lnpeer.maybe_fulfill_htlc: follow BOLTs re some errors

related: https://github.com/lightningnetwork/lightning-rfc/pull/608

electrum/lnpeer.py

@@ -1507,13 +1507,21 @@ class Peer(Logger):
 
         asyncio.ensure_future(forward_trampoline_payment())
 
-
     def maybe_fulfill_htlc(
             self, *,
             chan: Channel,
             htlc: UpdateAddHtlc,
             processed_onion: ProcessedOnionPacket,
-            is_trampoline:bool = False) -> Optional[bytes]:
+            is_trampoline: bool = False,
+    ) -> Optional[bytes]:
+        """As a final recipient of an HTLC, decide if we should fulfill it.
+        Returns the preimage if yes, or None.
+        """
+
+        try:
+            amt_to_forward = processed_onion.hop_data.payload["amt_to_forward"]["amt_to_forward"]
+        except:
+            raise OnionRoutingFailure(code=OnionFailureCode.INVALID_ONION_PAYLOAD, data=b'\x00\x00\x00')
 
         # Check that our blockchain tip is sufficiently recent so that we have an approx idea of the height.
         # We should not release the preimage for an HTLC that its sender could already time out as
@@ -1522,8 +1530,11 @@ class Peer(Logger):
         if chain.is_tip_stale():
             raise OnionRoutingFailure(code=OnionFailureCode.TEMPORARY_NODE_FAILURE, data=b'')
         local_height = chain.height()
+        exc_incorrect_or_unknown_pd = OnionRoutingFailure(
+                code=OnionFailureCode.INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS,
+                data=amt_to_forward.to_bytes(8, byteorder="big") + local_height.to_bytes(4, byteorder="big"))
         if local_height + MIN_FINAL_CLTV_EXPIRY_ACCEPTED > htlc.cltv_expiry:
-            raise OnionRoutingFailure(code=OnionFailureCode.FINAL_EXPIRY_TOO_SOON, data=b'')
+            raise exc_incorrect_or_unknown_pd
         try:
             cltv_from_onion = processed_onion.hop_data.payload["outgoing_cltv_value"]["outgoing_cltv_value"]
         except:
@@ -1534,10 +1545,6 @@ class Peer(Logger):
                 raise OnionRoutingFailure(
                     code=OnionFailureCode.FINAL_INCORRECT_CLTV_EXPIRY,
                     data=htlc.cltv_expiry.to_bytes(4, byteorder="big"))
-        try:
-            amt_to_forward = processed_onion.hop_data.payload["amt_to_forward"]["amt_to_forward"]
-        except:
-            raise OnionRoutingFailure(code=OnionFailureCode.INVALID_ONION_PAYLOAD, data=b'\x00\x00\x00')
         try:
             total_msat = processed_onion.hop_data.payload["payment_data"]["total_msat"]
         except:
@@ -1554,21 +1561,21 @@ class Peer(Logger):
 
         info = self.lnworker.get_payment_info(htlc.payment_hash)
         if info is None:
-            raise OnionRoutingFailure(code=OnionFailureCode.INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS, data=b'')
+            raise exc_incorrect_or_unknown_pd
         preimage = self.lnworker.get_preimage(htlc.payment_hash)
         try:
             payment_secret_from_onion = processed_onion.hop_data.payload["payment_data"]["payment_secret"]
         except:
             if total_msat > amt_to_forward:
                 # payment_secret is required for MPP
-                raise OnionRoutingFailure(code=OnionFailureCode.INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS, data=b'')
+                raise exc_incorrect_or_unknown_pd
             # TODO fail here if invoice has set PAYMENT_SECRET_REQ
         else:
             if payment_secret_from_onion != derive_payment_secret_from_payment_preimage(preimage):
-                raise OnionRoutingFailure(code=OnionFailureCode.INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS, data=b'')
+                raise exc_incorrect_or_unknown_pd
         invoice_msat = info.amount_msat
         if not (invoice_msat is None or invoice_msat <= total_msat <= 2 * invoice_msat):
-            raise OnionRoutingFailure(code=OnionFailureCode.INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS, data=b'')
+            raise exc_incorrect_or_unknown_pd
         accepted, expired = self.lnworker.htlc_received(chan.short_channel_id, htlc, total_msat)
         if accepted:
             return preimage