wallet: Return change satoshis when selecting coins

We'd be computing them later most of the time anyway.

lightningd/build_utxos.c

@@ -172,22 +172,17 @@ const struct utxo **build_utxos(const tal_t *ctx,
 				u32 feerate_per_kw, u64 dust_limit,
 				u64 *change_satoshis, u32 *change_keyindex)
 {
-	u64 satoshi_in = 0;
 	u64 fee_estimate = 0;
 	u64 bip32_max_index = db_get_intvar(ld->wallet->db, "bip32_max_index", 0);
 	const struct utxo **utxos =
-		wallet_select_coins(ctx, ld->wallet, satoshi_out, feerate_per_kw, &fee_estimate);
+	    wallet_select_coins(ctx, ld->wallet, satoshi_out, feerate_per_kw,
+				&fee_estimate, change_satoshis);
 
 	/* Oops, didn't have enough coins available */
 	if (!utxos)
 		return NULL;
 
-	/* How much are we actually claiming? */
-	for (size_t i=0; i<tal_count(utxos); i++)
-		satoshi_in += utxos[i]->amount;
-
 	/* Do we need a change output? */
-	*change_satoshis = satoshi_in - (fee_estimate + satoshi_out);
 	if (*change_satoshis < dust_limit) {
 		*change_satoshis = 0;
 		*change_keyindex = 0;

wallet/wallet.c

@@ -129,17 +129,20 @@ void wallet_confirm_utxos(struct wallet *w, const struct utxo **utxos)
 const struct utxo **wallet_select_coins(const tal_t *ctx, struct wallet *w,
 					const u64 value,
 					const u32 feerate_per_kw,
-					u64 *fee_estimate)
+					u64 *fee_estimate, u64 *changesatoshi)
 {
 	size_t i = 0;
 	struct utxo **available;
 	const struct utxo **utxos = tal_arr(ctx, const struct utxo *, 0);
+	*fee_estimate = 0;
 
 	/* We assume two outputs for the weight. */
 	u64 satoshi_in = 0, weight = (4 + (8 + 22) * 2 + 4) * 4;
 	tal_add_destructor2(utxos, destroy_utxos, w);
 
-	db_begin_transaction(w->db);
+	if (!db_begin_transaction(w->db)) {
+		fatal("Unable to begin transaction: %s", w->db->err);
+	}
 	available = wallet_get_utxos(ctx, w, output_state_available);
 
 	for (i = 0; i < tal_count(available); i++) {
@@ -171,6 +174,8 @@ const struct utxo **wallet_select_coins(const tal_t *ctx, struct wallet *w,
 	} else {
 		/* Commit the db transaction to persist markings */
 		db_commit_transaction(w->db);
+		*changesatoshi = satoshi_in - value - *fee_estimate;
+
 	}
 	return utxos;
 }

wallet/wallet.h

@@ -77,7 +77,8 @@ struct utxo **wallet_get_utxos(const tal_t *ctx, struct wallet *w,
 const struct utxo **wallet_select_coins(const tal_t *ctx, struct wallet *w,
 					const u64 value,
 					const u32 feerate_per_kw,
-					u64 *fee_estimate);
+					u64 *fee_estimate,
+					u64 *change_satoshi);
 
 /**
  * wallet_confirm_utxos - Once we've spent a set of utxos, mark them confirmed.