Merge #6870: refactor: allow wallet to select fresh change address to prevent address reuse in coinjoin createdenoms

efc689c Apply suggestions from code review (PastaPastaPasta)
cfae1e6 refactor: update CoinJoin change address handling to respect wallet's avoid_reuse setting (pasta)
4800e2f chore: run clang-format (pasta)
d4044d4 feat: introduce -coinjoinfreshchange option to enhance CoinJoin change address handling (pasta)
6ca08f6 docs: add release notes for CoinJoin change address improvement in wallet (pasta)
739d9f1 refactor: allow wallet to select fresh change address to prevent address reuse in coinjoin createdenoms (pasta)

Pull request description:

  ## Issue being fixed or feature implemented
  Coinjoin CreateDenoms will currently intentionally re-use the input address for change; I see no reason to do this, and it can be non-ideal for certain instances such as if you receive on X and are tracking that address X for further receipts. Say every time you receive on X you send to Y as a part of an exchange or something.

  The only time I can image where it's not ideal is if you receive to X multiple times, maybe you receive 1.9 and 0.1 (contrived) in this case, say your first create denoms makes 1.9 -> 1 and 0.9; because that 0.9 is kept in address X, the second 0.1 can be combined with the 0.9 to make another 1 Dash denom.

  ## What was done?
  Use fresh change address instead of re-using

  ## How Has This Been Tested?
  NOT TESTED (@kwvg please test and confirm functionality of createdenoms)

  ## Breaking Changes

  ## Checklist:
    _Go over all the following points, and put an `x` in all the boxes that apply._
  - [ ] I have performed a self-review of my own code
  - [ ] I have commented my code, particularly in hard-to-understand areas
  - [ ] I have added or updated relevant unit/integration/functional/e2e tests
  - [ ] I have made corresponding changes to the documentation
  - [x] I have assigned this pull request to a milestone _(for repository code-owners and collaborators only)_

ACKs for top commit:
  UdjinM6:
    utACK efc689c

Tree-SHA512: cd6e79b7a699253832181235da171501e153e26177e584d7bea62f7849133ad65bbc439b99664b90b7300e63cbce74da36d611ac46a3ee32637369685e6262ce

Author: PastaPastaPasta

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/util.cpp

@@ -9,6 +9,7 @@
 #include <wallet/fees.h>
 #include <wallet/spend.h>
 #include <wallet/wallet.h>
+#include <wallet/walletutil.h>
 
 #include <numeric>
 
@@ -125,8 +126,13 @@ 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