add optional_fields to KademliaDatagramBase

-update KademliaDatagramBase.bencode and decode_datagram

lbry/lbry/dht/serialization/datagram.py

@@ -7,9 +7,13 @@ REQUEST_TYPE = 0
 RESPONSE_TYPE = 1
 ERROR_TYPE = 2
 
+OPTIONAL_ARG_OFFSET = 100
+
 # bencode representation of argument keys
 PAGE_KEY = b'p'
 
+OPTIONAL_FIELDS = ()
+
 
 class KademliaDatagramBase:
     """
@@ -18,7 +22,7 @@ class KademliaDatagramBase:
     these correspond to the packet_type, rpc_id, and node_id args
     """
 
-    fields = [
+    required_fields = [
         'packet_type',
         'rpc_id',
         'node_id'
@@ -38,13 +42,18 @@ class KademliaDatagramBase:
         self.node_id = node_id
 
     def bencode(self) -> bytes:
-        return bencode({
-           i: getattr(self, k) for i, k in enumerate(self.fields)
-        })
+        datagram = {
+            i: getattr(self, k) for i, k in enumerate(self.required_fields)
+        }
+        for i, k in enumerate(OPTIONAL_FIELDS):
+            v = getattr(self, k, None)
+            if v is not None:
+                datagram[i + OPTIONAL_ARG_OFFSET] = v
+        return bencode(datagram)
 
 
 class RequestDatagram(KademliaDatagramBase):
-    fields = [
+    required_fields = [
         'packet_type',
         'rpc_id',
         'node_id',
@@ -104,7 +113,7 @@ class RequestDatagram(KademliaDatagramBase):
 
 
 class ResponseDatagram(KademliaDatagramBase):
-    fields = [
+    required_fields = [
         'packet_type',
         'rpc_id',
         'node_id',
@@ -119,7 +128,7 @@ class ResponseDatagram(KademliaDatagramBase):
 
 
 class ErrorDatagram(KademliaDatagramBase):
-    fields = [
+    required_fields = [
         'packet_type',
         'rpc_id',
         'node_id',
@@ -148,12 +157,15 @@ def decode_datagram(datagram: bytes) -> typing.Union[RequestDatagram, ResponseDa
     else:
         raise ValueError("invalid datagram type")
     datagram_class = msg_types[datagram_type]
-    return datagram_class(**{
-            k: primitive[i]  # pylint: disable=unsubscriptable-object
-            for i, k in enumerate(datagram_class.fields)
-            if i in primitive  # pylint: disable=unsupported-membership-test
-        }
-    )
+    decoded = {
+        k: primitive[i]  # pylint: disable=unsubscriptable-object
+        for i, k in enumerate(datagram_class.required_fields)
+        if i in primitive  # pylint: disable=unsupported-membership-test
+    }
+    for i, k in enumerate(OPTIONAL_FIELDS):
+        if i + OPTIONAL_ARG_OFFSET in primitive:
+            decoded[i + OPTIONAL_ARG_OFFSET] = primitive[i + OPTIONAL_ARG_OFFSET]
+    return datagram_class(**decoded)
 
 
 def make_compact_ip(address: str) -> bytearray: