Separate between fee related NoPathFound and other causes when doing lightning payments/forwards

electrum/lnpeer.py

@@ -44,7 +44,8 @@
                      ln_compare_features, MIN_FINAL_CLTV_DELTA_ACCEPTED,
                      RemoteMisbehaving, ShortChannelID,
                      IncompatibleLightningFeatures, derive_payment_secret_from_payment_preimage,
-                     ChannelType, LNProtocolWarning, validate_features, IncompatibleOrInsaneFeatures)
+                     ChannelType, LNProtocolWarning, validate_features,
+                     IncompatibleOrInsaneFeatures, FeeBudgetExceeded)
 from .lnutil import FeeUpdate, channel_id_from_funding_tx, PaymentFeeBudget
 from .lnutil import serialize_htlc_key, Keypair
 from .lntransport import LNTransport, LNTransportBase, LightningPeerConnectionClosed, HandshakeFailed
@@ -2063,11 +2064,12 @@ async def maybe_forward_trampoline(
             )
         except OnionRoutingFailure as e:
             raise
+        except FeeBudgetExceeded:
+            raise OnionRoutingFailure(code=OnionFailureCode.TRAMPOLINE_FEE_INSUFFICIENT, data=b'')
         except PaymentFailure as e:
             self.logger.debug(
                 f"maybe_forward_trampoline. PaymentFailure for {payment_hash.hex()=}, {payment_secret.hex()=}: {e!r}")
-            # FIXME: adapt the error code
-            raise OnionRoutingFailure(code=OnionFailureCode.TRAMPOLINE_FEE_INSUFFICIENT, data=b'')
+            raise OnionRoutingFailure(code=OnionFailureCode.UNKNOWN_NEXT_PEER, data=b'')
 
     def _maybe_refuse_to_forward_htlc_that_corresponds_to_payreq_we_created(self, payment_hash: bytes) -> bool:
         """Returns True if the HTLC should be failed.

electrum/lnutil.py

@@ -452,6 +452,9 @@ class PaymentFailure(UserFacingException): pass
 class NoPathFound(PaymentFailure):
     def __str__(self):
         return _('No path found')
+class FeeBudgetExceeded(PaymentFailure):
+    def __str__(self):
+        return _('Fee budget exceeded')
 
 
 class LNProtocolError(Exception):

electrum/lnworker.py

@@ -67,7 +67,7 @@
                      NUM_MAX_EDGES_IN_PAYMENT_PATH, SENT, RECEIVED, HTLCOwner,
                      UpdateAddHtlc, Direction, LnFeatures, ShortChannelID,
                      HtlcLog, derive_payment_secret_from_payment_preimage,
-                     NoPathFound, InvalidGossipMsg)
+                     NoPathFound, InvalidGossipMsg, FeeBudgetExceeded)
 from .lnutil import ln_compare_features, IncompatibleLightningFeatures, PaymentFeeBudget
 from .transaction import PartialTxOutput, PartialTransaction, PartialTxInput
 from .lnonion import decode_onion_error, OnionFailureCode, OnionRoutingFailure, OnionPacket
@@ -1880,6 +1880,7 @@ async def create_routes_for_payment(
         and mpp is supported by the receiver, we will split the payment."""
         trampoline_features = LnFeatures.VAR_ONION_OPT
         local_height = self.network.get_local_height()
+        fee_related_error = None  # type: Optional[FeeBudgetExceeded]
         if channels:
             my_active_channels = channels
         else:
@@ -1970,8 +1971,9 @@ async def create_routes_for_payment(
                             )
                             routes.append((shi, per_trampoline_cltv_delta, trampoline_onion))
                         if per_trampoline_fees != 0:
-                            self.logger.info('not enough margin to pay trampoline fee')
-                            raise NoPathFound()
+                            e = 'not enough margin to pay trampoline fee'
+                            self.logger.info(e)
+                            raise FeeBudgetExceeded(e)
                 else:
                     # We atomically loop through a split configuration. If there was
                     # a failure to find a path for a single part, we try the next configuration
@@ -2004,9 +2006,14 @@ async def create_routes_for_payment(
                             routes.append((shi, paysession.min_final_cltv_delta, fwd_trampoline_onion))
             except NoPathFound:
                 continue
+            except FeeBudgetExceeded as e:
+                fee_related_error = e
+                continue
             for route in routes:
                 yield route
             return
+        if fee_related_error is not None:
+            raise fee_related_error
         raise NoPathFound()
 
     @profiler
@@ -2079,7 +2086,7 @@ def create_route_for_single_htlc(
             route, budget=budget, amount_msat_for_dest=amount_msat, cltv_delta_for_dest=min_final_cltv_delta,
         ):
             self.logger.info(f"rejecting route (exceeds budget): {route=}. {budget=}")
-            raise NoPathFound()
+            raise FeeBudgetExceeded()
         assert len(route) > 0
         if route[-1].end_node != invoice_pubkey:
             raise LNPathInconsistent("last node_id != invoice pubkey")

electrum/trampoline.py

@@ -3,7 +3,7 @@
 import random
 from typing import Mapping, DefaultDict, Tuple, Optional, Dict, List, Iterable, Sequence, Set, Any
 
-from .lnutil import LnFeatures, PaymentFeeBudget
+from .lnutil import LnFeatures, PaymentFeeBudget, FeeBudgetExceeded
 from .lnonion import calc_hops_data_for_payment, new_onion_packet, OnionPacket
 from .lnrouter import RouteEdge, TrampolineEdge, LNPaymentRoute, is_route_within_budget, LNPaymentTRoute
 from .lnutil import NoPathFound
@@ -161,7 +161,7 @@ def _allocate_fee_along_route(
         assert trampoline_fee_level > 0
         MAX_LEVEL = 6
         if trampoline_fee_level > MAX_LEVEL:
-            raise NoPathFound()
+            raise FeeBudgetExceeded("highest trampoline fee level reached")
         budget_to_use = budget.fee_msat // (2 ** (MAX_LEVEL - trampoline_fee_level))
     _logger.debug(f"_allocate_fee_along_route(). {trampoline_fee_level=}, {budget.fee_msat=}, {budget_to_use=}")
     # replace placeholder fees
@@ -264,7 +264,7 @@ def create_trampoline_route(
         amount_msat_for_dest=amount_msat,
         cltv_delta_for_dest=min_final_cltv_delta,
     ):
-        raise NoPathFound("route exceeds budget")
+        raise FeeBudgetExceeded(f"route exceeds budget: budget: {budget}")
     return route