askrene: refine: disable HTLC min violations

diff --git a/plugins/askrene/askrene.c b/plugins/askrene/askrene.c
index 31126f82..ad6bd088 100644
--- a/plugins/askrene/askrene.c
+++ b/plugins/askrene/askrene.c
@@ -564,6 +564,13 @@ static struct command_result *do_getroutes(struct command *cmd,
 	/* we temporarily apply localmods */
 	gossmap_apply_localmods(askrene->gossmap, localmods);
 
+	/* I want to be able to disable channels while working on this query.
+	 * Layers are for user interaction and cannot be used for this purpose.
+	 */
+	rq->disabled_chans =
+	    tal_arrz(rq, bitmap,
+		     2 * BITMAP_NWORDS(gossmap_max_chan_idx(askrene->gossmap)));
+
 	/* localmods can add channels, so we need to allocate biases array
 	 * *afterwards* */
 	rq->biases =
diff --git a/plugins/askrene/askrene.h b/plugins/askrene/askrene.h
index 8688835e..7f1352f6 100644
--- a/plugins/askrene/askrene.h
+++ b/plugins/askrene/askrene.h
@@ -2,6 +2,7 @@
 #define LIGHTNING_PLUGINS_ASKRENE_ASKRENE_H
 #include "config.h"
 #include <bitcoin/short_channel_id.h>
+#include <ccan/bitmap/bitmap.h>
 #include <ccan/htable/htable_type.h>
 #include <ccan/list/list.h>
 #include <common/amount.h>
@@ -60,6 +61,9 @@ struct route_query {
 
 	/* Additional per-htlc cost for local channels */
 	const struct additional_cost_htable *additional_costs;
+
+	/* channels we disable during computation to meet constraints */
+	bitmap *disabled_chans;
 };
 
 /* Given a gossmap channel, get the current known min/max */
diff --git a/plugins/askrene/mcf.c b/plugins/askrene/mcf.c
index 696b810c..6d50cb8b 100644
--- a/plugins/askrene/mcf.c
+++ b/plugins/askrene/mcf.c
@@ -319,6 +319,15 @@ static void set_capacity(s64 *capacity, u64 value, u64 *cap_on_capacity)
 	*cap_on_capacity -= *capacity;
 }
 
+/* Helper to check whether a channel is available */
+static bool channel_is_available(const struct route_query *rq,
+				 const struct gossmap_chan *chan, const int dir)
+{
+	const u32 c_idx = gossmap_chan_idx(rq->gossmap, chan);
+	return gossmap_chan_set(chan, dir) && chan->half[dir].enabled &&
+	       !bitmap_test_bit(rq->disabled_chans, c_idx * 2 + dir);
+}
+
 /* FIXME: unit test this */
 /* The probability of forwarding a payment amount given a high and low liquidity
  * bounds.
@@ -568,7 +577,7 @@ static void init_linear_network(const tal_t *ctx,
 			const struct gossmap_chan *c = gossmap_nth_chan(gossmap,
 			                                                node, j, &half);
 
-			if (!gossmap_chan_set(c, half) || !c->half[half].enabled)
+			if (!channel_is_available(params->rq, c, half))
 				continue;
 
 			/* If a channel insists on more than our total, remove it */
@@ -644,7 +653,7 @@ struct chan_flow
  * */
 static struct node find_path_or_cycle(
 		const tal_t *working_ctx,
-		const struct gossmap *gossmap,
+		const struct route_query *rq,
 		const struct chan_flow *chan_flow,
 		const struct node source,
 		const s64 *balance,
@@ -653,6 +662,7 @@ static struct node find_path_or_cycle(
 		int *prev_dir,
 		u32 *prev_idx)
 {
+	const struct gossmap *gossmap = rq->gossmap;
 	const size_t max_num_nodes = gossmap_max_node_idx(gossmap);
 	bitmap *visited =
 	    tal_arrz(working_ctx, bitmap, BITMAP_NWORDS(max_num_nodes));
@@ -671,7 +681,7 @@ static struct node find_path_or_cycle(
 			const struct gossmap_chan *c =
 			    gossmap_nth_chan(gossmap, cur, i, &dir);
 
-			if (!gossmap_chan_set(c, dir) || !c->half[dir].enabled)
+			if (!channel_is_available(rq, c, dir))
 				continue;
 
 			const u32 c_idx = gossmap_chan_idx(gossmap, c);
@@ -877,7 +887,7 @@ get_flow_paths(const tal_t *ctx,
 		while (balance[source.idx] < 0) {
 			prev_chan[source.idx] = NULL;
 			struct node sink = find_path_or_cycle(
-			    working_ctx, params->rq->gossmap, chan_flow, source,
+			    working_ctx, params->rq, chan_flow, source,
 			    balance, prev_chan, prev_dir, prev_idx);
 
 			if (balance[sink.idx] > 0)
@@ -1107,8 +1117,7 @@ static void init_linear_network_single_path(
 			    gossmap_nth_chan(gossmap, node, j, &half);
                         struct amount_msat mincap, maxcap;
 
-			if (!gossmap_chan_set(c, half) ||
-			    !c->half[half].enabled)
+			if (!channel_is_available(params->rq, c, half))
 				continue;
 
 			/* If a channel cannot forward the total amount we don't
diff --git a/plugins/askrene/refine.c b/plugins/askrene/refine.c
index e93d0953..97472443 100644
--- a/plugins/askrene/refine.c
+++ b/plugins/askrene/refine.c
@@ -735,6 +735,38 @@ static struct amount_msat path_min_deliverable(struct channel_data *path)
 	return least_destination;
 }
 
+static const char *
+remove_htlc_min_violations(const tal_t *ctx, struct route_query *rq,
+			   const struct flow *flow,
+			   const struct channel_data *channels)
+{
+	const char *error_message = NULL;
+	struct amount_msat msat = flow->delivers;
+	for (size_t i = tal_count(flow->path) - 1; i < tal_count(flow->path);
+	     i--) {
+		if (amount_msat_less(msat, channels[i].htlc_min)) {
+			rq_log(
+			    ctx, rq, LOG_INFORM,
+			    "Sending %s across %s would violate htlc_min "
+			    "(~%s), disabling this channel",
+			    fmt_amount_msat(ctx, msat),
+			    fmt_short_channel_id_dir(ctx, &channels[i].scidd),
+			    fmt_amount_msat(ctx, channels[i].htlc_min));
+			bitmap_set_bit(rq->disabled_chans, channels[i].idx);
+			break;
+		}
+		if (!amount_msat_add_fee(
+			&msat, channels[i].fee_base_msat,
+			channels[i].fee_proportional_millionths)) {
+			error_message =
+			    rq_log(ctx, rq, LOG_BROKEN,
+				   "%s: Adding fee to amount", __func__);
+			break;
+		}
+	}
+	return error_message;
+}
+
 static struct amount_msat sum_all_deliver(struct flow **flows,
 					  size_t *flows_index)
 {
@@ -819,6 +851,7 @@ const char *refine_flows(const tal_t *ctx, struct route_query *rq,
 			 struct amount_msat deliver, struct flow ***flows)
 {
 	const tal_t *working_ctx = tal(ctx, tal_t);
+	const char *error_message = NULL;
 	struct amount_msat *max_deliverable;
 	struct amount_msat *min_deliverable;
 	struct channel_data **channel_mpp_cache;
@@ -863,6 +896,10 @@ const char *refine_flows(const tal_t *ctx, struct route_query *rq,
 		}
 		/* htlc_min is not met for this flow */
 		tal_arr_remove(&flows_index, i);
+		error_message = remove_htlc_min_violations(
+		    working_ctx, rq, (*flows)[k], channel_mpp_cache[k]);
+		if (error_message)
+			goto fail;
 	}
 
 	/* remove 0 amount flows if any */
@@ -889,4 +926,8 @@ const char *refine_flows(const tal_t *ctx, struct route_query *rq,
 
 	tal_free(working_ctx);
 	return NULL;
+
+fail:
+	tal_free(working_ctx);
+	return error_message;
 }