rpc/wallet/bumpfee: allow specifying output index

Currently, bumpfee cannot be used to bump a single output transaction. This should be possible, as single output transactions are special in that the exact amount doesn't usually matter (the user is emptying a wallet or using coin control to move UTXOs).

src/interfaces/wallet.cpp

@@ -268,12 +268,13 @@ class WalletImpl : public Wallet
     bool createBumpTransaction(const uint256& txid,
         const CCoinControl& coin_control,
         CAmount total_fee,
+        int32_t reduce_output,
         std::vector<std::string>& errors,
         CAmount& old_fee,
         CAmount& new_fee,
         CMutableTransaction& mtx) override
     {
-        return feebumper::CreateTransaction(&m_wallet, txid, coin_control, total_fee, errors, old_fee, new_fee, mtx) ==
+        return feebumper::CreateTransaction(&m_wallet, txid, coin_control, total_fee, reduce_output, errors, old_fee, new_fee, mtx) ==
                feebumper::Result::OK;
     }
     bool signBumpTransaction(CMutableTransaction& mtx) override { return feebumper::SignTransaction(&m_wallet, mtx); }

src/interfaces/wallet.h

@@ -152,6 +152,7 @@ class Wallet
     virtual bool createBumpTransaction(const uint256& txid,
         const CCoinControl& coin_control,
         CAmount total_fee,
+        int32_t reduce_output,
         std::vector<std::string>& errors,
         CAmount& old_fee,
         CAmount& new_fee,

src/qt/walletmodel.cpp

@@ -509,7 +509,7 @@ bool WalletModel::bumpFee(uint256 hash, uint256& new_hash)
     CAmount old_fee;
     CAmount new_fee;
     CMutableTransaction mtx;
-    if (!m_wallet->createBumpTransaction(hash, coin_control, 0 /* totalFee */, errors, old_fee, new_fee, mtx)) {
+    if (!m_wallet->createBumpTransaction(hash, coin_control, 0 /* totalFee */, -1 /* reduce_output */, errors, old_fee, new_fee, mtx)) {
         QMessageBox::critical(nullptr, tr("Fee bump error"), tr("Increasing transaction fee failed") + "<br />(" +
             (errors.size() ? QString::fromStdString(errors[0]) : "") +")");
          return false;

src/wallet/feebumper.cpp

@@ -75,7 +75,8 @@ bool TransactionCanBeBumped(const CWallet* wallet, const uint256& txid)
     return res == feebumper::Result::OK;
 }
 
-Result CreateTransaction(const CWallet* wallet, const uint256& txid, const CCoinControl& coin_control, CAmount total_fee, std::vector<std::string>& errors,
+Result CreateTransaction(const CWallet* wallet, const uint256& txid, const CCoinControl& coin_control,
+                         CAmount total_fee, int32_t reduce_output, std::vector<std::string>& errors,
                          CAmount& old_fee, CAmount& new_fee, CMutableTransaction& mtx)
 {
     auto locked_chain = wallet->chain().lock();
@@ -87,22 +88,28 @@ Result CreateTransaction(const CWallet* wallet, const uint256& txid, const CCoin
         return Result::INVALID_ADDRESS_OR_KEY;
     }
     const CWalletTx& wtx = it->second;
+    if (reduce_output < -1 || reduce_output >= int64_t(wtx.tx->vout.size())) {
+        errors.push_back(strprintf("Change output out of bounds [0..%zu]", wtx.tx->vout.size()-1));
+        return Result::INVALID_PARAMETER;
+    }
 
     Result result = PreconditionChecks(*locked_chain, wallet, wtx, errors);
     if (result != Result::OK) {
         return result;
     }
 
-    // figure out which output was change
-    // if there was no change output or multiple change outputs, fail
-    int nOutput = -1;
-    for (size_t i = 0; i < wtx.tx->vout.size(); ++i) {
-        if (wallet->IsChange(wtx.tx->vout[i])) {
-            if (nOutput != -1) {
-                errors.push_back("Transaction has multiple change outputs");
-                return Result::WALLET_ERROR;
+    int nOutput = reduce_output;
+    if (nOutput == -1) {
+        // figure out which output was change
+        // if there was no change output or multiple change outputs, fail
+        for (size_t i = 0; i < wtx.tx->vout.size(); ++i) {
+            if (wallet->IsChange(wtx.tx->vout[i])) {
+                if (nOutput != -1) {
+                    errors.push_back("Transaction has multiple change outputs");
+                    return Result::WALLET_ERROR;
+                }
+                nOutput = i;
             }
-            nOutput = i;
         }
     }
     if (nOutput == -1) {
@@ -161,11 +168,11 @@ Result CreateTransaction(const CWallet* wallet, const uint256& txid, const CCoin
     }
 
     // Check that in all cases the new fee doesn't violate maxTxFee
-     if (new_fee > maxTxFee) {
-         errors.push_back(strprintf("Specified or calculated fee %s is too high (cannot be higher than maxTxFee %s)",
-                               FormatMoney(new_fee), FormatMoney(maxTxFee)));
-         return Result::WALLET_ERROR;
-     }
+    if (new_fee > maxTxFee) {
+        errors.push_back(strprintf("Specified or calculated fee %s is too high (cannot be higher than maxTxFee %s)",
+                         FormatMoney(new_fee), FormatMoney(maxTxFee)));
+        return Result::WALLET_ERROR;
+    }
 
     // check that fee rate is higher than mempool's minimum fee
     // (no point in bumping fee if we know that the new tx won't be accepted to the mempool)

src/wallet/feebumper.h

@@ -33,6 +33,7 @@ Result CreateTransaction(const CWallet* wallet,
                          const uint256& txid,
                          const CCoinControl& coin_control,
                          CAmount total_fee,
+                         int32_t reduce_output,
                          std::vector<std::string>& errors,
                          CAmount& old_fee,
                          CAmount& new_fee,

src/wallet/rpcwallet.cpp

@@ -3168,7 +3168,8 @@ static UniValue bumpfee(const JSONRPCRequest& request)
                 "\nBumps the fee of an opt-in-RBF transaction T, replacing it with a new transaction B.\n"
                 "An opt-in RBF transaction with the given txid must be in the wallet.\n"
                 "The command will pay the additional fee by decreasing (or perhaps removing) its change output.\n"
-                "If the change output is not big enough to cover the increased fee, the command will currently fail\n"
+                "If an explicit \"reduce_output\" has been picked, that will be decreased instead.\n"
+                "If the selected output is not big enough to cover the increased fee, the command will currently fail\n"
                 "instead of adding new inputs to compensate. (A future implementation could improve this.)\n"
                 "The command will fail if the wallet or mempool contains a transaction that spends one of T's outputs.\n"
                 "By default, the new fee will be calculated automatically using estimatesmartfee.\n"
@@ -3196,6 +3197,8 @@ static UniValue bumpfee(const JSONRPCRequest& request)
             "         \"UNSET\"\n"
             "         \"ECONOMICAL\"\n"
             "         \"CONSERVATIVE\""},
+                            {"reduce_output", RPCArg::Type::NUM, /* opt */ true, /* default_val */ "", "Increase the fee by reducing the output value of the output at\n"
+            "                         the given index. NOTE: The recipient of the given output will receive a smaller amount!"},
                         },
                         "options"},
                 }}
@@ -3217,6 +3220,7 @@ static UniValue bumpfee(const JSONRPCRequest& request)
 
     // optional parameters
     CAmount totalFee = 0;
+    int32_t reduce_output = -1;
     CCoinControl coin_control;
     coin_control.m_signal_bip125_rbf = true;
     if (!request.params[1].isNull()) {
@@ -3227,6 +3231,7 @@ static UniValue bumpfee(const JSONRPCRequest& request)
                 {"totalFee", UniValueType(UniValue::VNUM)},
                 {"replaceable", UniValueType(UniValue::VBOOL)},
                 {"estimate_mode", UniValueType(UniValue::VSTR)},
+                {"reduce_output", UniValueType(UniValue::VNUM)},
             },
             true, true);
 
@@ -3249,6 +3254,12 @@ static UniValue bumpfee(const JSONRPCRequest& request)
                 throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid estimate_mode parameter");
             }
         }
+        if (options.exists("reduce_output")) {
+            reduce_output = options["reduce_output"].get_int64();
+            if (reduce_output < 0) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid reduce_output parameter (cannot be negative)");
+            }
+        }
     }
 
     // Make sure the results are valid at least up to the most recent block
@@ -3264,7 +3275,7 @@ static UniValue bumpfee(const JSONRPCRequest& request)
     CAmount old_fee;
     CAmount new_fee;
     CMutableTransaction mtx;
-    feebumper::Result res = feebumper::CreateTransaction(pwallet, hash, coin_control, totalFee, errors, old_fee, new_fee, mtx);
+    feebumper::Result res = feebumper::CreateTransaction(pwallet, hash, coin_control, totalFee, reduce_output, errors, old_fee, new_fee, mtx);
     if (res != feebumper::Result::OK) {
         switch(res) {
             case feebumper::Result::INVALID_ADDRESS_OR_KEY: