fix lightning "init" msg assertion error

closes #5962

electrum/lnmsg.py

@@ -44,12 +44,12 @@ def _eval_exp_with_ctx(exp, ctx: dict) -> int:
         return result
     return sum(_eval_length_term(x, ctx) for x in exp.split("+"))
 
-def _make_handler(k: str, v: dict) -> Callable[[bytes], Tuple[str, dict]]:
+def _make_handler(msg_name: str, v: dict) -> Callable[[bytes], Tuple[str, dict]]:
     """
     Generate a message handler function (taking bytes)
-    for message type `k` with specification `v`
+    for message type `msg_name` with specification `v`
 
-    Check lib/lightning.json, `k` could be 'init',
+    Check lib/lightning.json, `msg_name` could be 'init',
     and `v` could be
 
       { type: 16, payload: { 'gflen': ..., ... }, ... }
@@ -57,8 +57,8 @@ def _make_handler(k: str, v: dict) -> Callable[[bytes], Tuple[str, dict]]:
     Returns function taking bytes
     """
     def handler(data: bytes) -> Tuple[str, dict]:
-        nonlocal k, v
-        ma = {}
+        nonlocal msg_name, v
+        ma = {}  # map of field name -> field data; after parsing msg
         pos = 0
         for fieldname in v["payload"]:
             poslenMap = v["payload"][fieldname]
@@ -69,8 +69,9 @@ def _make_handler(k: str, v: dict) -> Callable[[bytes], Tuple[str, dict]]:
             length = _eval_exp_with_ctx(length, ma)
             ma[fieldname] = data[pos:pos+length]
             pos += length
-        assert pos == len(data), (k, pos, len(data))
-        return k, ma
+        # BOLT-01: "MUST ignore any additional data within a message beyond the length that it expects for that type."
+        assert pos <= len(data), (msg_name, pos, len(data))
+        return msg_name, ma
     return handler
 
 class LNSerializer:
@@ -80,12 +81,12 @@ class LNSerializer:
         with open(path) as f:
             structured = json.loads(f.read(), object_pairs_hook=OrderedDict)
 
-        for k in structured:
-            v = structured[k]
+        for msg_name in structured:
+            v = structured[msg_name]
             # these message types are skipped since their types collide
             # (for example with pong, which also uses type=19)
             # we don't need them yet
-            if k in ["final_incorrect_cltv_expiry", "final_incorrect_htlc_amount"]:
+            if msg_name in ["final_incorrect_cltv_expiry", "final_incorrect_htlc_amount"]:
                 continue
             if len(v["payload"]) == 0:
                 continue
@@ -95,11 +96,11 @@ class LNSerializer:
                 #print("skipping", k)
                 continue
             byts = num.to_bytes(2, 'big')
-            assert byts not in message_types, (byts, message_types[byts].__name__, k)
+            assert byts not in message_types, (byts, message_types[byts].__name__, msg_name)
             names = [x.__name__ for x in message_types.values()]
-            assert k + "_handler" not in names, (k, names)
-            message_types[byts] = _make_handler(k, v)
-            message_types[byts].__name__ = k + "_handler"
+            assert msg_name + "_handler" not in names, (msg_name, names)
+            message_types[byts] = _make_handler(msg_name, v)
+            message_types[byts].__name__ = msg_name + "_handler"
 
         assert message_types[b"\x00\x10"].__name__ == "init_handler"
         self.structured = structured