send: refactor CreateTransaction flow to return a BResult<CTransactionRef>

9 files changed, 81 insertions, 93 deletions

diff --git a/src/interfaces/wallet.h b/src/interfaces/wallet.h
index b3cb0ae387..00d238f186 100644
--- a/src/interfaces/wallet.h
+++ b/src/interfaces/wallet.h
@@ -12,6 +12,7 @@
 #include <script/standard.h>           // For CTxDestination
 #include <support/allocators/secure.h> // For SecureString
 #include <util/message.h>
+#include <util/result.h>
 #include <util/ui_change_type.h>
 
 #include <cstdint>
@@ -138,12 +139,11 @@ public:
     virtual void listLockedCoins(std::vector<COutPoint>& outputs) = 0;
 
     //! Create transaction.
-    virtual CTransactionRef createTransaction(const std::vector<wallet::CRecipient>& recipients,
+    virtual BResult<CTransactionRef> createTransaction(const std::vector<wallet::CRecipient>& recipients,
         const wallet::CCoinControl& coin_control,
         bool sign,
         int& change_pos,
-        CAmount& fee,
-        bilingual_str& fail_reason) = 0;
+        CAmount& fee) = 0;
 
     //! Commit transaction.
     virtual void commitTransaction(CTransactionRef tx,
diff --git a/src/qt/walletmodel.cpp b/src/qt/walletmodel.cpp
index ab4d1cae3f..bb6079afee 100644
--- a/src/qt/walletmodel.cpp
+++ b/src/qt/walletmodel.cpp
@@ -204,10 +204,10 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
     {
         CAmount nFeeRequired = 0;
         int nChangePosRet = -1;
-        bilingual_str error;
 
         auto& newTx = transaction.getWtx();
-        newTx = m_wallet->createTransaction(vecSend, coinControl, !wallet().privateKeysDisabled() /* sign */, nChangePosRet, nFeeRequired, error);
+        const auto& res = m_wallet->createTransaction(vecSend, coinControl, !wallet().privateKeysDisabled() /* sign */, nChangePosRet, nFeeRequired);
+        newTx = res ? res.GetObj() : nullptr;
         transaction.setTransactionFee(nFeeRequired);
         if (fSubtractFeeFromAmount && newTx)
             transaction.reassignAmounts(nChangePosRet);
@@ -218,7 +218,7 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
             {
                 return SendCoinsReturn(AmountWithFeeExceedsBalance);
             }
-            Q_EMIT message(tr("Send Coins"), QString::fromStdString(error.translated),
+            Q_EMIT message(tr("Send Coins"), QString::fromStdString(res.GetError().translated),
                 CClientUIInterface::MSG_ERROR);
             return TransactionCreationFailed;
         }
diff --git a/src/wallet/feebumper.cpp b/src/wallet/feebumper.cpp
index 9bd7e62c28..43fdcee48d 100644
--- a/src/wallet/feebumper.cpp
+++ b/src/wallet/feebumper.cpp
@@ -219,18 +219,18 @@ Result CreateRateBumpTransaction(CWallet& wallet, const uint256& txid, const CCo
     new_coin_control.m_min_depth = 1;
 
     constexpr int RANDOM_CHANGE_POSITION = -1;
-    bilingual_str fail_reason;
-    std::optional<CreatedTransactionResult> txr = CreateTransaction(wallet, recipients, RANDOM_CHANGE_POSITION, fail_reason, new_coin_control, false);
-    if (!txr) {
-        errors.push_back(Untranslated("Unable to create transaction.") + Untranslated(" ") + fail_reason);
+    auto res = CreateTransaction(wallet, recipients, RANDOM_CHANGE_POSITION, new_coin_control, false);
+    if (!res) {
+        errors.push_back(Untranslated("Unable to create transaction.") + Untranslated(" ") + res.GetError());
         return Result::WALLET_ERROR;
     }
 
+    const auto& txr = res.GetObj();
     // Write back new fee if successful
-    new_fee = txr->fee;
+    new_fee = txr.fee;
 
     // Write back transaction
-    mtx = CMutableTransaction(*txr->tx);
+    mtx = CMutableTransaction(*txr.tx);
 
     return Result::OK;
 }
diff --git a/src/wallet/interfaces.cpp b/src/wallet/interfaces.cpp
index cec103b02b..424d441488 100644
--- a/src/wallet/interfaces.cpp
+++ b/src/wallet/interfaces.cpp
@@ -250,21 +250,21 @@ public:
         LOCK(m_wallet->cs_wallet);
         return m_wallet->ListLockedCoins(outputs);
     }
-    CTransactionRef createTransaction(const std::vector<CRecipient>& recipients,
+    BResult<CTransactionRef> createTransaction(const std::vector<CRecipient>& recipients,
         const CCoinControl& coin_control,
         bool sign,
         int& change_pos,
-        CAmount& fee,
-        bilingual_str& fail_reason) override
+        CAmount& fee) override
     {
         LOCK(m_wallet->cs_wallet);
-        std::optional<CreatedTransactionResult> txr = CreateTransaction(*m_wallet, recipients, change_pos,
-                fail_reason, coin_control, sign);
-        if (!txr) return {};
-        fee = txr->fee;
-        change_pos = txr->change_pos;
+        const auto& res = CreateTransaction(*m_wallet, recipients, change_pos,
+                                     coin_control, sign);
+        if (!res) return res.GetError();
+        const auto& txr = res.GetObj();
+        fee = txr.fee;
+        change_pos = txr.change_pos;
 
-        return txr->tx;
+        return txr.tx;
     }
     void commitTransaction(CTransactionRef tx,
         WalletValueMap value_map,
diff --git a/src/wallet/rpc/spend.cpp b/src/wallet/rpc/spend.cpp
index 362e9113fb..22c1f2bdc2 100644
--- a/src/wallet/rpc/spend.cpp
+++ b/src/wallet/rpc/spend.cpp
@@ -156,17 +156,16 @@ UniValue SendMoney(CWallet& wallet, const CCoinControl &coin_control, std::vecto
 
     // Send
     constexpr int RANDOM_CHANGE_POSITION = -1;
-    bilingual_str error;
-    std::optional<CreatedTransactionResult> txr = CreateTransaction(wallet, recipients, RANDOM_CHANGE_POSITION, error, coin_control, true);
-    if (!txr) {
-        throw JSONRPCError(RPC_WALLET_INSUFFICIENT_FUNDS, error.original);
+    auto res = CreateTransaction(wallet, recipients, RANDOM_CHANGE_POSITION, coin_control, true);
+    if (!res) {
+        throw JSONRPCError(RPC_WALLET_INSUFFICIENT_FUNDS, res.GetError().original);
     }
-    CTransactionRef tx = txr->tx;
+    const CTransactionRef& tx = res.GetObj().tx;
     wallet.CommitTransaction(tx, std::move(map_value), {} /* orderForm */);
     if (verbose) {
         UniValue entry(UniValue::VOBJ);
         entry.pushKV("txid", tx->GetHash().GetHex());
-        entry.pushKV("fee_reason", StringForFeeReason(txr->fee_calc.reason));
+        entry.pushKV("fee_reason", StringForFeeReason(res.GetObj().fee_calc.reason));
         return entry;
     }
     return tx->GetHash().GetHex();
diff --git a/src/wallet/spend.cpp b/src/wallet/spend.cpp
index 9846335da7..4a54a5f671 100644
--- a/src/wallet/spend.cpp
+++ b/src/wallet/spend.cpp
@@ -656,18 +656,16 @@ static void DiscourageFeeSniping(CMutableTransaction& tx, FastRandomContext& rng
     }
 }
 
-static std::optional<CreatedTransactionResult> CreateTransactionInternal(
+static BResult<CreatedTransactionResult> CreateTransactionInternal(
         CWallet& wallet,
         const std::vector<CRecipient>& vecSend,
         int change_pos,
-        bilingual_str& error,
         const CCoinControl& coin_control,
         bool sign) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet)
 {
     AssertLockHeld(wallet.cs_wallet);
 
     // out variables, to be packed into returned result structure
-    CTransactionRef tx;
     CAmount nFeeRet;
     int nChangePosInOut = change_pos;
 
@@ -696,6 +694,7 @@ static std::optional<CreatedTransactionResult> CreateTransactionInternal(
 
     // Create change script that will be used if we need change
     CScript scriptChange;
+    bilingual_str error; // possible error str
 
     // coin control: send change to custom address
     if (!std::get_if<CNoDestination>(&coin_control.destChange)) {
@@ -743,13 +742,11 @@ static std::optional<CreatedTransactionResult> CreateTransactionInternal(
     // Do not, ever, assume that it's fine to change the fee rate if the user has explicitly
     // provided one
     if (coin_control.m_feerate && coin_selection_params.m_effective_feerate > *coin_control.m_feerate) {
-        error = strprintf(_("Fee rate (%s) is lower than the minimum fee rate setting (%s)"), coin_control.m_feerate->ToString(FeeEstimateMode::SAT_VB), coin_selection_params.m_effective_feerate.ToString(FeeEstimateMode::SAT_VB));
-        return std::nullopt;
+        return strprintf(_("Fee rate (%s) is lower than the minimum fee rate setting (%s)"), coin_control.m_feerate->ToString(FeeEstimateMode::SAT_VB), coin_selection_params.m_effective_feerate.ToString(FeeEstimateMode::SAT_VB));
     }
     if (feeCalc.reason == FeeReason::FALLBACK && !wallet.m_allow_fallback_fee) {
         // eventually allow a fallback fee
-        error = _("Fee estimation failed. Fallbackfee is disabled. Wait a few blocks or enable -fallbackfee.");
-        return std::nullopt;
+        return _("Fee estimation failed. Fallbackfee is disabled. Wait a few blocks or enable -fallbackfee.");
     }
 
     // Calculate the cost of change
@@ -774,10 +771,8 @@ static std::optional<CreatedTransactionResult> CreateTransactionInternal(
             coin_selection_params.tx_noinputs_size += ::GetSerializeSize(txout, PROTOCOL_VERSION);
         }
 
-        if (IsDust(txout, wallet.chain().relayDustFee()))
-        {
-            error = _("Transaction amount too small");
-            return std::nullopt;
+        if (IsDust(txout, wallet.chain().relayDustFee())) {
+            return _("Transaction amount too small");
         }
         txNew.vout.push_back(txout);
     }
@@ -798,8 +793,7 @@ static std::optional<CreatedTransactionResult> CreateTransactionInternal(
     // Choose coins to use
     std::optional<SelectionResult> result = SelectCoins(wallet, res_available_coins.coins, /*nTargetValue=*/selection_target, coin_control, coin_selection_params);
     if (!result) {
-        error = _("Insufficient funds");
-        return std::nullopt;
+        return _("Insufficient funds");
     }
     TRACE5(coin_selection, selected_coins, wallet.GetName().c_str(), GetAlgorithmName(result->m_algo).c_str(), result->m_target, result->GetWaste(), result->GetSelectedValue());
 
@@ -813,10 +807,8 @@ static std::optional<CreatedTransactionResult> CreateTransactionInternal(
         // Insert change txn at random position:
         nChangePosInOut = rng_fast.randrange(txNew.vout.size() + 1);
     }
-    else if ((unsigned int)nChangePosInOut > txNew.vout.size())
-    {
-        error = _("Transaction change output index out of range");
-        return std::nullopt;
+    else if ((unsigned int)nChangePosInOut > txNew.vout.size()) {
+        return _("Transaction change output index out of range");
     }
 
     assert(nChangePosInOut != -1);
@@ -843,8 +835,7 @@ static std::optional<CreatedTransactionResult> CreateTransactionInternal(
     TxSize tx_sizes = CalculateMaximumSignedTxSize(CTransaction(txNew), &wallet, &coin_control);
     int nBytes = tx_sizes.vsize;
     if (nBytes == -1) {
-        error = _("Missing solving data for estimating transaction size");
-        return std::nullopt;
+        return _("Missing solving data for estimating transaction size");
     }
     nFeeRet = coin_selection_params.m_effective_feerate.GetFee(nBytes);
 
@@ -904,11 +895,10 @@ static std::optional<CreatedTransactionResult> CreateTransactionInternal(
                 // Error if this output is reduced to be below dust
                 if (IsDust(txout, wallet.chain().relayDustFee())) {
                     if (txout.nValue < 0) {
-                        error = _("The transaction amount is too small to pay the fee");
+                        return _("The transaction amount is too small to pay the fee");
                     } else {
-                        error = _("The transaction amount is too small to send after the fee has been deducted");
+                        return _("The transaction amount is too small to send after the fee has been deducted");
                     }
-                    return std::nullopt;
                 }
             }
             ++i;
@@ -918,35 +908,31 @@ static std::optional<CreatedTransactionResult> CreateTransactionInternal(
 
     // Give up if change keypool ran out and change is required
     if (scriptChange.empty() && nChangePosInOut != -1) {
-        return std::nullopt;
+        return error;
     }
 
     if (sign && !wallet.SignTransaction(txNew)) {
-        error = _("Signing transaction failed");
-        return std::nullopt;
+        return _("Signing transaction failed");
     }
 
     // Return the constructed transaction data.
-    tx = MakeTransactionRef(std::move(txNew));
+    CTransactionRef tx = MakeTransactionRef(std::move(txNew));
 
     // Limit size
     if ((sign && GetTransactionWeight(*tx) > MAX_STANDARD_TX_WEIGHT) ||
         (!sign && tx_sizes.weight > MAX_STANDARD_TX_WEIGHT))
     {
-        error = _("Transaction too large");
-        return std::nullopt;
+        return _("Transaction too large");
     }
 
     if (nFeeRet > wallet.m_default_max_tx_fee) {
-        error = TransactionErrorString(TransactionError::MAX_FEE_EXCEEDED);
-        return std::nullopt;
+        return TransactionErrorString(TransactionError::MAX_FEE_EXCEEDED);
     }
 
     if (gArgs.GetBoolArg("-walletrejectlongchains", DEFAULT_WALLET_REJECT_LONG_CHAINS)) {
         // Lastly, ensure this tx will pass the mempool's chain limits
         if (!wallet.chain().checkChainLimits(tx)) {
-            error = _("Transaction has too long of a mempool chain");
-            return std::nullopt;
+            return _("Transaction has too long of a mempool chain");
         }
     }
 
@@ -965,48 +951,47 @@ static std::optional<CreatedTransactionResult> CreateTransactionInternal(
     return CreatedTransactionResult(tx, nFeeRet, nChangePosInOut, feeCalc);
 }
 
-std::optional<CreatedTransactionResult> CreateTransaction(
+BResult<CreatedTransactionResult> CreateTransaction(
         CWallet& wallet,
         const std::vector<CRecipient>& vecSend,
         int change_pos,
-        bilingual_str& error,
         const CCoinControl& coin_control,
         bool sign)
 {
     if (vecSend.empty()) {
-        error = _("Transaction must have at least one recipient");
-        return std::nullopt;
+        return _("Transaction must have at least one recipient");
     }
 
     if (std::any_of(vecSend.cbegin(), vecSend.cend(), [](const auto& recipient){ return recipient.nAmount < 0; })) {
-        error = _("Transaction amounts must not be negative");
-        return std::nullopt;
+        return _("Transaction amounts must not be negative");
     }
 
     LOCK(wallet.cs_wallet);
 
-    std::optional<CreatedTransactionResult> txr_ungrouped = CreateTransactionInternal(wallet, vecSend, change_pos, error, coin_control, sign);
-    TRACE4(coin_selection, normal_create_tx_internal, wallet.GetName().c_str(), txr_ungrouped.has_value(),
-           txr_ungrouped.has_value() ? txr_ungrouped->fee : 0, txr_ungrouped.has_value() ? txr_ungrouped->change_pos : 0);
-    if (!txr_ungrouped) return std::nullopt;
+    auto res = CreateTransactionInternal(wallet, vecSend, change_pos, coin_control, sign);
+    TRACE4(coin_selection, normal_create_tx_internal, wallet.GetName().c_str(), res.HasRes(),
+           res ? res.GetObj().fee : 0, res ? res.GetObj().change_pos : 0);
+    if (!res) return res;
+    const auto& txr_ungrouped = res.GetObj();
     // try with avoidpartialspends unless it's enabled already
-    if (txr_ungrouped->fee > 0 /* 0 means non-functional fee rate estimation */ && wallet.m_max_aps_fee > -1 && !coin_control.m_avoid_partial_spends) {
+    if (txr_ungrouped.fee > 0 /* 0 means non-functional fee rate estimation */ && wallet.m_max_aps_fee > -1 && !coin_control.m_avoid_partial_spends) {
         TRACE1(coin_selection, attempting_aps_create_tx, wallet.GetName().c_str());
         CCoinControl tmp_cc = coin_control;
         tmp_cc.m_avoid_partial_spends = true;
-        bilingual_str error2; // fired and forgotten; if an error occurs, we discard the results
-        std::optional<CreatedTransactionResult> txr_grouped = CreateTransactionInternal(wallet, vecSend, change_pos, error2, tmp_cc, sign);
+        auto res_tx_grouped = CreateTransactionInternal(wallet, vecSend, change_pos, tmp_cc, sign);
+        // Helper optional class for now
+        std::optional<CreatedTransactionResult> txr_grouped{res_tx_grouped.HasRes() ? std::make_optional(res_tx_grouped.GetObj()) : std::nullopt};
         // if fee of this alternative one is within the range of the max fee, we use this one
-        const bool use_aps{txr_grouped.has_value() ? (txr_grouped->fee <= txr_ungrouped->fee + wallet.m_max_aps_fee) : false};
+        const bool use_aps{txr_grouped.has_value() ? (txr_grouped->fee <= txr_ungrouped.fee + wallet.m_max_aps_fee) : false};
         TRACE5(coin_selection, aps_create_tx_internal, wallet.GetName().c_str(), use_aps, txr_grouped.has_value(),
                txr_grouped.has_value() ? txr_grouped->fee : 0, txr_grouped.has_value() ? txr_grouped->change_pos : 0);
         if (txr_grouped) {
             wallet.WalletLogPrintf("Fee non-grouped = %lld, grouped = %lld, using %s\n",
-                txr_ungrouped->fee, txr_grouped->fee, use_aps ? "grouped" : "non-grouped");
-            if (use_aps) return txr_grouped;
+                txr_ungrouped.fee, txr_grouped->fee, use_aps ? "grouped" : "non-grouped");
+            if (use_aps) return res_tx_grouped;
         }
     }
-    return txr_ungrouped;
+    return res;
 }
 
 bool FundTransaction(CWallet& wallet, CMutableTransaction& tx, CAmount& nFeeRet, int& nChangePosInOut, bilingual_str& error, bool lockUnspents, const std::set<int>& setSubtractFeeFromOutputs, CCoinControl coinControl)
@@ -1042,11 +1027,15 @@ bool FundTransaction(CWallet& wallet, CMutableTransaction& tx, CAmount& nFeeRet,
         }
     }
 
-    std::optional<CreatedTransactionResult> txr = CreateTransaction(wallet, vecSend, nChangePosInOut, error, coinControl, false);
-    if (!txr) return false;
-    CTransactionRef tx_new = txr->tx;
-    nFeeRet = txr->fee;
-    nChangePosInOut = txr->change_pos;
+    auto res = CreateTransaction(wallet, vecSend, nChangePosInOut, coinControl, false);
+    if (!res) {
+        error = res.GetError();
+        return false;
+    }
+    const auto& txr = res.GetObj();
+    CTransactionRef tx_new = txr.tx;
+    nFeeRet = txr.fee;
+    nChangePosInOut = txr.change_pos;
 
     if (nChangePosInOut != -1) {
         tx.vout.insert(tx.vout.begin() + nChangePosInOut, tx_new->vout[nChangePosInOut]);
diff --git a/src/wallet/spend.h b/src/wallet/spend.h
index ecf9b695ee..e9078bd792 100644
--- a/src/wallet/spend.h
+++ b/src/wallet/spend.h
@@ -7,6 +7,7 @@
 
 #include <consensus/amount.h>
 #include <policy/fees.h> // for FeeCalculation
+#include <util/result.h>
 #include <wallet/coinselection.h>
 #include <wallet/transaction.h>
 #include <wallet/wallet.h>
@@ -120,7 +121,7 @@ struct CreatedTransactionResult
  * selected by SelectCoins(); Also create the change output, when needed
  * @note passing change_pos as -1 will result in setting a random position
  */
-std::optional<CreatedTransactionResult> CreateTransaction(CWallet& wallet, const std::vector<CRecipient>& vecSend, int change_pos, bilingual_str& error, const CCoinControl& coin_control, bool sign = true);
+BResult<CreatedTransactionResult> CreateTransaction(CWallet& wallet, const std::vector<CRecipient>& vecSend, int change_pos, const CCoinControl& coin_control, bool sign = true);
 
 /**
  * Insert additional inputs into the transaction by
diff --git a/src/wallet/test/spend_tests.cpp b/src/wallet/test/spend_tests.cpp
index 435c139170..53c3b5d2ae 100644
--- a/src/wallet/test/spend_tests.cpp
+++ b/src/wallet/test/spend_tests.cpp
@@ -28,18 +28,18 @@ BOOST_FIXTURE_TEST_CASE(SubtractFee, TestChain100Setup)
     auto check_tx = [&wallet](CAmount leftover_input_amount) {
         CRecipient recipient{GetScriptForRawPubKey({}), 50 * COIN - leftover_input_amount, true /* subtract fee */};
         constexpr int RANDOM_CHANGE_POSITION = -1;
-        bilingual_str error;
         CCoinControl coin_control;
         coin_control.m_feerate.emplace(10000);
         coin_control.fOverrideFeeRate = true;
         // We need to use a change type with high cost of change so that the leftover amount will be dropped to fee instead of added as a change output
         coin_control.m_change_type = OutputType::LEGACY;
-        std::optional<CreatedTransactionResult> txr = CreateTransaction(*wallet, {recipient}, RANDOM_CHANGE_POSITION, error, coin_control);
-        BOOST_CHECK(txr.has_value());
-        BOOST_CHECK_EQUAL(txr->tx->vout.size(), 1);
-        BOOST_CHECK_EQUAL(txr->tx->vout[0].nValue, recipient.nAmount + leftover_input_amount - txr->fee);
-        BOOST_CHECK_GT(txr->fee, 0);
-        return txr->fee;
+        auto res = CreateTransaction(*wallet, {recipient}, RANDOM_CHANGE_POSITION, coin_control);
+        BOOST_CHECK(res);
+        const auto& txr = res.GetObj();
+        BOOST_CHECK_EQUAL(txr.tx->vout.size(), 1);
+        BOOST_CHECK_EQUAL(txr.tx->vout[0].nValue, recipient.nAmount + leftover_input_amount - txr.fee);
+        BOOST_CHECK_GT(txr.fee, 0);
+        return txr.fee;
     };
 
     // Send full input amount to recipient, check that only nonzero fee is
diff --git a/src/wallet/test/wallet_tests.cpp b/src/wallet/test/wallet_tests.cpp
index 9ed840031d..877de89cc1 100644
--- a/src/wallet/test/wallet_tests.cpp
+++ b/src/wallet/test/wallet_tests.cpp
@@ -519,13 +519,12 @@ public:
     CWalletTx& AddTx(CRecipient recipient)
     {
         CTransactionRef tx;
-        bilingual_str error;
         CCoinControl dummy;
         {
             constexpr int RANDOM_CHANGE_POSITION = -1;
-            std::optional<CreatedTransactionResult> txr = CreateTransaction(*wallet, {recipient}, RANDOM_CHANGE_POSITION, error, dummy);
-            BOOST_CHECK(txr.has_value());
-            tx = txr->tx;
+            auto res = CreateTransaction(*wallet, {recipient}, RANDOM_CHANGE_POSITION, dummy);
+            BOOST_CHECK(res);
+            tx = res.GetObj().tx;
         }
         wallet->CommitTransaction(tx, {}, {});
         CMutableTransaction blocktx;