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askrene: fixed a timeout corner case

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Changelog-Fixed: askrene: fixed a class of corner cases that cause askrene main loop to timeout instead of quickly failing, thus wasting runtime.

Signed-off-by: Lagrang3 <lagrang3@protonmail.com>
This commit is contained in:
Lagrang3
2026-01-27 12:36:18 +01:00
committed by Rusty Russell
parent 597da1fef9
commit f1bf66c91f
2 changed files with 75 additions and 4 deletions
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78
plugins/askrene/refine.c
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@@ -303,6 +303,45 @@ remove_htlc_min_violations(const tal_t *ctx, struct route_query *rq,
return error_message;
}
/* Loop over the channels in the path and disable the one with the least
* permiting amount based on htlc_max and known max liquidity. */
static const char *remove_bottleneck(const tal_t *ctx, struct route_query *rq,
const struct flow *flow)
{
const char *error_message = NULL;
struct amount_msat min = AMOUNT_MSAT(-1);
u32 min_pos = UINT32_MAX;
struct amount_msat htlc_max, known_max, unused;
struct short_channel_id_dir scidd;
size_t idx;
for (u32 i = 0; i < tal_count(flow->path); i++) {
htlc_max = get_chan_htlc_max(rq, flow->path[i], flow->dirs[i]);
get_constraints(rq, flow->path[i], flow->dirs[i], &unused,
&known_max);
known_max = amount_msat_min(known_max, htlc_max);
if (amount_msat_less(known_max, min)) {
min = known_max;
min_pos = i;
}
}
if (min_pos >= tal_count(flow->path)) {
error_message = rq_log(
ctx, rq, LOG_BROKEN,
"%s: failed to find any bottleneck, flow has no hops? %s",
__func__, fmt_flow_full(tmpctx, rq, flow));
} else {
get_scidd(rq->gossmap, flow, min_pos, &scidd);
rq_log(ctx, rq, LOG_INFORM,
"Disabling bottleneck channel %s with "
"htlc_max/known_max at %s",
fmt_short_channel_id_dir(ctx, &scidd),
fmt_amount_msat(ctx, min));
idx = flow->dirs[min_pos] +
2 * gossmap_chan_idx(rq->gossmap, flow->path[min_pos]);
bitmap_set_bit(rq->disabled_chans, idx);
}
return error_message;
}
static struct amount_msat sum_all_deliver(struct flow **flows)
{
@@ -494,10 +533,43 @@ const char *refine_flows(const tal_t *ctx, struct route_query *rq,
size_t *flows_index;
/* do not deliver more than HTLC_MAX allow us */
for (size_t i = 0; i < tal_count(*flows); i++) {
for (size_t i = 0; i < tal_count(*flows);) {
struct amount_msat try_deliver = (*flows)[i]->delivers;
struct amount_msat deliverable =
flow_max_deliverable(rq, (*flows)[i]);
/* We don't expect to have a zero flow amount here. Just report
* it. */
if (amount_msat_is_zero(try_deliver)) {
rq_log(ctx, rq, LOG_UNUSUAL,
"Tried to refine a flow with zero amount: %s",
fmt_flow_full(tmpctx, rq, (*flows)[i]));
del_flow_from_arr(flows, i);
continue;
}
/* A path with a very small deliverable amount is not worth the
* effort, and we don't want either a path that for fees and
* HTLC max constraints removes too much from the actual
* delivery amount. In theory the MCF already has partitioned
* the payment in different paths. The refinement step is not
* expected to change the flow by much. */
if (amount_msat_less(deliverable, AMOUNT_MSAT(1000)) ||
amount_msat_ratio(deliverable, try_deliver) < 0.2) {
error_message = remove_bottleneck(ctx, rq, (*flows)[i]);
if (error_message)
goto fail;
del_flow_from_arr(flows, i);
continue;
}
(*flows)[i]->delivers =
amount_msat_min((*flows)[i]->delivers,
flow_max_deliverable(rq, (*flows)[i]));
amount_msat_min(try_deliver, deliverable);
i++;
}
if (tal_count(*flows) == 0) {
/* No flows left to complete the next steps, early exit. */
goto fail;
}
/* remove excess from MCF granularity if any */