move lightning message encoding to new lnmsg module

electrum/lnmsg.py

@@ -0,0 +1,155 @@
+import json
+import os
+from typing import Callable, Tuple
+from collections import OrderedDict
+
+def _eval_length_term(x, ma: dict) -> int:
+    """
+    Evaluate a term of the simple language used
+    to specify lightning message field lengths.
+
+    If `x` is an integer, it is returned as is,
+    otherwise it is treated as a variable and
+    looked up in `ma`.
+
+    If the value in `ma` was no integer, it is
+    assumed big-endian bytes and decoded.
+
+    Returns evaluated result as int
+    """
+    try:
+        x = int(x)
+    except ValueError:
+        x = ma[x]
+    try:
+        x = int(x)
+    except ValueError:
+        x = int.from_bytes(x, byteorder='big')
+    return x
+
+def _eval_exp_with_ctx(exp, ctx: dict) -> int:
+    """
+    Evaluate simple mathematical expression given
+    in `exp` with context (variables assigned)
+    from the dict `ctx`.
+
+    Returns evaluated result as int
+    """
+    exp = str(exp)
+    if "*" in exp:
+        assert "+" not in exp
+        result = 1
+        for term in exp.split("*"):
+            result *= _eval_length_term(term, ctx)
+        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]]:
+    """
+    Generate a message handler function (taking bytes)
+    for message type `k` with specification `v`
+
+    Check lib/lightning.json, `k` could be 'init',
+    and `v` could be
+
+      { type: 16, payload: { 'gflen': ..., ... }, ... }
+
+    Returns function taking bytes
+    """
+    def handler(data: bytes) -> Tuple[str, dict]:
+        nonlocal k, v
+        ma = {}
+        pos = 0
+        for fieldname in v["payload"]:
+            poslenMap = v["payload"][fieldname]
+            if "feature" in poslenMap and pos == len(data):
+                continue
+            assert pos == _eval_exp_with_ctx(poslenMap["position"], ma)
+            length = poslenMap["length"]
+            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
+    return handler
+
+class LNSerializer:
+    def __init__(self):
+        message_types = {}
+        path = os.path.join(os.path.dirname(__file__), 'lightning.json')
+        with open(path) as f:
+            structured = json.loads(f.read(), object_pairs_hook=OrderedDict)
+
+        for k in structured:
+            v = structured[k]
+            # 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"]:
+                continue
+            if len(v["payload"]) == 0:
+                continue
+            try:
+                num = int(v["type"])
+            except ValueError:
+                #print("skipping", k)
+                continue
+            byts = num.to_bytes(2, 'big')
+            assert byts not in message_types, (byts, message_types[byts].__name__, k)
+            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 message_types[b"\x00\x10"].__name__ == "init_handler"
+        self.structured = structured
+        self.message_types = message_types
+
+    def encode_msg(self, msg_type : str, **kwargs) -> bytes:
+        """
+        Encode kwargs into a Lightning message (bytes)
+        of the type given in the msg_type string
+        """
+        typ = self.structured[msg_type]
+        data = int(typ["type"]).to_bytes(2, 'big')
+        lengths = {}
+        for k in typ["payload"]:
+            poslenMap = typ["payload"][k]
+            if "feature" in poslenMap: continue
+            leng = _eval_exp_with_ctx(poslenMap["length"], lengths)
+            try:
+                clone = dict(lengths)
+                clone.update(kwargs)
+                leng = _eval_exp_with_ctx(poslenMap["length"], clone)
+            except KeyError:
+                pass
+            try:
+                param = kwargs[k]
+            except KeyError:
+                param = 0
+            try:
+                if not isinstance(param, bytes):
+                    assert isinstance(param, int), "field {} is neither bytes or int".format(k)
+                    param = param.to_bytes(leng, 'big')
+            except ValueError:
+                raise Exception("{} does not fit in {} bytes".format(k, leng))
+            lengths[k] = len(param)
+            if lengths[k] != leng:
+                raise Exception("field {} is {} bytes long, should be {} bytes long".format(k, lengths[k], leng))
+            data += param
+        return data
+
+    def decode_msg(self, data : bytes) -> Tuple[str, dict]:
+        """
+        Decode Lightning message by reading the first
+        two bytes to determine message type.
+
+        Returns message type string and parsed message contents dict
+        """
+        typ = data[:2]
+        k, parsed = self.message_types[typ](data[2:])
+        return k, parsed
+
+_inst = LNSerializer()
+encode_msg = _inst.encode_msg
+decode_msg = _inst.decode_msg