common/withdraw_tx: tell caller where we put the change output.

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

common/withdraw_tx.c

@@ -10,11 +10,12 @@
 
 struct bitcoin_tx *withdraw_tx(const tal_t *ctx,
 			       const struct utxo **utxos,
-			       u8 *destination,
+			       const u8 *destination,
 			       struct amount_sat withdraw_amount,
 			       const struct pubkey *changekey,
 			       struct amount_sat change,
-			       const struct ext_key *bip32_base)
+			       const struct ext_key *bip32_base,
+			       int *change_outnum)
 {
 	struct bitcoin_tx *tx;
 
@@ -30,7 +31,10 @@ struct bitcoin_tx *withdraw_tx(const tal_t *ctx,
 		bitcoin_tx_add_output(tx, scriptpubkey_p2wpkh(tx, changekey),
 				      &change);
 		permute_outputs(tx, NULL, map);
-	}
+		if (change_outnum)
+			*change_outnum = ptr2int(map[1]);
+	} else if (change_outnum)
+		*change_outnum = -1;
 	permute_inputs(tx, (const void **)utxos);
 	assert(bitcoin_tx_check(tx));
 	return tx;

common/withdraw_tx.h

@@ -22,13 +22,15 @@ struct utxo;
  * @changekey: (in) key to send change to (only used if change_satoshis != 0).
  * @change: (in) amount to send as change.
  * @bip32_base: (in) bip32 base for key derivation, or NULL.
+ * @change_outnum: (out) set to output index of change output or -1 if none, unless NULL.
  */
 struct bitcoin_tx *withdraw_tx(const tal_t *ctx,
 			       const struct utxo **utxos,
-			       u8 *destination,
+			       const u8 *destination,
 			       struct amount_sat withdraw_amount,
 			       const struct pubkey *changekey,
 			       struct amount_sat change,
-			       const struct ext_key *bip32_base);
+			       const struct ext_key *bip32_base,
+			       int *change_outnum);
 
 #endif /* LIGHTNING_COMMON_WITHDRAW_TX_H */

hsmd/hsmd.c

@@ -1456,7 +1456,7 @@ static struct io_plan *handle_sign_withdrawal_tx(struct io_conn *conn,
 	pubkey_from_der(ext.pub_key, sizeof(ext.pub_key), &changekey);
 	tx = withdraw_tx(tmpctx, cast_const2(const struct utxo **, utxos),
 			 scriptpubkey, satoshi_out,
-			 &changekey, change_out, NULL);
+			 &changekey, change_out, NULL, NULL);
 
 	sign_all_inputs(tx, utxos);