bitcoin: make input witness weight calculation explicit.

This is inspired by a patch from @whitslack, which overlapped with this series.
Most importantly, there was only one call to bitcoin_tx_simple_input_weight(),
and it is better to be explicit with that one.

This also changes our funder calculation to assume our own input is taproot,
which it is likely to be given we've defaulted to taproot for outputs for
change addresses since 23.08.

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

bitcoin/tx.c

@@ -913,17 +913,32 @@ size_t bitcoin_tx_input_weight(bool p2sh, size_t witness_weight)
 	return weight;
 }
 
-size_t bitcoin_tx_simple_input_witness_weight(void)
-{
-	/* Account for witness (sig + key) */
-	return bitcoin_tx_input_sig_weight() + 1 + 33;
-}
-
-/* We only do segwit inputs, and we assume witness is sig + key  */
-size_t bitcoin_tx_simple_input_weight(bool p2sh)
-{
-	return bitcoin_tx_input_weight(p2sh,
-				       bitcoin_tx_simple_input_witness_weight());
+size_t bitcoin_tx_input_witness_weight(enum utxotype utxotype)
+{
+	switch (utxotype) {
+	case UTXO_P2SH_P2WPKH:
+	case UTXO_P2WPKH:
+		/* Account for witness (sig + key) */
+		return bitcoin_tx_input_sig_weight() + 1 + 33;
+	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`.
+		 */
+		/* In practice, these predate anchors, so: */
+		return 1 + 1 + bitcoin_tx_input_sig_weight();
+	case UTXO_P2TR:
+		return 1 + 64;
+	}
+	abort();
 }
 
 size_t bitcoin_tx_2of2_input_witness_weight(void)

bitcoin/tx.h

@@ -48,6 +48,17 @@ struct bitcoin_tx_output {
 	u8 *script;
 };
 
+enum utxotype {
+	/* Obsolete: we used to have P2SH-wrapped outputs (removed in 24.02, though can still have old UTXOs) */
+	UTXO_P2SH_P2WPKH = 1,
+	/* "bech32" addresses */
+	UTXO_P2WPKH = 2,
+	/* Used for closing addresses: implies ->close_info is non-NULL */
+	UTXO_P2WSH_FROM_CLOSE = 3,
+	/* "p2tr" addresses. */
+	UTXO_P2TR = 4,
+};
+
 struct bitcoin_tx_output *new_tx_output(const tal_t *ctx,
 					struct amount_sat amount,
 					const u8 *script);
@@ -320,11 +331,8 @@ size_t bitcoin_tx_input_sig_weight(void);
  * but not the varint_size() for the number of elements. */
 size_t bitcoin_tx_input_weight(bool p2sh, size_t witness_weight);
 
-/* The witness weight for a simple (sig + key) input */
-size_t bitcoin_tx_simple_input_witness_weight(void);
-
-/* We only do segwit inputs, and we assume witness is sig + key  */
-size_t bitcoin_tx_simple_input_weight(bool p2sh);
+/* The witness weight */
+size_t bitcoin_tx_input_witness_weight(enum utxotype utxotype);
 
 /* The witness for our 2of2 input (closing or commitment tx). */
 size_t bitcoin_tx_2of2_input_witness_weight(void);

common/utxo.c

@@ -65,44 +65,10 @@ 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 witness_weight;
-	bool p2sh;
+	bool p2sh = (utxo->utxotype == UTXO_P2SH_P2WPKH);
 
-	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();
+	witness_weight = bitcoin_tx_input_witness_weight(utxo->utxotype);
 
-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 (witness_weight < min_witness_weight)

common/utxo.h

@@ -31,17 +31,6 @@ enum output_status {
 	OUTPUT_STATE_ANY = 255
 };
 
-enum utxotype {
-	/* Obsolete: we used to have P2SH-wrapped outputs (removed in 24.02, though can still have old UTXOs) */
-	UTXO_P2SH_P2WPKH = 1,
-	/* "bech32" addresses */
-	UTXO_P2WPKH = 2,
-	/* Used for closing addresses: implies ->close_info is non-NULL */
-	UTXO_P2WSH_FROM_CLOSE = 3,
-	/* "p2tr" addresses. */
-	UTXO_P2TR = 4,
-};
-
 const char *utxotype_to_str(enum utxotype utxotype);
 
 struct utxo {

plugins/funder_policy.c

@@ -127,9 +127,9 @@ default_lease_rates(const tal_t *ctx)
 	rates->channel_fee_max_base_msat = 5000000;
 
 	/* Let's set our default max weight to two inputs + an output
-	 * (use helpers b/c elements) */
+	 * (use helpers b/c elements).  We're mainly taproot now. */
 	rates->funding_weight
-		= 2 * bitcoin_tx_simple_input_weight(false)
+		= 2 * bitcoin_tx_input_weight(false, bitcoin_tx_input_witness_weight(UTXO_P2TR))
 		+ bitcoin_tx_output_weight(BITCOIN_SCRIPTPUBKEY_P2WPKH_LEN);
 
 	return rates;