common: fix utxo_spend_weight to understand how cheap P2TR is.

We previously treated it as a P2WPKH, which is wrong.

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

Author: rustyrussell

common/utxo.c

@@ -64,14 +64,52 @@ struct utxo *fromwire_utxo(const tal_t *ctx, const u8 **ptr, size_t *max)
 
 size_t utxo_spend_weight(const struct utxo *utxo, size_t min_witness_weight)
 {
-	size_t wit_weight = bitcoin_tx_simple_input_witness_weight();
+	size_t witness_weight;
+	bool p2sh;
+
+	switch (utxo->utxotype) {
+	case UTXO_P2SH_P2WPKH:
+		witness_weight = bitcoin_tx_simple_input_witness_weight();
+		p2sh = true;
+		goto have_weight;
+	case UTXO_P2WPKH:
+		witness_weight = bitcoin_tx_simple_input_witness_weight();
+		p2sh = false;
+		goto have_weight;
+	case UTXO_P2WSH_FROM_CLOSE:
+		/* BOLT #3:
+		 * #### `to_remote` Output
+		 *
+		 * If `option_anchors` applies to the commitment
+		 * transaction, the `to_remote` output is encumbered by a one
+		 * block csv lock.
+		 *    <remotepubkey> OP_CHECKSIGVERIFY 1 OP_CHECKSEQUENCEVERIFY
+		 *
+		 * The output is spent by an input with `nSequence` field set
+		 * to `1` and witness: <remote_sig>
+		 * Otherwise, this output is a simple P2WPKH to `remotepubkey`.
+		 */
+		if (utxo->close_info->option_anchors)
+			witness_weight = 1 + 33 + 3 + 1 + 64;
+		else
+			witness_weight = 1 + 64;
+		p2sh = false;
+		goto have_weight;
+	case UTXO_P2TR:
+		witness_weight = 1 + 64;
+		p2sh = false;
+		goto have_weight;
+	}
+	abort();
+
+have_weight:
 	/* If the min is less than what we'd use for a 'normal' tx,
 	 * we return the value with the greater added/calculated */
-	if (wit_weight < min_witness_weight)
-		return bitcoin_tx_input_weight(utxo->utxotype == UTXO_P2SH_P2WPKH,
+	if (witness_weight < min_witness_weight)
+		return bitcoin_tx_input_weight(p2sh,
 					       min_witness_weight);
 
-	return bitcoin_tx_input_weight(utxo->utxotype == UTXO_P2SH_P2WPKH, wit_weight);
+	return bitcoin_tx_input_weight(p2sh, witness_weight);
 }
 
 u32 utxo_is_immature(const struct utxo *utxo, u32 blockheight)