Add unit test for trampoline MPP consolidation

This tests that a trampoline waits until all incoming HTLCs are
received, and fail or succeed them together

electrum/lnworker.py

@@ -1648,6 +1648,8 @@ class LNWallet(LNWorker):
             is_mpp = sum(len(x) for x in list(sc.config.values())) > 1
             if is_mpp and not invoice_features.supports(LnFeatures.BASIC_MPP_OPT):
                 continue
+            if not is_mpp and self.config.TEST_FORCE_MPP:
+                continue
             self.logger.info(f"trying split configuration: {sc.config.values()} rating: {sc.rating}")
             routes = []
             try:

electrum/simple_config.py

@@ -893,6 +893,7 @@ class SimpleConfig(Logger):
     EXPERIMENTAL_LN_FORWARD_TRAMPOLINE_PAYMENTS = ConfigVar('lightning_forward_trampoline_payments', default=False, type_=bool)
     TEST_FAIL_HTLCS_WITH_TEMP_NODE_FAILURE = ConfigVar('test_fail_htlcs_with_temp_node_failure', default=False, type_=bool)
     TEST_FAIL_HTLCS_AS_MALFORMED = ConfigVar('test_fail_malformed_htlc', default=False, type_=bool)
+    TEST_FORCE_MPP = ConfigVar('test_force_mpp', default=False, type_=bool)
     TEST_SHUTDOWN_FEE = ConfigVar('test_shutdown_fee', default=None, type_=int)
     TEST_SHUTDOWN_FEE_RANGE = ConfigVar('test_shutdown_fee_range', default=None)
     TEST_SHUTDOWN_LEGACY = ConfigVar('test_shutdown_legacy', default=False, type_=bool)

electrum/tests/test_lnpeer.py

@@ -1180,7 +1180,7 @@ 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):
+    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 pay(lnaddr, pay_req):
@@ -1217,7 +1217,16 @@ class TestPeer(ElectrumTestCase):
             # insert a channel from bob to carol
             graph_definition['bob']['channels']['carol'] = high_fee_channel
 
+        if test_mpp_consolidation:
+            # deplete alice to carol so that all htlcs go through bob
+            graph_definition['alice']['channels']['carol'] = depleted_channel
+
         graph = self.prepare_chans_and_peers_in_graph(graph_definition)
+
+        if test_mpp_consolidation:
+            graph.workers['dave'].features |= LnFeatures.BASIC_MPP_OPT
+            graph.workers['alice'].network.config.TEST_FORCE_MPP = True
+
         peers = graph.peers.values()
         if is_legacy:
             # turn off trampoline features in invoice
@@ -1231,6 +1240,11 @@ class TestPeer(ElectrumTestCase):
 
         await f()
 
+    @needs_test_with_all_chacha20_implementations
+    async def test_trampoline_mpp_consolidation(self):
+        with self.assertRaises(PaymentDone):
+            await self._run_trampoline_payment(is_legacy=True, direct=False, test_mpp_consolidation=True)
+
     @needs_test_with_all_chacha20_implementations
     async def test_payment_trampoline_legacy(self):
         with self.assertRaises(PaymentDone):