wallet: reuse change dest when recreating TX with avoidpartialspends

test/functional/wallet_change_address.py

@@ -0,0 +1,108 @@
+#!/usr/bin/env python3
+# Copyright (c) 2023 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test wallet change address selection"""
+
+import re
+
+from test_framework.blocktools import COINBASE_MATURITY
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import (
+    assert_equal,
+)
+
+
+class WalletChangeAddressTest(BitcoinTestFramework):
+    def add_options(self, parser):
+        self.add_wallet_options(parser)
+
+    def set_test_params(self):
+        self.setup_clean_chain = True
+        self.num_nodes = 3
+        # discardfee is used to make change outputs less likely in the change_pos test
+        self.extra_args = [
+            [],
+            ["-discardfee=1"],
+            ["-avoidpartialspends", "-discardfee=1"]
+        ]
+
+    def skip_test_if_missing_module(self):
+        self.skip_if_no_wallet()
+
+    def assert_change_index(self, node, tx, index):
+        change_index = None
+        for vout in tx["vout"]:
+            info = node.getaddressinfo(vout["scriptPubKey"]["address"])
+            if (info["ismine"] and info["ischange"]):
+                change_index = int(re.findall(r'\d+', info["hdkeypath"])[-1])
+                break
+        assert_equal(change_index, index)
+
+    def assert_change_pos(self, wallet, tx, pos):
+        change_pos = None
+        for index, output in enumerate(tx["vout"]):
+            info = wallet.getaddressinfo(output["scriptPubKey"]["address"])
+            if (info["ismine"] and info["ischange"]):
+                change_pos = index
+                break
+        assert_equal(change_pos, pos)
+
+    def run_test(self):
+        self.log.info("Setting up")
+        # Mine some coins
+        self.generate(self.nodes[0], COINBASE_MATURITY + 1)
+
+        # Get some addresses from the two nodes
+        addr1 = [self.nodes[1].getnewaddress() for _ in range(3)]
+        addr2 = [self.nodes[2].getnewaddress() for _ in range(3)]
+        addrs = addr1 + addr2
+
+        # Send 1 + 0.5 coin to each address
+        [self.nodes[0].sendtoaddress(addr, 1.0) for addr in addrs]
+        [self.nodes[0].sendtoaddress(addr, 0.5) for addr in addrs]
+        self.generate(self.nodes[0], 1)
+
+        for i in range(20):
+            for n in [1, 2]:
+                self.log.debug(f"Send transaction from node {n}: expected change index {i}")
+                txid = self.nodes[n].sendtoaddress(self.nodes[0].getnewaddress(), 0.2)
+                tx = self.nodes[n].getrawtransaction(txid, True)
+                # find the change output and ensure that expected change index was used
+                self.assert_change_index(self.nodes[n], tx, i)
+
+        # Start next test with fresh wallets and new coins
+        self.nodes[1].createwallet("w1")
+        self.nodes[2].createwallet("w2")
+        w1 = self.nodes[1].get_wallet_rpc("w1")
+        w2 = self.nodes[2].get_wallet_rpc("w2")
+        addr1 = w1.getnewaddress()
+        addr2 = w2.getnewaddress()
+        self.nodes[0].sendtoaddress(addr1, 3.0)
+        self.nodes[0].sendtoaddress(addr1, 0.1)
+        self.nodes[0].sendtoaddress(addr2, 3.0)
+        self.nodes[0].sendtoaddress(addr2, 0.1)
+        self.generate(self.nodes[0], 1)
+
+        sendTo1 = self.nodes[0].getnewaddress()
+        sendTo2 = self.nodes[0].getnewaddress()
+        sendTo3 = self.nodes[0].getnewaddress()
+
+        # The avoid partial spends wallet will always create a change output
+        node = self.nodes[2]
+        res = w2.send({sendTo1: "1.0", sendTo2: "1.0", sendTo3: "0.9999"}, options={"change_position": 0})
+        tx = node.getrawtransaction(res["txid"], True)
+        self.assert_change_pos(w2, tx, 0)
+
+        # The default wallet will internally create a tx without change first,
+        # then create a second candidate using APS that requires a change output.
+        # Ensure that the user-configured change position is kept
+        node = self.nodes[1]
+        res = w1.send({sendTo1: "1.0", sendTo2: "1.0", sendTo3: "0.9999"}, options={"change_position": 0})
+        tx = node.getrawtransaction(res["txid"], True)
+        # If the wallet ignores the user's change_position there is still a 25%
+        # that the random change position passes the test
+        self.assert_change_pos(w1, tx, 0)
+
+if __name__ == '__main__':
+    WalletChangeAddressTest().main()