elements: Fix fee estimation for the htlc success and timeout txs

Signed-off-by: Christian Decker <decker.christian@gmail.com>

common/htlc_tx.h

@@ -4,6 +4,7 @@
 #include <bitcoin/chainparams.h>
 #include <common/amount.h>
 #include <common/htlc.h>
+#include <common/utils.h>
 
 struct bitcoin_signature;
 struct bitcoin_txid;
@@ -12,6 +13,32 @@ struct preimage;
 struct pubkey;
 struct ripemd160;
 
+/** Attempt to compute the elements overhead given a base bitcoin size.
+ *
+ * The overhead consists of 2 empty proofs for the transaction, 6 bytes of
+ * proofs per input and 35 bytes per output. In addition the explicit fee
+ * output will add 9 bytes and the per output overhead as well.
+ */
+static inline size_t elements_add_overhead(size_t weight, size_t incount,
+					   size_t outcount)
+{
+	if (is_elements) {
+		/* Each transaction has surjection and rangeproof (both empty
+		 * for us as long as we use unblinded L-BTC transactions). */
+		weight += 2 * 4;
+		/* For elements we also need to add the fee output and the
+		 * overhead for rangeproofs into the mix. */
+		weight += (8 + 1) * 4; /* Bitcoin style output */
+
+		/* All outputs have a bit of elements overhead */
+		weight += (32 + 1 + 1 + 1) * 4 * (outcount + 1); /* Elements added fields */
+
+		/* Inputs have 6 bytes of blank proofs attached. */
+		weight += 6 * incount;
+	}
+	return weight;
+}
+
 static inline struct amount_sat htlc_timeout_fee(u32 feerate_per_kw)
 {
 	/* BOLT #3:
@@ -21,7 +48,7 @@ static inline struct amount_sat htlc_timeout_fee(u32 feerate_per_kw)
 	 *    1. Multiply `feerate_per_kw` by 663 and divide by 1000 (rounding
 	 *       down).
 	 */
-	return amount_tx_fee(663, feerate_per_kw);
+	return amount_tx_fee(elements_add_overhead(663, 1, 1), feerate_per_kw);
 }
 
 static inline struct amount_sat htlc_success_fee(u32 feerate_per_kw)
@@ -33,7 +60,7 @@ static inline struct amount_sat htlc_success_fee(u32 feerate_per_kw)
 	 *     1. Multiply `feerate_per_kw` by 703 and divide by 1000 (rounding
 	 *     down).
 	 */
-	return amount_tx_fee(703, feerate_per_kw);
+	return amount_tx_fee(elements_add_overhead(703, 1, 1), feerate_per_kw);
 }
 
 /* Create HTLC-success tx to spend a received HTLC commitment tx

onchaind/onchaind.c

@@ -165,6 +165,8 @@ static bool set_htlc_timeout_fee(struct bitcoin_tx *tx,
 {
 	static struct amount_sat fee = AMOUNT_SAT_INIT(UINT64_MAX);
 	struct amount_sat amount = bitcoin_tx_output_get_amount(tx, 0);
+	size_t weight = elements_add_overhead(663, tx->wtx->num_inputs,
+					      tx->wtx->num_outputs);
 
 	/* BOLT #3:
 	 *
@@ -175,7 +177,7 @@ static bool set_htlc_timeout_fee(struct bitcoin_tx *tx,
 	 */
 	if (amount_sat_eq(fee, AMOUNT_SAT(UINT64_MAX))) {
 		struct amount_sat grindfee;
-		if (grind_htlc_tx_fee(&grindfee, tx, remotesig, wscript, 663)) {
+		if (grind_htlc_tx_fee(&grindfee, tx, remotesig, wscript, weight)) {
 			/* Cache this for next time */
 			fee = grindfee;
 			return true;
@@ -200,7 +202,8 @@ static void set_htlc_success_fee(struct bitcoin_tx *tx,
 				 const u8 *wscript)
 {
 	static struct amount_sat amt, fee = AMOUNT_SAT_INIT(UINT64_MAX);
-
+	size_t weight = elements_add_overhead(703, tx->wtx->num_inputs,
+					      tx->wtx->num_outputs);
 	/* BOLT #3:
 	 *
 	 * The fee for an HTLC-success transaction:
@@ -209,7 +212,7 @@ static void set_htlc_success_fee(struct bitcoin_tx *tx,
 	 *    (rounding down).
 	 */
 	if (amount_sat_eq(fee, AMOUNT_SAT(UINT64_MAX))) {
-		if (!grind_htlc_tx_fee(&fee, tx, remotesig, wscript, 703))
+		if (!grind_htlc_tx_fee(&fee, tx, remotesig, wscript, weight))
 			status_failed(STATUS_FAIL_INTERNAL_ERROR,
 				      "htlc_success_fee can't be found "
 				      " for tx %s, signature %s, wscript %s",