Fix weight calculation, rename cost->weight.

Reported-by: Jon Griffiths
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

bitcoin/tx.c

@@ -260,14 +260,17 @@ static void push_measure(const void *data, size_t len, void *lenp)
 	*(size_t *)lenp += len;
 }
 
-size_t measure_tx_cost(const struct bitcoin_tx *tx)
+size_t measure_tx_weight(const struct bitcoin_tx *tx)
 {
 	size_t non_witness_len = 0, witness_len = 0;
 	push_tx(tx, push_measure, &non_witness_len, false);
-	if (uses_witness(tx))
+	if (uses_witness(tx)) {
 		push_witnesses(tx, push_measure, &witness_len);
+		/* Include BIP 144 marker and flag bytes in witness length */
+		witness_len += 2;
+	}
 
-	/* Witness bytes only push 1/4 of normal bytes, for cost. */
+	/* Normal bytes weigh 4 times more than Witness bytes */
 	return non_witness_len * 4 + witness_len;
 }
 

bitcoin/tx.h

@@ -47,8 +47,8 @@ void sha256_tx_for_sig(struct sha256_double *h, const struct bitcoin_tx *tx,
 /* Linear bytes of tx. */
 u8 *linearize_tx(const tal_t *ctx, const struct bitcoin_tx *tx);
 
-/* Get cost of tx in (x4 of non-witness bytecount). */
-size_t measure_tx_cost(const struct bitcoin_tx *tx);
+/* Get weight of tx in Sipa. */
+size_t measure_tx_weight(const struct bitcoin_tx *tx);
 
 /* Allocate a tx: you just need to fill in inputs and outputs (they're
  * zeroed with inputs' sequence_number set to FFFFFFFF) */

lightningd/peer_control.c

@@ -1786,7 +1786,7 @@ static bool better_closing_fee(struct peer *peer, const struct bitcoin_tx *tx)
 		 " vs previous %"PRIu64, fee, last_fee);
 
 	/* Weight once we add in sigs. */
-	weight = measure_tx_cost(tx) + 74 * 2;
+	weight = measure_tx_weight(tx) + 74 * 2;
 
 	min_fee = get_feerate(peer->ld->topology, FEERATE_SLOW) * weight / 1000;
 	if (fee < min_fee) {

onchaind/onchain.c

@@ -236,7 +236,7 @@ static struct bitcoin_tx *tx_to_us(const tal_t *ctx,
 						   &our_wallet_pubkey);
 
 	/* Worst-case sig is 73 bytes */
-	fee = feerate_per_kw * (measure_tx_cost(tx)
+	fee = feerate_per_kw * (measure_tx_weight(tx)
 			 + 1 + 3 + 73 + 0 + tal_len(wscript))
 		/ 1000;