wallet: avoid reuse flags

Add m_avoid_address_reuse flag to coin control object.
Add avoid_reuse wallet flag and accompanying strings/caveats.

src/wallet/coincontrol.cpp

@@ -13,6 +13,7 @@ void CCoinControl::SetNull()
     fAllowOtherInputs = false;
     fAllowWatchOnly = false;
     m_avoid_partial_spends = gArgs.GetBoolArg("-avoidpartialspends", DEFAULT_AVOIDPARTIALSPENDS);
+    m_avoid_address_reuse = false;
     setSelected.clear();
     m_feerate.reset();
     fOverrideFeeRate = false;

src/wallet/coincontrol.h

@@ -34,6 +34,8 @@ public:
     boost::optional<bool> m_signal_bip125_rbf;
     //! Avoid partial use of funds sent to a given address
     bool m_avoid_partial_spends;
+    //! Forbids inclusion of dirty (previously used) addresses
+    bool m_avoid_address_reuse;
     //! Fee estimation mode to control arguments to estimateSmartFee
     FeeEstimateMode m_fee_mode;
     //! Minimum chain depth value for coin availability

src/wallet/rpcwallet.cpp

@@ -47,6 +47,17 @@
 
 static const std::string WALLET_ENDPOINT_BASE = "/wallet/";
 
+static inline bool GetAvoidReuseFlag(CWallet * const pwallet, const UniValue& param) {
+    bool can_avoid_reuse = pwallet->IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE);
+    bool avoid_reuse = param.isNull() ? can_avoid_reuse : param.get_bool();
+
+    if (avoid_reuse && !can_avoid_reuse) {
+        throw JSONRPCError(RPC_WALLET_ERROR, "wallet does not have the \"avoid reuse\" feature enabled");
+    }
+
+    return avoid_reuse;
+}
+
 bool GetWalletNameFromJSONRPCRequest(const JSONRPCRequest& request, std::string& wallet_name)
 {
     if (request.URI.substr(0, WALLET_ENDPOINT_BASE.size()) == WALLET_ENDPOINT_BASE) {

src/wallet/wallet.cpp

@@ -41,6 +41,14 @@
 
 #include <boost/algorithm/string/replace.hpp>
 
+const std::map<uint64_t,std::string> WALLET_FLAG_CAVEATS{
+    {WALLET_FLAG_AVOID_REUSE,
+        "You need to rescan the blockchain in order to correctly mark used "
+        "destinations in the past. Until this is done, some destinations may "
+        "be considered unused, even if the opposite is the case."
+    },
+};
+
 static const size_t OUTPUT_GROUP_MAX_ENTRIES = 10;
 
 static CCriticalSection cs_wallets;

src/wallet/wallet.h

@@ -120,6 +120,10 @@ enum WalletFlags : uint64_t {
     // wallet flags in the upper section (> 1 << 31) will lead to not opening the wallet if flag is unknown
     // unknown wallet flags in the lower section <= (1 << 31) will be tolerated
 
+    // will categorize coins as clean (not reused) and dirty (reused), and handle
+    // them with privacy considerations in mind
+    WALLET_FLAG_AVOID_REUSE = (1ULL << 0),
+
     // Indicates that the metadata has already been upgraded to contain key origins
     WALLET_FLAG_KEY_ORIGIN_METADATA = (1ULL << 1),
 
@@ -139,7 +143,20 @@ enum WalletFlags : uint64_t {
     WALLET_FLAG_BLANK_WALLET = (1ULL << 33),
 };
 
-static constexpr uint64_t KNOWN_WALLET_FLAGS = WALLET_FLAG_DISABLE_PRIVATE_KEYS | WALLET_FLAG_BLANK_WALLET | WALLET_FLAG_KEY_ORIGIN_METADATA;
+static constexpr uint64_t KNOWN_WALLET_FLAGS =
+        WALLET_FLAG_AVOID_REUSE
+    |   WALLET_FLAG_BLANK_WALLET
+    |   WALLET_FLAG_KEY_ORIGIN_METADATA
+    |   WALLET_FLAG_DISABLE_PRIVATE_KEYS;
+
+static const std::map<std::string,WalletFlags> WALLET_FLAG_MAP{
+    {"avoid_reuse", WALLET_FLAG_AVOID_REUSE},
+    {"blank", WALLET_FLAG_BLANK_WALLET},
+    {"key_origin_metadata", WALLET_FLAG_KEY_ORIGIN_METADATA},
+    {"disable_private_keys", WALLET_FLAG_DISABLE_PRIVATE_KEYS},
+};
+
+extern const std::map<uint64_t,std::string> WALLET_FLAG_CAVEATS;
 
 /** A key from a CWallet's keypool
  *