -205,6 +205,10 @@ class Attribute(object):
         return raw
 
     def encode(self):
+
+        if not self.LEN:
+            raise Exception('Please define an encode() method in your child attribute class, or do not use AttributeGeneric')
+
         length = self.HEADER_LEN + self.LEN
         attr_type_with_flags = self.atype
 
@@ -447,6 +451,14 @@ class AttributeGeneric(Attribute):
             raise
 
 
+class AttributeOneByteValue(AttributeGeneric):
+
+    def __init__(self, atype, string, logger):
+        Attribute.__init__(self, atype, string, logger)
+        self.PACK = '=B'
+        self.LEN = calcsize(self.PACK)
+
+
 class AttributeIFLA_AF_SPEC(Attribute):
     """
     value will be a dictionary such as:
@@ -1638,7 +1650,7 @@ class Link(NetlinkPacket):
         IFLA_PHYS_SWITCH_ID  : ('IFLA_PHYS_SWITCH_ID', AttributeGeneric),
         IFLA_LINK_NETNSID    : ('IFLA_LINK_NETNSID', AttributeGeneric),
         IFLA_PHYS_PORT_NAME  : ('IFLA_PHYS_PORT_NAME', AttributeGeneric),
-        IFLA_PROTO_DOWN      : ('IFLA_PROTO_DOWN', AttributeGeneric),
+        IFLA_PROTO_DOWN      : ('IFLA_PROTO_DOWN', AttributeOneByteValue),
         IFLA_LINKPROTODOWN   : ('IFLA_LINKPROTODOWN', AttributeGeneric)
     }