[wallet] use P2WPKH change output if any destination is P2WPKH or P2WSH

Only if -changetype is not set and -addresstype is not "legacy".

src/qt/paymentserver.cpp

@@ -643,8 +643,9 @@ void PaymentServer::fetchPaymentACK(CWallet* wallet, const SendCoinsRecipient& r
         // use for change. Despite an actual payment and not change, this is a close match:
         // it's the output type we use subject to privacy issues, but not restricted by what
         // other software supports.
-        wallet->LearnRelatedScripts(newKey, g_change_type);
-        CTxDestination dest = GetDestinationForKey(newKey, g_change_type);
+        const OutputType change_type = g_change_type != OUTPUT_TYPE_NONE ? g_change_type : g_address_type;
+        wallet->LearnRelatedScripts(newKey, change_type);
+        CTxDestination dest = GetDestinationForKey(newKey, change_type);
         wallet->SetAddressBook(dest, strAccount, "refund");
 
         CScript s = GetScriptForDestination(dest);

src/wallet/init.cpp

@@ -17,7 +17,7 @@ std::string GetWalletHelpString(bool showDebug)
 {
     std::string strUsage = HelpMessageGroup(_("Wallet options:"));
     strUsage += HelpMessageOpt("-addresstype", strprintf(_("What type of addresses to use (\"legacy\", \"p2sh-segwit\", or \"bech32\", default: \"%s\")"), FormatOutputType(OUTPUT_TYPE_DEFAULT)));
-    strUsage += HelpMessageOpt("-changetype", _("What type of change to use (\"legacy\", \"p2sh-segwit\", or \"bech32\", default is same as -addresstype)"));
+    strUsage += HelpMessageOpt("-changetype", _("What type of change to use (\"legacy\", \"p2sh-segwit\", or \"bech32\"). Default is same as -addresstype, except when -addresstype=p2sh-segwit a native segwit output is used when sending to a native segwit address)"));
     strUsage += HelpMessageOpt("-disablewallet", _("Do not load the wallet and disable wallet RPC calls"));
     strUsage += HelpMessageOpt("-keypool=<n>", strprintf(_("Set key pool size to <n> (default: %u)"), DEFAULT_KEYPOOL_SIZE));
     strUsage += HelpMessageOpt("-fallbackfee=<amt>", strprintf(_("A fee rate (in %s/kB) that will be used when fee estimation has insufficient data (default: %s)"),
@@ -182,8 +182,10 @@ bool WalletParameterInteraction()
         return InitError(strprintf(_("Unknown address type '%s'"), gArgs.GetArg("-addresstype", "")));
     }
 
-    g_change_type = ParseOutputType(gArgs.GetArg("-changetype", ""), g_address_type);
-    if (g_change_type == OUTPUT_TYPE_NONE) {
+    // If changetype is set in config file or parameter, check that it's valid.
+    // Default to OUTPUT_TYPE_NONE if not set.
+    g_change_type = ParseOutputType(gArgs.GetArg("-changetype", ""), OUTPUT_TYPE_NONE);
+    if (g_change_type == OUTPUT_TYPE_NONE && !gArgs.GetArg("-changetype", "").empty()) {
         return InitError(strprintf(_("Unknown change type '%s'"), gArgs.GetArg("-changetype", "")));
     }
 

src/wallet/rpcwallet.cpp

@@ -256,9 +256,9 @@ UniValue getrawchangeaddress(const JSONRPCRequest& request)
         pwallet->TopUpKeyPool();
     }
 
-    OutputType output_type = g_change_type;
+    OutputType output_type = g_change_type != OUTPUT_TYPE_NONE ? g_change_type : g_address_type;
     if (!request.params[0].isNull()) {
-        output_type = ParseOutputType(request.params[0].get_str(), g_change_type);
+        output_type = ParseOutputType(request.params[0].get_str(), output_type);
         if (output_type == OUTPUT_TYPE_NONE) {
             throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("Unknown address type '%s'", request.params[0].get_str()));
         }

src/wallet/wallet.cpp

@@ -2644,6 +2644,34 @@ bool CWallet::FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, int& nC
     return true;
 }
 
+OutputType CWallet::TransactionChangeType(const std::vector<CRecipient>& vecSend)
+{
+    // If -changetype is specified, always use that change type.
+    if (g_change_type != OUTPUT_TYPE_NONE) {
+        return g_change_type;
+    }
+
+    // if g_address_type is legacy, use legacy address as change (even
+    // if some of the outputs are P2WPKH or P2WSH).
+    if (g_address_type == OUTPUT_TYPE_LEGACY) {
+        return OUTPUT_TYPE_LEGACY;
+    }
+
+    // if any destination is P2WPKH or P2WSH, use P2WPKH for the change
+    // output.
+    for (const auto& recipient : vecSend) {
+        // Check if any destination contains a witness program:
+        int witnessversion = 0;
+        std::vector<unsigned char> witnessprogram;
+        if (recipient.scriptPubKey.IsWitnessProgram(witnessversion, witnessprogram)) {
+            return OUTPUT_TYPE_BECH32;
+        }
+    }
+
+    // else use g_address_type for change
+    return g_address_type;
+}
+
 bool CWallet::CreateTransaction(const std::vector<CRecipient>& vecSend, CWalletTx& wtxNew, CReserveKey& reservekey, CAmount& nFeeRet,
                                 int& nChangePosInOut, std::string& strFailReason, const CCoinControl& coin_control, bool sign)
 {
@@ -2739,8 +2767,10 @@ bool CWallet::CreateTransaction(const std::vector<CRecipient>& vecSend, CWalletT
                     return false;
                 }
 
-                LearnRelatedScripts(vchPubKey, g_change_type);
-                scriptChange = GetScriptForDestination(GetDestinationForKey(vchPubKey, g_change_type));
+                const OutputType change_type = TransactionChangeType(vecSend);
+
+                LearnRelatedScripts(vchPubKey, change_type);
+                scriptChange = GetScriptForDestination(GetDestinationForKey(vchPubKey, change_type));
             }
             CTxOut change_prototype_txout(0, scriptChange);
             size_t change_prototype_size = GetSerializeSize(change_prototype_txout, SER_DISK, 0);

src/wallet/wallet.h

@@ -961,6 +961,8 @@ public:
     CAmount GetLegacyBalance(const isminefilter& filter, int minDepth, const std::string* account) const;
     CAmount GetAvailableBalance(const CCoinControl* coinControl = nullptr) const;
 
+    OutputType TransactionChangeType(const std::vector<CRecipient>& vecSend);
+
     /**
      * Insert additional inputs into the transaction by
      * calling CreateTransaction();

test/functional/address_types.py

@@ -4,16 +4,24 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Test that the wallet can send and receive using all combinations of address types.
 
-There are 4 nodes-under-test:
+There are 5 nodes-under-test:
     - node0 uses legacy addresses
     - node1 uses p2sh/segwit addresses
     - node2 uses p2sh/segwit addresses and bech32 addresses for change
     - node3 uses bech32 addresses
+    - node4 uses a p2sh/segwit addresses for change
 
-node4 exists to generate new blocks.
+node5 exists to generate new blocks.
 
-The script is a series of tests, iterating over the 4 nodes. In each iteration
-of the test, one node sends:
+## Multisig address test
+
+Test that adding a multisig address with:
+    - an uncompressed pubkey always gives a legacy address
+    - only compressed pubkeys gives the an `-addresstype` address
+
+## Sending to address types test
+
+A series of tests, iterating over node0-node4. In each iteration of the test, one node sends:
     - 10/101th of its balance to itself (using getrawchangeaddress for single key addresses)
     - 20/101th to the next node
     - 30/101th to the node after that
@@ -21,21 +29,51 @@ of the test, one node sends:
     - 1/101th remains as fee+change
 
 Iterate over each node for single key addresses, and then over each node for
-multisig addresses. In a second iteration, the same is done, but with explicit address_type
-parameters passed to getnewaddress and getrawchangeaddress. Node0 and node3 send to p2sh,
-node 1 sends to bech32, and node2 sends to legacy. As every node sends coins after receiving,
-this also verifies that spending coins sent to all these address types works."""
+multisig addresses.
+
+Repeat test, but with explicit address_type parameters passed to getnewaddress
+and getrawchangeaddress:
+    - node0 and node3 send to p2sh.
+    - node1 sends to bech32.
+    - node2 sends to legacy.
+
+As every node sends coins after receiving, this also
+verifies that spending coins sent to all these address types works.
+
+## Change type test
+
+Test that the nodes generate the correct change address type:
+    - node0 always uses a legacy change address.
+    - node1 uses a bech32 addresses for change if any destination address is bech32.
+    - node2 always uses a bech32 address for change
+    - node3 always uses a bech32 address for change
+    - node4 always uses p2sh/segwit output for change.
+"""
 
 from decimal import Decimal
 import itertools
 
 from test_framework.test_framework import BitcoinTestFramework
-from test_framework.util import assert_equal, assert_greater_than, connect_nodes_bi, sync_blocks, sync_mempools
+from test_framework.util import (
+    assert_equal,
+    assert_greater_than,
+    assert_raises_rpc_error,
+    connect_nodes_bi,
+    sync_blocks,
+    sync_mempools,
+)
 
 class AddressTypeTest(BitcoinTestFramework):
     def set_test_params(self):
-        self.num_nodes = 5
-        self.extra_args = [["-addresstype=legacy"], ["-addresstype=p2sh-segwit"], ["-addresstype=p2sh-segwit", "-changetype=bech32"], ["-addresstype=bech32"], []]
+        self.num_nodes = 6
+        self.extra_args = [
+            ["-addresstype=legacy"],
+            ["-addresstype=p2sh-segwit"],
+            ["-addresstype=p2sh-segwit", "-changetype=bech32"],
+            ["-addresstype=bech32"],
+            ["-changetype=p2sh-segwit"],
+            []
+        ]
 
     def setup_network(self):
         self.setup_nodes()
@@ -104,10 +142,26 @@ class AddressTypeTest(BitcoinTestFramework):
             # Unknown type
             assert(False)
 
+    def test_change_output_type(self, node_sender, destinations, expected_type):
+        txid = self.nodes[node_sender].sendmany(fromaccount="", amounts=dict.fromkeys(destinations, 0.001))
+        raw_tx = self.nodes[node_sender].getrawtransaction(txid)
+        tx = self.nodes[node_sender].decoderawtransaction(raw_tx)
+
+        # Make sure the transaction has change:
+        assert_equal(len(tx["vout"]), len(destinations) + 1)
+
+        # Make sure the destinations are included, and remove them:
+        output_addresses = [vout['scriptPubKey']['addresses'][0] for vout in tx["vout"]]
+        change_addresses = [d for d in output_addresses if d not in destinations]
+        assert_equal(len(change_addresses), 1)
+
+        self.log.debug("Check if change address " + change_addresses[0] + " is " + expected_type)
+        self.test_address(node_sender, change_addresses[0], multisig=False, typ=expected_type)
+
     def run_test(self):
-        # Mine 101 blocks on node4 to bring nodes out of IBD and make sure that
+        # Mine 101 blocks on node5 to bring nodes out of IBD and make sure that
         # no coinbases are maturing for the nodes-under-test during the test
-        self.nodes[4].generate(101)
+        self.nodes[5].generate(101)
         sync_blocks(self.nodes)
 
         uncompressed_1 = "0496b538e853519c726a2c91e61ec11600ae1390813a627c66fb8be7947be63c52da7589379515d4e0a604f8141781e62294721166bf621e73a82cbf2342c858ee"
@@ -182,8 +236,8 @@ class AddressTypeTest(BitcoinTestFramework):
                 to_node %= 4
                 assert_equal(unconf_balances[to_node], to_send * 10 * (2 + n))
 
-            # node4 collects fee and block subsidy to keep accounting simple
-            self.nodes[4].generate(1)
+            # node5 collects fee and block subsidy to keep accounting simple
+            self.nodes[5].generate(1)
             sync_blocks(self.nodes)
 
             new_balances = self.get_balances()
@@ -195,5 +249,46 @@ class AddressTypeTest(BitcoinTestFramework):
                 to_node %= 4
                 assert_equal(new_balances[to_node], old_balances[to_node] + to_send * 10 * (2 + n))
 
+        # Get one p2sh/segwit address from node2 and two bech32 addresses from node3:
+        to_address_p2sh = self.nodes[2].getnewaddress()
+        to_address_bech32_1 = self.nodes[3].getnewaddress()
+        to_address_bech32_2 = self.nodes[3].getnewaddress()
+
+        # Fund node 4:
+        self.nodes[5].sendtoaddress(self.nodes[4].getnewaddress(), Decimal("1"))
+        self.nodes[5].generate(1)
+        sync_blocks(self.nodes)
+        assert_equal(self.nodes[4].getbalance(), 1)
+
+        self.log.info("Nodes with addresstype=legacy never use a P2WPKH change output")
+        self.test_change_output_type(0, [to_address_bech32_1], 'legacy')
+
+        self.log.info("Nodes with addresstype=p2sh-segwit only use a P2WPKH change output if any destination address is bech32:")
+        self.test_change_output_type(1, [to_address_p2sh], 'p2sh-segwit')
+        self.test_change_output_type(1, [to_address_bech32_1], 'bech32')
+        self.test_change_output_type(1, [to_address_p2sh, to_address_bech32_1], 'bech32')
+        self.test_change_output_type(1, [to_address_bech32_1, to_address_bech32_2], 'bech32')
+
+        self.log.info("Nodes with change_type=bech32 always use a P2WPKH change output:")
+        self.test_change_output_type(2, [to_address_bech32_1], 'bech32')
+        self.test_change_output_type(2, [to_address_p2sh], 'bech32')
+
+        self.log.info("Nodes with addresstype=bech32 always use a P2WPKH change output (unless changetype is set otherwise):")
+        self.test_change_output_type(3, [to_address_bech32_1], 'bech32')
+        self.test_change_output_type(3, [to_address_p2sh], 'bech32')
+
+        self.log.info('getrawchangeaddress defaults to addresstype if -changetype is not set and argument is absent')
+        self.test_address(3, self.nodes[3].getrawchangeaddress(), multisig=False, typ='bech32')
+
+        self.log.info('getrawchangeaddress fails with invalid changetype argument')
+        assert_raises_rpc_error(-5, "Unknown address type 'bech23'", self.nodes[3].getrawchangeaddress, 'bech23')
+
+        self.log.info("Nodes with changetype=p2sh-segwit never use a P2WPKH change output")
+        self.test_change_output_type(4, [to_address_bech32_1], 'p2sh-segwit')
+        self.test_address(4, self.nodes[4].getrawchangeaddress(), multisig=False, typ='p2sh-segwit')
+        self.log.info("Except for getrawchangeaddress if specified:")
+        self.test_address(4, self.nodes[4].getrawchangeaddress(), multisig=False, typ='p2sh-segwit')
+        self.test_address(4, self.nodes[4].getrawchangeaddress('bech32'), multisig=False, typ='bech32')
+
 if __name__ == '__main__':
     AddressTypeTest().main()