lnworker: collect failed htlcs during payment attempt, fix todo

[f321x]

This makes LNWallet.pay_to_node() wait
PaySession.TIMEOUT_WAIT_FOR_NEXT_RESOLVED_HTLC (0.5 sec) for another htlc to get resolved after receiving a htlc failure during a payment attempt.
This seems to make payments more reliable in scenarios where we receive multiple htlc failures closely after each other as LNWallet.create_route_for_payment() then has access to the failed routes/failure information of all these htlcs when trying to re-split the outstanding amount.
I especially noticed the improvement when running this test on top of #10230 which would otherwise fail regularly:

Unittest

diff --git a/tests/test_lnpeer.py b/tests/test_lnpeer.py
index dee22229e..c7fa6314d 100644
--- a/tests/test_lnpeer.py
+++ b/tests/test_lnpeer.py
@@ -1960,7 +1960,8 @@ class TestPeerForwarding(TestPeer):
                     bob_pubkey=keys[b].pubkey,
                     local_msat=channel_def['local_balance_msat'],
                     remote_msat=channel_def['remote_balance_msat'],
-                    anchor_outputs=self.TEST_ANCHOR_CHANNELS
+                    anchor_outputs=self.TEST_ANCHOR_CHANNELS,
+                    max_accepted_htlcs=20,
                 )
                 channels[(a, b)], channels[(b, a)] = channel_ab, channel_ba
                 transport_ab, transport_ba = transport_pair(keys[a], keys[b], channel_ab.name, channel_ba.name)
@@ -2585,6 +2586,35 @@ class TestPeerForwarding(TestPeer):
             await self._run_trampoline_payment(
                 graph, sender_name='alice', destination_name='edward',tf_names=('bob', 'dave'))
 
+    async def test_multi_trampoline_payment(self):
+        """
+        Alice splits her payment to Dave between two trampoline forwarding nodes Carol and Bob.
+        This should test Multi-Trampoline MPP:
+        https://github.com/lightning/bolts/blob/bc7a1a0bc97b2293e7f43dd8a06529e5fdcf7cd2/proposals/trampoline.md#multi-trampoline-mpp
+        """
+        graph_definition = self.GRAPH_DEFINITIONS['square_graph']
+        # payment amount is 100_000_000 msat, size the channels so that alice must use both to succeed
+        graph_definition['alice']['channels']['bob']['local_balance_msat'] = int(100_000_000 * 0.75)
+        graph_definition['alice']['channels']['carol']['local_balance_msat'] = int(100_000_000 * 0.75)
+        g = self.prepare_chans_and_peers_in_graph(graph_definition)
+        w = g.workers['alice'], g.workers['carol'], g.workers['bob'], g.workers['dave']
+        alice_w, carol_w, bob_w, dave_w = w
+
+        alice_w.config.TEST_FORCE_MPP = True
+        bob_w.config.TEST_FORCE_MPP = True
+        carol_w.config.TEST_FORCE_MPP = True
+        dave_w.features |= LnFeatures.BASIC_MPP_OPT
+        # alice_w.config.INITIAL_TRAMPOLINE_FEE_LEVEL = 6
+
+        with self.assertRaises(PaymentDone):
+            await self._run_trampoline_payment(
+                g,
+                sender_name='alice',
+                destination_name='dave',
+                tf_names=('bob', 'carol'),
+                attempts=30,  # the default used in LNWallet.pay_invoice()
+            )
+
     async def test_forwarder_fails_for_inconsistent_trampoline_onions(self):
         """
         verify that the receiver of a trampoline forwarding fails the mpp set

[SomberNight]

Thanks. I added a follow-up that split things up a bit for more readability.