Merge #12119: [wallet] use P2WPKH change output if any destination is P2WPKH or P2WSH

596c446 [wallet] use P2WPKH change output if any destination is P2WPKH or P2WSH (Sjors Provoost)

Pull request description:

  If `-changetype` is not explicitly set, then regardless of `-addresstype`, the wallet will use a ~`bech32` change address~ `P2WPKH` change output if any destination is `P2WPKH` or `P2WSH`.

  This seems more intuitive to me and more in line with the spirit of [BIP-69](https://github.com/bitcoin/bips/blob/master/bip-0069.mediawiki).

  When combined with #11991 a QT user could opt to use `bech32` exclusively without having to figure out how to launch with `-changetype=bech32`, although so would #11937.

Tree-SHA512: 9238d3ccd1f3be8dfdd43444ccf45d6bdc6584ced3172a3045f3ecfec4a7cc8999db0cdb76ae49236492a84e6dbf3a1fdf18544d3eaf6d518e1f8bd241db33e7

1 files changed, 32 insertions, 2 deletions

diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index dd9bc64728..7fb26900e1 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -2674,6 +2674,34 @@ 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)
 {
@@ -2769,8 +2797,10 @@ 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);