tx: add measure_tx_len() helper.

We currently linearize and then measure the string; this is better since
we're about to do it in a second place.

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

bitcoin/tx.c

@@ -295,6 +295,18 @@ u8 *linearize_tx_force_extended(const tal_t *ctx,
 	return arr;
 }
 
+static void add_measure(const void *data, size_t len, void *lenp)
+{
+	*(size_t *)lenp += len;
+}
+
+size_t measure_tx_len(const struct bitcoin_tx *tx)
+{
+	size_t len = 0;
+	add_tx(tx, add_measure, &len, uses_witness(tx));
+	return len;
+}
+
 void bitcoin_txid(const struct bitcoin_tx *tx, struct sha256_double *txid)
 {
 	struct sha256_ctx ctx = SHA256_INIT;

bitcoin/tx.h

@@ -53,6 +53,9 @@ u8 *linearize_tx(const tal_t *ctx, const struct bitcoin_tx *tx);
 u8 *linearize_tx_force_extended(const tal_t *ctx,
 				const struct bitcoin_tx *tx);
 
+/* Get length of tx in bytes. */
+size_t measure_tx_len(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) */
 struct bitcoin_tx *bitcoin_tx(const tal_t *ctx, varint_t input_count,

daemon/peer.c

@@ -1105,7 +1105,7 @@ const struct bitcoin_tx *bitcoin_close(struct peer *peer)
 /* Create a bitcoin spend tx (to spend our commit's outputs) */
 const struct bitcoin_tx *bitcoin_spend_ours(struct peer *peer)
 {
-	u8 *redeemscript, *linear;
+	u8 *redeemscript;
 	const struct bitcoin_tx *commit = peer->us.commit->tx;
 	struct bitcoin_signature sig;
 	struct bitcoin_tx *tx;
@@ -1146,10 +1146,8 @@ const struct bitcoin_tx *bitcoin_spend_ours(struct peer *peer)
 
 	/* Now, calculate the fee, given length. */
 	/* FIXME: Dynamic fees! */
-	linear = linearize_tx(peer, tx);
-	fee = fee_by_feerate(tal_count(linear),
+	fee = fee_by_feerate(measure_tx_len(tx),
 				 peer->dstate->config.closing_fee_rate);
-	tal_free(linear);
 
 	/* FIXME: Fail gracefully in these cases (not worth collecting) */
 	if (fee > tx->output[0].amount