Merge bitcoin-core/gui#807: refactor: interfaces, make 'createTransaction' less error-prone

4c0d4f6f93 refactor: interfaces, make 'createTransaction' less error-prone (furszy)
e2c3ec9bf4 refactor: move CreatedTransactionResult to types.h (furszy)
45372175c3 gui: remove AmountWithFeeExceedsBalance error special case (furszy)

Pull request description:

  Bundle all function's outputs inside the `util::Result` returned object.

  Removals:
  - The input-output 'change_pos' ref arg from `createTransaction`, which has been a source of bugs in the past.
  - The 'fee' ref arg from `createTransaction`, which is currently only set when the transaction creation process succeeds.
  - The no longer needed `AmountWithFeeExceedsBalance` error (more info about its re-introduction at [bitcoin#25269](https://github.com/bitcoin/bitcoin/pull/25269) and [bitcoin#34299](https://github.com/bitcoin/bitcoin/pull/34299).

  Additionally, this PR moves the `CreatedTransactionResult` struct into its own file. This change is made to avoid further expanding the GUI dependencies on `wallet.h`. Structurally, the GUI should only access the model/interfaces and never the wallet directly.

ACKs for top commit:
  stratospher:
    ACK 4c0d4f6.
  hebasto:
    ACK 4c0d4f6f93.

Tree-SHA512: 4fc61f08ca2e66e46001defb3a2e852265713e75006c98f0c465bd48afe42e7b0d626d28d578741906fdd26e907d6919f06dc640c55c44efc3dfa766fdbf38a4

src/qt/sendcoinsdialog.cpp

@@ -742,9 +742,6 @@ void SendCoinsDialog::processSendCoinsReturn(const WalletModel::SendCoinsReturn
     case WalletModel::AmountExceedsBalance:
         msgParams.first = tr("The amount exceeds your balance.");
         break;
-    case WalletModel::AmountWithFeeExceedsBalance:
-        msgParams.first = tr("The total exceeds your balance when the %1 transaction fee is included.").arg(msgArg);
-        break;
     case WalletModel::DuplicateAddress:
         msgParams.first = tr("Duplicate address found: addresses should only be used once each.");
         break;

src/qt/walletmodel.cpp

@@ -23,6 +23,7 @@
 #include <psbt.h>
 #include <util/translation.h>
 #include <wallet/coincontrol.h>
+#include <wallet/types.h>
 #include <wallet/wallet.h>
 
 #include <cstdint>
@@ -149,6 +150,8 @@ bool WalletModel::validateAddress(const QString& address) const
 
 WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransaction &transaction, const CCoinControl& coinControl)
 {
+    transaction.getWtx() = nullptr; // reset tx output
+
     CAmount total = 0;
     bool fSubtractFeeFromAmount = false;
     QList<SendCoinsRecipient> recipients = transaction.getRecipients();
@@ -199,27 +202,21 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
     }
 
     try {
-        CAmount nFeeRequired = 0;
-        int nChangePosRet = -1;
-
         auto& newTx = transaction.getWtx();
-        const auto& res = m_wallet->createTransaction(vecSend, coinControl, /*sign=*/!wallet().privateKeysDisabled(), nChangePosRet, nFeeRequired);
-        newTx = res ? *res : nullptr;
-        transaction.setTransactionFee(nFeeRequired);
-        if (fSubtractFeeFromAmount && newTx)
-            transaction.reassignAmounts(nChangePosRet);
-
-        if(!newTx)
-        {
-            if(!fSubtractFeeFromAmount && (total + nFeeRequired) > nBalance)
-            {
-                return SendCoinsReturn(AmountWithFeeExceedsBalance);
-            }
+        const auto& res = m_wallet->createTransaction(vecSend, coinControl, /*sign=*/!wallet().privateKeysDisabled(), /*change_pos=*/std::nullopt);
+        if (!res) {
             Q_EMIT message(tr("Send Coins"), QString::fromStdString(util::ErrorString(res).translated),
-                CClientUIInterface::MSG_ERROR);
+                           CClientUIInterface::MSG_ERROR);
             return TransactionCreationFailed;
         }
 
+        newTx = res->tx;
+        CAmount nFeeRequired = res->fee;
+        transaction.setTransactionFee(nFeeRequired);
+        if (fSubtractFeeFromAmount && newTx) {
+            transaction.reassignAmounts(static_cast<int>(res->change_pos.value_or(-1)));
+        }
+
         // Reject absurdly high fee. (This can never happen because the
         // wallet never creates transactions with fee greater than
         // m_default_max_tx_fee. This merely a belt-and-suspenders check).

src/qt/walletmodel.h

@@ -59,7 +59,6 @@ public:
         InvalidAmount,
         InvalidAddress,
         AmountExceedsBalance,
-        AmountWithFeeExceedsBalance,
         DuplicateAddress,
         TransactionCreationFailed, // Error returned when wallet is still locked
         AbsurdFee