refactor: allow wallet to select fresh change address to prevent address reuse in coinjoin createdenoms

doc/release-notes-6870.md

@@ -0,0 +1,9 @@
+Wallet
+------
+
+- CoinJoin denomination creation now respects the wallet's "avoid_reuse"
+  setting. When the wallet has `avoid_reuse` enabled, change is sent to a
+  fresh change address to avoid address/public key reuse. Otherwise, change
+  goes back to the source address (legacy behavior). (#6870)
+
+

src/coinjoin/options.cpp

@@ -61,6 +61,7 @@ void CCoinJoinClientOptions::SetDenomsHardCap(int denoms_hardcap)
     options.nCoinJoinDenomsHardCap = denoms_hardcap;
 }
 
+
 void CCoinJoinClientOptions::Init()
 {
     assert(!CCoinJoinClientOptions::_instance);

src/coinjoin/util.cpp

@@ -2,13 +2,15 @@
 // Distributed under the MIT/X11 software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <coinjoin/options.h>
 #include <coinjoin/util.h>
 #include <policy/fees.h>
 #include <policy/policy.h>
 #include <util/translation.h>
 #include <wallet/fees.h>
 #include <wallet/spend.h>
 #include <wallet/wallet.h>
+#include <wallet/walletutil.h>
 
 #include <numeric>
 
@@ -125,8 +127,13 @@ CTransactionBuilder::CTransactionBuilder(CWallet& wallet, const CompactTallyItem
     coinControl.m_discard_feerate = ::GetDiscardRate(m_wallet);
     // Generate a feerate which will be used by calculations of this class and also by CWallet::CreateTransaction
     coinControl.m_feerate = std::max(GetRequiredFeeRate(m_wallet), m_wallet.m_pay_tx_fee);
-    // Change always goes back to origin
-    coinControl.destChange = tallyItemIn.txdest;
+    // If wallet does not have the avoid-reuse feature enabled, keep legacy
+    // behavior: force change to go back to the origin address. When
+    // WALLET_FLAG_AVOID_REUSE is enabled, let the wallet select a fresh
+    // change destination to avoid address reuse.
+    if (!m_wallet.IsWalletFlagSet(wallet::WALLET_FLAG_AVOID_REUSE)) {
+        coinControl.destChange = tallyItemIn.txdest;
+    }
     // Only allow tallyItems inputs for tx creation
     coinControl.m_allow_other_inputs = false;
     // Create dummy tx to calculate the exact required fees upfront for accurate amount and fee calculations

src/wallet/init.cpp

@@ -105,6 +105,7 @@ void WalletInit::AddWalletOptions(ArgsManager& argsman) const
     argsman.AddArg("-coinjoinmultisession", strprintf("Enable multiple CoinJoin mixing sessions per block, experimental (0-1, default: %u)", DEFAULT_COINJOIN_MULTISESSION), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET_COINJOIN);
     argsman.AddArg("-coinjoinrounds=<n>", strprintf("Use N separate masternodes for each denominated input to mix funds (%u-%u, default: %u)", MIN_COINJOIN_ROUNDS, MAX_COINJOIN_ROUNDS, DEFAULT_COINJOIN_ROUNDS), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET_COINJOIN);
     argsman.AddArg("-coinjoinsessions=<n>", strprintf("Use N separate masternodes in parallel to mix funds (%u-%u, default: %u)", MIN_COINJOIN_SESSIONS, MAX_COINJOIN_SESSIONS, DEFAULT_COINJOIN_SESSIONS), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET_COINJOIN);
+
 
 #ifdef USE_BDB
     argsman.AddArg("-dblogsize=<n>", strprintf("Flush wallet database activity from memory to disk log every <n> megabytes (default: %u)", DatabaseOptions().max_log_mb), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::WALLET_DEBUG_TEST);