bitcoin: fix out-by-one-error in bitcoin_tx_input_weight.

We need one byte for the number of witness elements.  Some callers added it themselves,
but it's always needed.  So document and fix the callers.

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

Author: rustyrussell

bitcoin/tx.c

@@ -893,7 +893,8 @@ size_t bitcoin_tx_input_sig_weight(void)
 /* Input weight */
 size_t bitcoin_tx_input_weight(bool p2sh, size_t witness_weight)
 {
-	size_t weight = witness_weight;
+	/* We assume < 253 witness elements */
+	size_t weight = 1 + witness_weight;
 
 	/* Input weight: txid + index + sequence */
 	weight += (32 + 4 + 4) * 4;
@@ -914,8 +915,8 @@ size_t bitcoin_tx_input_weight(bool p2sh, size_t witness_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);
+	/* 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  */

bitcoin/tx.h

@@ -315,7 +315,9 @@ 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) */
+/* Segwit input, but with parameter for witness weight (size).
+ * witness_weight must include the varint_size() for each witness element,
+ * 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 */

onchaind/onchaind.c

@@ -186,9 +186,9 @@ static void trim_maximum_feerate(struct amount_sat funding,
 	 *   * `txin[0]` script bytes: 0
 	 *   * `txin[0]` witness: `0 <signature_for_pubkey1> <signature_for_pubkey2>`
 	 */
-	/* Account for witness (1 byte count + 1 empty + sig + sig) */
+	/* Account for witness (1 empty + sig + sig) */
 	assert(tal_count(commitment->inputs) == 1);
-	weight += bitcoin_tx_input_weight(false, 1 + 1 + 2 * bitcoin_tx_input_sig_weight());
+	weight += bitcoin_tx_input_weight(false, 1 + 2 * bitcoin_tx_input_sig_weight());
 
 	for (size_t i = 0; i < tal_count(commitment->outputs); i++) {
 		struct amount_asset amt;