txprepare fixes

common/withdraw_tx.c

@@ -19,24 +19,37 @@ struct bitcoin_tx *withdraw_tx(const tal_t *ctx,
 			       int *change_outnum)
 {
 	struct bitcoin_tx *tx;
+	int output_count;
 
 	tx = tx_spending_utxos(ctx, chainparams, utxos, bip32_base,
 			       !amount_sat_eq(change, AMOUNT_SAT(0)),
 			       tal_count(outputs));
 
-	bitcoin_tx_add_multi_outputs(tx, outputs);
+	output_count = bitcoin_tx_add_multi_outputs(tx, outputs);
+	assert(output_count == tal_count(outputs));
 
 	if (!amount_sat_eq(change, AMOUNT_SAT(0))) {
-		const void *map[2];
-		map[0] = int2ptr(0);
-		map[1] = int2ptr(1);
+		/* Add one to the output_count, for the change */
+		output_count++;
+
+		const void *map[output_count];
+		for (size_t i = 0; i < output_count; i++)
+			map[i] = int2ptr(i);
+
 		bitcoin_tx_add_output(tx, scriptpubkey_p2wpkh(tmpctx, changekey),
 				      change);
+
+		assert(tx->wtx->num_outputs == output_count);
 		permute_outputs(tx, NULL, map);
+
+		/* The change is the last output added, so the last position
+		 * in the map */
 		if (change_outnum)
-			*change_outnum = ptr2int(map[1]);
+			*change_outnum = ptr2int(map[output_count - 1]);
+
 	} else if (change_outnum)
 		*change_outnum = -1;
+
 	permute_inputs(tx, (const void **)utxos);
 	elements_tx_add_fee_output(tx);
 	assert(bitcoin_tx_check(tx));

contrib/pyln-client/pyln/client/lightning.py

@@ -990,7 +990,7 @@ def txprepare(self, *args, **kwargs):
         if 'destination' in kwargs or 'satoshi' in kwargs:
             return self._deprecated_txprepare(*args, **kwargs)
 
-        if not isinstance(args[0], list):
+        if len(args) and not isinstance(args[0], list):
             return self._deprecated_txprepare(*args, **kwargs)
 
         def _txprepare(outputs, feerate=None, minconf=None, utxos=None):

tests/test_wallet.py

@@ -262,6 +262,21 @@ def test_deprecated_txprepare(node_factory, bitcoind):
     l1.rpc.call('txprepare', {'destination': addr, 'satoshi': Millisatoshi(amount * 100)})
 
 
+def test_txprepare_multi(node_factory, bitcoind):
+    amount = 10000000
+    l1 = node_factory.get_node(random_hsm=True)
+
+    bitcoind.rpc.sendtoaddress(l1.rpc.newaddr()['bech32'], amount / 10**8)
+    bitcoind.generate_block(1)
+    wait_for(lambda: len(l1.rpc.listfunds()['outputs']) == 1)
+
+    outputs = []
+    for i in range(9):
+        outputs.append({l1.rpc.newaddr()['bech32']: Millisatoshi(amount * 100)})
+    prep = l1.rpc.txprepare(outputs=outputs)
+    l1.rpc.txdiscard(prep['txid'])
+
+
 def test_txprepare(node_factory, bitcoind, chainparams):
     amount = 1000000
     l1 = node_factory.get_node(random_hsm=True)
@@ -277,7 +292,7 @@ def test_txprepare(node_factory, bitcoind, chainparams):
     bitcoind.generate_block(1)
     wait_for(lambda: len(l1.rpc.listfunds()['outputs']) == 10)
 
-    prep = l1.rpc.txprepare([{addr: Millisatoshi(amount * 3 * 1000)}])
+    prep = l1.rpc.txprepare(outputs=[{addr: Millisatoshi(amount * 3 * 1000)}])
     decode = bitcoind.rpc.decoderawtransaction(prep['unsigned_tx'])
     assert decode['txid'] == prep['txid']
     # 4 inputs, 2 outputs (3 if we have a fee output).