lnpeer/lnworker: refactor htlc_switch

refactor `htlc_switch` to new architecture to make it more robust
against partial settlement of htlc sets and increase maintainability.
Htlcs are now processed in two steps, first the htlcs are collected into
sets from the channels, and potentially failed on their own already.
Then a second loop iterates over the htlc sets and finalizes only on
whole sets.

Author: f321x

electrum/lnchannel.py

@@ -783,7 +783,7 @@ def __init__(self, state: 'StoredDict', *, name=None, lnworker=None, initial_fee
         self.onion_keys = state['onion_keys']  # type: Dict[int, bytes]
         self.data_loss_protect_remote_pcp = state['data_loss_protect_remote_pcp']
         self.hm = HTLCManager(log=state['log'], initial_feerate=initial_feerate)
-        self.unfulfilled_htlcs = state["unfulfilled_htlcs"]  # type: Dict[int, Tuple[str, Optional[str]]]
+        self.unfulfilled_htlcs = state["unfulfilled_htlcs"]  # type: Dict[int, Optional[str]]
         # ^ htlc_id -> onion_packet_hex, forwarding_key
         self._state = ChannelState[state['state']]
         self.peer_state = PeerState.DISCONNECTED
@@ -1112,6 +1112,7 @@ def _assert_can_add_htlc(self, *, htlc_proposer: HTLCOwner, amount_msat: int,
             if amount_msat <= 0:
                 raise PaymentFailure("HTLC value must be positive")
             if amount_msat < chan_config.htlc_minimum_msat:
+                # todo: for incoming htlcs this could be handled more gracefully with `amount_below_minimum`
                 raise PaymentFailure(f'HTLC value too small: {amount_msat} msat')
 
         if self.htlc_slots_left(htlc_proposer) == 0:
@@ -1226,7 +1227,7 @@ def receive_htlc(self, htlc: UpdateAddHtlc, onion_packet:bytes = None) -> Update
         with self.db_lock:
             self.hm.recv_htlc(htlc)
             if onion_packet:
-                self.unfulfilled_htlcs[htlc.htlc_id] = onion_packet.hex(), None
+                self.unfulfilled_htlcs[htlc.htlc_id] = onion_packet.hex()
 
         self.logger.info("receive_htlc")
         return htlc