wallet: shuffle coins before grouping, where warranted

Issue brought up in https://github.com/bitcoin/bitcoin/pull/12257\#discussion_r204554549

Author: kallewoof

src/wallet/wallet.cpp

@@ -35,6 +35,8 @@
 
 #include <boost/algorithm/string/replace.hpp>
 
+static const size_t OUTPUT_GROUP_MAX_ENTRIES = 10;
+
 static CCriticalSection cs_wallets;
 static std::vector<std::shared_ptr<CWallet>> vpwallets GUARDED_BY(cs_wallets);
 
@@ -2525,6 +2527,12 @@ bool CWallet::SelectCoins(const std::vector<COutput>& vAvailableCoins, const CAm
 
     // form groups from remaining coins; note that preset coins will not
     // automatically have their associated (same address) coins included
+    if (coin_control.m_avoid_partial_spends && vCoins.size() > OUTPUT_GROUP_MAX_ENTRIES) {
+        // Cases where we have 11+ outputs all pointing to the same destination may result in
+        // privacy leaks as they will potentially be deterministically sorted. We solve that by
+        // explicitly shuffling the outputs before processing
+        std::shuffle(vCoins.begin(), vCoins.end(), FastRandomContext());
+    }
     std::vector<OutputGroup> groups = GroupOutputs(vCoins, !coin_control.m_avoid_partial_spends);
 
     size_t max_ancestors = (size_t)std::max<int64_t>(1, gArgs.GetArg("-limitancestorcount", DEFAULT_ANCESTOR_LIMIT));
@@ -4444,7 +4452,7 @@ std::vector<OutputGroup> CWallet::GroupOutputs(const std::vector<COutput>& outpu
                 // Limit output groups to no more than 10 entries, to protect
                 // against inadvertently creating a too-large transaction
                 // when using -avoidpartialspends
-                if (gmap[dst].m_outputs.size() >= 10) {
+                if (gmap[dst].m_outputs.size() >= OUTPUT_GROUP_MAX_ENTRIES) {
                     groups.push_back(gmap[dst]);
                     gmap.erase(dst);
                 }