bitcoin: break out method to calculate input weight

We have a required minimum witness weight for dual funded opens.

bitcoin/tx.c

@@ -831,13 +831,13 @@ size_t bitcoin_tx_input_sig_weight(void)
 	return 1 + 71;
 }
 
-/* We only do segwit inputs, and we assume witness is sig + key  */
-size_t bitcoin_tx_simple_input_weight(bool p2sh)
+/* Input weight */
+size_t bitcoin_tx_input_weight(bool p2sh, size_t witness_weight)
 {
-	size_t weight;
+	size_t weight = witness_weight;
 
 	/* Input weight: txid + index + sequence */
-	weight = (32 + 4 + 4) * 4;
+	weight += (32 + 4 + 4) * 4;
 
 	/* We always encode the length of the script, even if empty */
 	weight += 1 * 4;
@@ -846,9 +846,6 @@ size_t bitcoin_tx_simple_input_weight(bool p2sh)
 	if (p2sh)
 		weight += 23 * 4;
 
-	/* Account for witness (1 byte count + sig + key) */
-	weight += 1 + (bitcoin_tx_input_sig_weight() + 1 + 33);
-
 	/* Elements inputs have 6 bytes of blank proofs attached. */
 	if (chainparams->is_elements)
 		weight += 6;
@@ -856,6 +853,19 @@ size_t bitcoin_tx_simple_input_weight(bool p2sh)
 	return weight;
 }
 
+size_t bitcoin_tx_simple_input_witness_weight(void)
+{
+	/* Account for witness (1 byte count + sig + key) */
+	return 1 + (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());
+}
+
 struct amount_sat change_amount(struct amount_sat excess, u32 feerate_perkw)
 {
 	size_t outweight;

bitcoin/tx.h

@@ -255,6 +255,12 @@ size_t bitcoin_tx_output_weight(size_t outscript_len);
 /* Weight to push sig on stack. */
 size_t bitcoin_tx_input_sig_weight(void);
 
+/* Segwit input, but with parameter for witness weight (size) */
+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);