lnchan.receive_revocation: tolerate not having htlc fail reason

If we get a revack after reestablish, but the fail_htlc was already
committed in a previous app-session, the fail_htlc will not be re-sent and
we will not have the reason (as it's not persisted).

fixes #6675

electrum/lnchannel.py

@@ -997,14 +997,22 @@ class Channel(AbstractChannel):
                 self.lnworker.payment_sent(self, htlc.payment_hash)
             failed = self.hm.failed_in_ctn(new_ctn)
             for htlc in failed:
+                try:
                     error_bytes, failure_message = self._receive_fail_reasons.pop(htlc.htlc_id)
+                except KeyError:
+                    error_bytes, failure_message = None, None
                 # if we are forwarding, save error message to disk
                 if self.lnworker.get_payment_info(htlc.payment_hash) is None:
                     self.save_fail_htlc_reason(htlc.htlc_id, error_bytes, failure_message)
                 else:
                     self.lnworker.payment_failed(self, htlc.payment_hash, error_bytes, failure_message)
 
-    def save_fail_htlc_reason(self, htlc_id, error_bytes, failure_message):
+    def save_fail_htlc_reason(
+            self,
+            htlc_id: int,
+            error_bytes: Optional[bytes],
+            failure_message: Optional['OnionRoutingFailureMessage'],
+    ):
         error_hex = error_bytes.hex() if error_bytes else None
         failure_hex = failure_message.to_bytes().hex() if failure_message else None
         self.hm.log['fail_htlc_reasons'][htlc_id] = (error_hex, failure_hex)

electrum/lnworker.py

@@ -59,7 +59,7 @@ from .lnutil import (Outpoint, LNPeerAddr,
                      BarePaymentAttemptLog, derive_payment_secret_from_payment_preimage)
 from .lnutil import ln_dummy_address, ln_compare_features, IncompatibleLightningFeatures
 from .transaction import PartialTxOutput, PartialTransaction, PartialTxInput
-from .lnonion import OnionFailureCode, process_onion_packet, OnionPacket
+from .lnonion import OnionFailureCode, process_onion_packet, OnionPacket, OnionRoutingFailureMessage
 from .lnmsg import decode_msg
 from .i18n import _
 from .lnrouter import (RouteEdge, LNPaymentRoute, LNPaymentPath, is_route_sane_to_use,
@@ -1241,7 +1241,13 @@ class LNWallet(LNWorker):
         info = info._replace(status=status)
         self.save_payment_info(info)
 
-    def payment_failed(self, chan, payment_hash: bytes, error_bytes: bytes, failure_message):
+    def payment_failed(
+            self,
+            chan: Channel,
+            payment_hash: bytes,
+            error_bytes: Optional[bytes],
+            failure_message: Optional['OnionRoutingFailureMessage'],
+    ):
         self.set_payment_status(payment_hash, PR_UNPAID)
         f = self.pending_payments.get(payment_hash)
         if f and not f.cancelled():