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askrene: re-check min_htlc violations after correcting for MCF rounding.

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Thanks to @Lagrang3 for spotting this!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This commit is contained in:
Rusty Russell 2024-09-19 10:11:53 +09:30
parent 8c551ae48a
commit 9b60f6cc6d
2 changed files with 64 additions and 6 deletions
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56
plugins/askrene/refine.c
View file

@ -267,6 +267,32 @@ static struct flow *pick_most_likely_flow(struct route_query *rq,
return best_flow;
}
/* A secondary check for htlc_min violations, after excess trimming. */
static const char *flow_violates_min(const tal_t *ctx,
struct route_query *rq,
const struct flow *flow)
{
struct amount_msat msat = flow->delivers;
for (int i = tal_count(flow->path) - 1; i >= 0; i--) {
const struct half_chan *h = flow_edge(flow, i);
struct amount_msat min = amount_msat(fp16_to_u64(h->htlc_min));
plugin_log(rq->plugin, LOG_DBG, "flow_violates_min: %u/%zu amt=%s, min=%s",
i, tal_count(flow->path), fmt_amount_msat(tmpctx, msat), fmt_amount_msat(tmpctx, min));
if (amount_msat_less(msat, min)) {
struct short_channel_id_dir scidd;
get_scidd(rq->gossmap, flow, i, &scidd);
return tal_fmt(ctx, "Sending %s across %s would violate htlc_min (~%s)",
fmt_amount_msat(tmpctx, msat),
fmt_short_channel_id_dir(tmpctx, &scidd),
fmt_amount_msat(tmpctx, min));
}
if (!amount_msat_add_fee(&msat, h->base_fee, h->proportional_fee))
plugin_err(rq->plugin, "Adding fee to amount");
}
return NULL;
}
const char *
refine_with_fees_and_limits(const tal_t *ctx,
struct route_query *rq,
@ -314,20 +340,38 @@ refine_with_fees_and_limits(const tal_t *ctx,
abort();
for (size_t i = 0; i < tal_count(*flows); i++) {
if (amount_msat_sub(&(*flows)[i]->delivers, (*flows)[i]->delivers, excess)) {
const char *err;
plugin_log(rq->plugin, LOG_DBG,
"Flows delivered %s extra, trimming %zu/%zu",
fmt_amount_msat(tmpctx, excess),
i, tal_count(*flows));
/* In theory, this can violate min_htlc! Thanks @Lagrang3! */
err = flow_violates_min(tmpctx, rq, (*flows)[i]);
if (err) {
/* This flow was reduced to 0 / impossible, remove */
tal_arr_remove(flows, i);
i--;
/* If this causes failure, indicate why! */
flow_constraint_error = err;
continue;
}
break;
}
}
if (!amount_msat_eq(flowset_delivers(rq->plugin, *flows), deliver)) {
plugin_err(rq->plugin,
"Flowset delivers %s, can't shed excess?",
fmt_amount_msat(tmpctx, flowset_delivers(rq->plugin, *flows)),
fmt_amount_msat(tmpctx, deliver));
/* Usually this should shed excess, *BUT* maybe one
* was deleted instead for being below minimum */
if (!amount_msat_sub(&more_to_deliver, deliver,
flowset_delivers(rq->plugin, *flows))) {
ret = tal_fmt(ctx,
"Flowset delivers %s instead of %s, can't shed excess?",
fmt_amount_msat(tmpctx, flowset_delivers(rq->plugin, *flows)),
fmt_amount_msat(tmpctx, deliver));
goto out;
}
more_to_deliver = AMOUNT_MSAT(0);
plugin_log(rq->plugin, LOG_DBG, "After dealing with excess, more_to_deliver=%s",
fmt_amount_msat(tmpctx, more_to_deliver));
}
/* The residual is minimal. In theory we could add one msat at a time

14
tests/test_askrene.py
View file

@ -663,3 +663,17 @@ def test_min_htlc(node_factory, bitcoind):
layers=[],
maxfee_msat=20_000_000,
final_cltv=10)
def test_min_htlc_after_excess(node_factory, bitcoind):
gsfile, nodemap = generate_gossip_store([GenChannel(0, 1, capacity_sats=500_000,
forward=GenChannel.Half(htlc_min=2_000))])
l1 = node_factory.get_node(gossip_store_file=gsfile.name)
with pytest.raises(RpcError, match=r"ending 1999msat across 0x1x0/1 would violate htlc_min \(~2000msat\)"):
l1.rpc.getroutes(source=nodemap[0],
destination=nodemap[1],
amount_msat=1999,
layers=[],
maxfee_msat=20_000_000,
final_cltv=10)