lnpeer: reestablish_channel - replay un-acked local updates

Replay un-acked local updates (including commitment_signed) byte-for-byte.
If we have sent them a commitment signature that they "lost" (due to disconnect),
we need to make sure we replay the same local updates, as otherwise they could
end up with two (or more) signed valid commitment transactions at the same ctn.
Multiple valid ctxs at the same ctn is a major headache for pre-signing spending txns,
e.g. for watchtowers, hence we must ensure these ctxs coincide.

electrum/lnhtlc.py

@@ -2,7 +2,7 @@ from copy import deepcopy
 from typing import Optional, Sequence, Tuple, List
 
 from .lnutil import SENT, RECEIVED, LOCAL, REMOTE, HTLCOwner, UpdateAddHtlc, Direction, FeeUpdate
-from .util import bh2u
+from .util import bh2u, bfh
 
 
 class HTLCManager:
@@ -34,6 +34,9 @@ class HTLCManager:
                 log[sub]['fails'] = {int(htlc_id): coerceHtlcOwner2IntMap(ctns) for htlc_id, ctns in log[sub]['fails'].items()}
                 # "side who initiated fee update" -> action -> list of FeeUpdates
                 log[sub]['fee_updates'] = [FeeUpdate.from_dict(fee_upd) for fee_upd in log[sub]['fee_updates']]
+        if 'unacked_local_updates' not in log:
+            log['unacked_local_updates'] = []
+        log['unacked_local_updates'] = [bfh(upd) for upd in log['unacked_local_updates']]
         # maybe bootstrap fee_updates if initial_feerate was provided
         if initial_feerate is not None:
             assert type(initial_feerate) is int
@@ -68,6 +71,7 @@ class HTLCManager:
             log[sub]['adds'] = d
             # fee_updates
             log[sub]['fee_updates'] = [FeeUpdate.to_dict(fee_upd) for fee_upd in log[sub]['fee_updates']]
+        log['unacked_local_updates'] = [bh2u(upd) for upd in log['unacked_local_updates']]
         return log
 
     ##### Actions on channel:
@@ -166,6 +170,8 @@ class HTLCManager:
         for fee_update in self.log[LOCAL]['fee_updates']:
             if fee_update.ctns[LOCAL] is None and fee_update.ctns[REMOTE] <= self.ctn_latest(REMOTE):
                 fee_update.ctns[LOCAL] = self.ctn_latest(LOCAL) + 1
+        # no need to keep local update raw msgs anymore, they have just been ACKed.
+        self.log['unacked_local_updates'].clear()
 
     def discard_unsigned_remote_updates(self):
         """Discard updates sent by the remote, that the remote itself
@@ -186,6 +192,12 @@ class HTLCManager:
             if fee_update.ctns[LOCAL] > self.ctn_latest(LOCAL):
                 del self.log[REMOTE]['fee_updates'][i]
 
+    def store_local_update_raw_msg(self, raw_update_msg: bytes):
+        self.log['unacked_local_updates'].append(raw_update_msg)
+
+    def get_unacked_local_updates(self) -> Sequence[bytes]:
+        return self.log['unacked_local_updates']
+
     ##### Queries re HTLCs:
 
     def htlcs_by_direction(self, subject: HTLCOwner, direction: Direction,

electrum/lnpeer.py

@@ -93,7 +93,19 @@ class Peer(Logger):
     def send_message(self, message_name: str, **kwargs):
         assert type(message_name) is str
         self.logger.debug(f"Sending {message_name.upper()}")
-        self.transport.send_bytes(encode_msg(message_name, **kwargs))
+        raw_msg = encode_msg(message_name, **kwargs)
+        self._store_raw_msg_if_local_update(raw_msg, message_name=message_name, channel_id=kwargs.get("channel_id"))
+        self.transport.send_bytes(raw_msg)
+
+    def _store_raw_msg_if_local_update(self, raw_msg: bytes, *, message_name: str, channel_id: Optional[bytes]):
+        if not (message_name.startswith("update_") or message_name == "commitment_signed"):
+            return
+        assert channel_id
+        chan = self.lnworker.channels[channel_id]  # type: Channel
+        chan.hm.store_local_update_raw_msg(raw_msg)
+        if message_name == "commitment_signed":
+            # saving now, to ensure replaying updates works (in case of channel reestablishment)
+            self.lnworker.save_channel(chan)
 
     async def initialize(self):
         if isinstance(self.transport, LNTransport):
@@ -733,6 +745,16 @@ class Peer(Logger):
         should_close_they_are_ahead = False
         # compare remote ctns
         if next_remote_ctn != their_next_local_ctn:
+            if their_next_local_ctn == latest_remote_ctn and chan.hm.is_revack_pending(REMOTE):
+                # Replay un-acked local updates (including commitment_signed) byte-for-byte.
+                # If we have sent them a commitment signature that they "lost" (due to disconnect),
+                # we need to make sure we replay the same local updates, as otherwise they could
+                # end up with two (or more) signed valid commitment transactions at the same ctn.
+                # Multiple valid ctxs at the same ctn is a major headache for pre-signing spending txns,
+                # e.g. for watchtowers, hence we must ensure these ctxs coincide.
+                for raw_upd_msg in chan.hm.get_unacked_local_updates():
+                    self.transport.send_bytes(raw_upd_msg)
+            else:
                 self.logger.warning(f"channel_reestablish: expected remote ctn {next_remote_ctn}, got {their_next_local_ctn}")
                 if their_next_local_ctn < next_remote_ctn:
                     should_close_we_are_ahead = True