From 596c44633fd03e76cc12f2fd37452e223ba43115 Mon Sep 17 00:00:00 2001
From: Sjors Provoost <sjors@sprovoost.nl>
Date: Tue, 23 Jan 2018 17:56:15 +0100
Subject: [wallet] use P2WPKH change output if any destination is P2WPKH or
 P2WSH

Only if -changetype is not set and -addresstype is not "legacy".
---
 src/wallet/wallet.cpp | 34 ++++++++++++++++++++++++++++++++--
 1 file changed, 32 insertions(+), 2 deletions(-)

(limited to 'src/wallet/wallet.cpp')

diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index da721ea00..c80433895 100644
--- a/src/wallet/wallet.cpp
+++ b/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);
-- 
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