trampoline forwarding: use routing hints

unit tests:
 - remove 'drop_dave' flag, replace it by depleted channel
 - add test for trampoline forwarding using routing hints
 - lower attempts to 2

electrum/lnpeer.py

@@ -51,6 +51,7 @@ from .lnrouter import fee_for_edge_msat
 from .json_db import StoredDict
 from .invoices import PR_PAID
 from .simple_config import FEE_LN_ETA_TARGET
+from .trampoline import decode_routing_info
 
 if TYPE_CHECKING:
     from .lnworker import LNGossip, LNWallet
@@ -1702,12 +1703,14 @@ class Peer(Logger):
                 next_trampoline_onion = None
                 invoice_features = payload["invoice_features"]["invoice_features"]
                 invoice_routing_info = payload["invoice_routing_info"]["invoice_routing_info"]
-                # TODO use invoice_routing_info
+                r_tags = decode_routing_info(invoice_routing_info)
+                self.logger.info(f'r_tags {r_tags}')
                 # TODO legacy mpp payment, use total_msat from trampoline onion
             else:
                 self.logger.info('forward_trampoline: end-to-end')
                 invoice_features = LnFeatures.BASIC_MPP_OPT
                 next_trampoline_onion = trampoline_onion.next_packet
+                r_tags = []
         except Exception as e:
             self.logger.exception('')
             raise OnionRoutingFailure(code=OnionFailureCode.INVALID_ONION_PAYLOAD, data=b'\x00\x00\x00')
@@ -1732,7 +1735,7 @@ class Peer(Logger):
                 payment_secret=payment_secret,
                 amount_to_pay=amt_to_forward,
                 min_cltv_expiry=cltv_from_onion,
-                r_tags=[],
+                r_tags=r_tags,
                 invoice_features=invoice_features,
                 fwd_trampoline_onion=next_trampoline_onion,
                 fwd_trampoline_fee=trampoline_fee,

electrum/tests/test_lnpeer.py

@@ -1407,23 +1407,24 @@ class TestPeer(ElectrumTestCase):
         graph = self.prepare_chans_and_peers_in_graph(self.GRAPH_DEFINITIONS['square_graph'])
         await self._run_mpp(graph, {'mpp_invoice': False}, {'mpp_invoice': True})
 
-    async def _run_trampoline_payment(self, is_legacy, direct, drop_dave=None, test_mpp_consolidation=False):
-        if drop_dave is None: drop_dave = []
+    async def _run_trampoline_payment(
+            self, *,
+            is_legacy=False,
+            direct=False,
+            test_mpp_consolidation=False,
+            include_routing_hints=True, # only relevant if is_legacy is True
+            attempts=2,
+    ):
 
         async def pay(lnaddr, pay_req):
             self.assertEqual(PR_UNPAID, graph.workers['dave'].get_payment_status(lnaddr.paymenthash))
-            result, log = await graph.workers['alice'].pay_invoice(pay_req, attempts=10)
+            result, log = await graph.workers['alice'].pay_invoice(pay_req, attempts=attempts)
             if result:
                 self.assertEqual(PR_PAID, graph.workers['dave'].get_payment_status(lnaddr.paymenthash))
                 raise PaymentDone()
             else:
                 raise NoPathFound()
 
-        def do_drop_dave(t):
-            # this will trigger UNKNOWN_NEXT_PEER
-            dave_node_id = graph.workers['dave'].node_keypair.pubkey
-            graph.workers[t].peers.pop(dave_node_id)
-
         async def f():
             await self._activate_trampoline(graph.workers['alice'])
             async with OldTaskGroup() as group:
@@ -1432,17 +1433,17 @@ class TestPeer(ElectrumTestCase):
                     await group.spawn(peer.htlc_switch())
                 for peer in peers:
                     await peer.initialized
-                lnaddr, pay_req = self.prepare_invoice(graph.workers['dave'], include_routing_hints=True)
-                for p in drop_dave:
-                    do_drop_dave(p)
+                lnaddr, pay_req = self.prepare_invoice(graph.workers['dave'], include_routing_hints=include_routing_hints)
                 await group.spawn(pay(lnaddr, pay_req))
 
         graph_definition = self.GRAPH_DEFINITIONS['square_graph']
         if not direct:
-            # deplete channel from alice to carol
+            # deplete channel from alice to carol and from bob to dave
             graph_definition['alice']['channels']['carol'] = depleted_channel
+            graph_definition['bob']['channels']['dave'] = depleted_channel
             # insert a channel from bob to carol
-            graph_definition['bob']['channels']['carol'] = high_fee_channel
+            graph_definition['bob']['channels']['carol'] = low_fee_channel
+            # now the only route possible is alice -> bob -> carol -> dave
 
         if test_mpp_consolidation:
             # deplete alice to carol so that all htlcs go through bob
@@ -1475,7 +1476,9 @@ class TestPeer(ElectrumTestCase):
     @needs_test_with_all_chacha20_implementations
     async def test_payment_trampoline_legacy(self):
         with self.assertRaises(PaymentDone):
-            await self._run_trampoline_payment(is_legacy=True, direct=False)
+            await self._run_trampoline_payment(is_legacy=True, direct=False, include_routing_hints=True)
+        with self.assertRaises(NoPathFound):
+            await self._run_trampoline_payment(is_legacy=True, direct=False, include_routing_hints=False)
 
     @needs_test_with_all_chacha20_implementations
     async def test_payment_trampoline_e2e_direct(self):
@@ -1486,10 +1489,7 @@ class TestPeer(ElectrumTestCase):
     async def test_payment_trampoline_e2e_indirect(self):
         # must use two trampolines
         with self.assertRaises(PaymentDone):
-            await self._run_trampoline_payment(is_legacy=False, direct=False, drop_dave=['bob'])
-        # both trampolines drop dave
-        with self.assertRaises(NoPathFound):
-            await self._run_trampoline_payment(is_legacy=False, direct=False, drop_dave=['bob', 'carol'])
+            await self._run_trampoline_payment(is_legacy=False, direct=False)
 
     @needs_test_with_all_chacha20_implementations
     async def test_payment_multipart_trampoline_e2e(self):

electrum/trampoline.py

@@ -92,10 +92,30 @@ def encode_routing_info(r_tags):
     for route in r_tags:
         result.append(bitstring.pack('uint:8', len(route)))
         for step in route:
-            pubkey, channel, feebase, feerate, cltv = step
-            result.append(bitstring.BitArray(pubkey) + bitstring.BitArray(channel) + bitstring.pack('intbe:32', feebase) + bitstring.pack('intbe:32', feerate) + bitstring.pack('intbe:16', cltv))
+            pubkey, scid, feebase, feerate, cltv = step
+            result.append(
+                bitstring.BitArray(pubkey) \
+                + bitstring.BitArray(scid)\
+                + bitstring.pack('intbe:32', feebase)\
+                + bitstring.pack('intbe:32', feerate)\
+                + bitstring.pack('intbe:16', cltv))
     return result.tobytes()
 
+def decode_routing_info(s: bytes):
+    s = bitstring.BitArray(s)
+    r_tags = []
+    n = 8*(33 + 8 + 4 + 4 + 2)
+    while s:
+        route = []
+        length, s = s[0:8], s[8:]
+        length = length.unpack('uint:8')[0]
+        for i in range(length):
+            chunk, s = s[0:n], s[n:]
+            item = chunk.unpack('bytes:33, bytes:8, intbe:32, intbe:32, intbe:16')
+            route.append(item)
+        r_tags.append(route)
+    return r_tags
+
 
 def is_legacy_relay(invoice_features, r_tags) -> Tuple[bool, Set[bytes]]:
     """Returns if we deal with a legacy payment and the list of trampoline pubkeys in the invoice.
@@ -213,6 +233,7 @@ def create_trampoline_route(
         # the last trampoline onion must contain routing hints for the last trampoline
         # node to find the recipient
         invoice_routing_info = encode_routing_info(r_tags)
+        assert invoice_routing_info == encode_routing_info(decode_routing_info(invoice_routing_info))
         # lnwire invoice_features for trampoline is u64
         invoice_features = invoice_features & 0xffffffffffffffff
         route[-1].invoice_routing_info = invoice_routing_info