fix: calc commit_txfee_spend/recv for spendable or receivable

lightningd/peer_control.c

@@ -509,9 +509,10 @@ static void json_add_sat_only(struct json_stream *result,
 				type_to_string(tmpctx, struct amount_msat, &msat));
 }
 
+/* Fee a commitment transaction would currently cost when spending */
 /* This is quite a lot of work to figure out what it would cost us! */
-static struct amount_sat commit_txfee(const struct channel *channel,
-				      struct amount_msat spendable)
+static struct amount_sat commit_txfee_spend(const struct channel *channel,
+					   struct amount_msat spendable)
 {
 	/* FIXME: make per-channel htlc maps! */
 	const struct htlc_in *hin;
@@ -567,6 +568,65 @@ static struct amount_sat commit_txfee(const struct channel *channel,
 	return commit_tx_base_fee(local_feerate * 2, num_untrimmed_htlcs + 1);
 }
 
+/* Fee a commitment transaction would currently cost when receiving */
+static struct amount_sat commit_txfee_recv(const struct channel *channel,
+					   struct amount_msat receivable)
+{
+	/* FIXME: make per-channel htlc maps! */
+	const struct htlc_in *hin;
+	struct htlc_in_map_iter ini;
+	const struct htlc_out *hout;
+	struct htlc_out_map_iter outi;
+	struct lightningd *ld = channel->peer->ld;
+	u32 feerate = get_feerate(channel->channel_info.fee_states,
+				  channel->funder, REMOTE);
+	size_t num_untrimmed_htlcs = 0;
+
+	/* Assume we tried to receive "receivable" */
+	if (!htlc_is_trimmed(REMOTE, receivable, feerate,
+			     channel->channel_info.their_config.dust_limit,
+			     REMOTE))
+		num_untrimmed_htlcs++;
+
+	for (hin = htlc_in_map_first(&ld->htlcs_in, &ini);
+	     hin;
+	     hin = htlc_in_map_next(&ld->htlcs_in, &ini)) {
+		if (hin->key.channel != channel)
+			continue;
+		if (!htlc_is_trimmed(LOCAL, hin->msat, feerate,
+				     channel->channel_info.their_config.dust_limit,
+				     REMOTE))
+			num_untrimmed_htlcs++;
+	}
+	for (hout = htlc_out_map_first(&ld->htlcs_out, &outi);
+	     hout;
+	     hout = htlc_out_map_next(&ld->htlcs_out, &outi)) {
+		if (hout->key.channel != channel)
+			continue;
+		if (!htlc_is_trimmed(REMOTE, hout->msat, feerate,
+				     channel->channel_info.their_config.dust_limit,
+				     REMOTE))
+			num_untrimmed_htlcs++;
+	}
+
+	/*
+	 * BOLT-95c74fef2fe590cb8adbd7b848743a229ffe825a #2:
+	 * Adding an HTLC: update_add_htlc
+	 *
+	 * A sending node:
+	 *   - if it is responsible for paying the Bitcoin fee:
+	 *     - SHOULD NOT offer `amount_msat` if, after adding that HTLC to
+	 *       its commitment transaction, its remaining balance doesn't allow
+	 *       it to pay the fee for a future additional non-dust HTLC at
+	 *       `N*feerate_per_kw` while maintaining its channel reserve
+	 *       ("fee spike buffer"), where `N` is a parameter chosen by the
+	 *       implementation (`N = 2` is recommended to ensure
+	 *       predictability).
+	 */
+	return commit_tx_base_fee(2 * feerate, num_untrimmed_htlcs + 1);
+}
+
+
 static void subtract_offered_htlcs(const struct channel *channel,
 				   struct amount_msat *amount)
 {
@@ -733,7 +793,7 @@ static void json_add_channel(struct lightningd *ld,
 	/* If we're funder, subtract txfees we'll need to spend this */
 	if (channel->funder == LOCAL) {
 		if (!amount_msat_sub_sat(&spendable, spendable,
-					 commit_txfee(channel, spendable)))
+					 commit_txfee_spend(channel, spendable)))
 			spendable = AMOUNT_MSAT(0);
 	}
 
@@ -764,7 +824,7 @@ static void json_add_channel(struct lightningd *ld,
 	/* If they're funder, subtract txfees they'll need to spend this */
 	if (channel->funder == REMOTE) {
 		if (!amount_msat_sub_sat(&receivable, receivable,
-					 commit_txfee(channel, receivable)))
+					 commit_txfee_recv(channel, receivable)))
 			receivable = AMOUNT_MSAT(0);
 	}