AaronFiore/electrum
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trampoline: fix off-by-one confusion of fees

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The convention is that edges (start_node -> edge_node) store
the policy/fees for the *start_node*.
This is what the non-trampoline edges were already using (for a long time),
but the trampoline ones were off-by-one (policy was for end_node),
which was then worked around in multiple places, to correct for...

i.e. I think because of all the workarounds, there was no actual bug,
but it was just very confusing.

Also note that the prior usage of trampoline edges would not work if
we (sender) were not directly connected to a TF (trampoline-forwarder)
but had extra edges in the route to even get to the first TF.
Having the policy corresponding to the start_node of the edge would work
even in that case.
This commit is contained in:
SomberNight
2023-10-27 14:22:01 +00:00
parent 097a10f84d
commit 53a8453e3b
4 changed files with 45 additions and 42 deletions
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9
electrum/lnonion.py
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@@ -241,10 +241,6 @@ def calc_hops_data_for_payment(
# payloads, backwards from last hop (but excluding the first edge):
for edge_index in range(len(route) - 1, 0, -1):
route_edge = route[edge_index]
is_trampoline = route_edge.is_trampoline()
if is_trampoline:
amt += route_edge.fee_for_edge(amt)
cltv_abs += route_edge.cltv_delta
hop_payload = {
"amt_to_forward": {"amt_to_forward": amt},
"outgoing_cltv_value": {"outgoing_cltv_value": cltv_abs},
@@ -252,9 +248,8 @@ def calc_hops_data_for_payment(
}
hops_data.append(
OnionHopsDataSingle(payload=hop_payload))
if not is_trampoline:
amt += route_edge.fee_for_edge(amt)
cltv_abs += route_edge.cltv_delta
amt += route_edge.fee_for_edge(amt)
cltv_abs += route_edge.cltv_delta
hops_data.reverse()
return hops_data, amt, cltv_abs