qt: Defer transaction signing until user clicks Send

This fixes issue #30070 where creating unsigned PSBTs from the GUI
would fail because the transaction was already signed during preparation,
causing legacy inputs to have non-empty scriptSig fields.

The fix defers signing until the user explicitly clicks 'Send', allowing
truly unsigned PSBTs to be created while still supporting fee calculation.

Author: 151henry151

src/qt/sendcoinsdialog.cpp

@@ -281,14 +281,16 @@ bool SendCoinsDialog::PrepareSendText(QString& question_string, QString& informa
     }
 
     // prepare transaction for getting txFee earlier
+    // Create unsigned transaction to support creating unsigned PSBTs.
+    // Signing is deferred until the user clicks "Send".
     m_current_transaction = std::make_unique<WalletModelTransaction>(recipients);
     WalletModel::SendCoinsReturn prepareStatus;
 
     updateCoinControlState();
 
     CCoinControl coin_control = *m_coin_control;
     coin_control.m_allow_other_inputs = !coin_control.HasSelected(); // future, could introduce a checkbox to customize this value.
-    prepareStatus = model->prepareTransaction(*m_current_transaction, coin_control);
+    prepareStatus = model->prepareTransaction(*m_current_transaction, coin_control, /*sign=*/false);
 
     // process prepareStatus and on error generate message shown to user
     processSendCoinsReturn(prepareStatus,
@@ -540,6 +542,24 @@ void SendCoinsDialog::sendButtonClicked([[maybe_unused]] bool checked)
                     presentPSBT(psbtx);
                 }
             }
+        } else {
+            // Sign the transaction now that the user has confirmed they want to send.
+            CMutableTransaction mtx = CMutableTransaction{*(m_current_transaction->getWtx())};
+            PartiallySignedTransaction psbtx(mtx);
+            bool complete = false;
+            // Fill and sign the PSBT
+            const auto err{model->wallet().fillPSBT(std::nullopt, /*sign=*/true, /*bip32derivs=*/false, /*n_signed=*/nullptr, psbtx, complete)};
+            if (err || !complete) {
+                Q_EMIT message(tr("Send Coins"), tr("Failed to sign transaction."),
+                    CClientUIInterface::MSG_ERROR);
+                send_failure = true;
+                broadcast = false;
+            } else {
+                // Extract the signed transaction
+                CHECK_NONFATAL(FinalizeAndExtractPSBT(psbtx, mtx));
+                const CTransactionRef tx = MakeTransactionRef(mtx);
+                m_current_transaction->setWtx(tx);
+            }
         }
 
         // Broadcast the transaction, unless an external signer was used and it

src/qt/walletmodel.cpp

@@ -148,7 +148,7 @@ bool WalletModel::validateAddress(const QString& address) const
     return IsValidDestinationString(address.toStdString());
 }
 
-WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransaction &transaction, const CCoinControl& coinControl)
+WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransaction &transaction, const CCoinControl& coinControl, bool sign)
 {
     transaction.getWtx() = nullptr; // reset tx output
 
@@ -203,7 +203,9 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
 
     try {
         auto& newTx = transaction.getWtx();
-        const auto& res = m_wallet->createTransaction(vecSend, coinControl, /*sign=*/!wallet().privateKeysDisabled(), /*change_pos=*/std::nullopt);
+        // Only sign if explicitly requested via the sign parameter (e.g. when user clicks Send).
+        const bool should_sign = sign && !wallet().privateKeysDisabled();
+        const auto& res = m_wallet->createTransaction(vecSend, coinControl, should_sign, /*change_pos=*/std::nullopt);
         if (!res) {
             Q_EMIT message(tr("Send Coins"), QString::fromStdString(util::ErrorString(res).translated),
                            CClientUIInterface::MSG_ERROR);
@@ -217,6 +219,10 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
             transaction.reassignAmounts(static_cast<int>(res->change_pos.value_or(-1)));
         }
 
+        if (!fSubtractFeeFromAmount && (total + nFeeRequired) > nBalance) {
+            return SendCoinsReturn(AmountWithFeeExceedsBalance);
+        }
+
         // 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

@@ -95,7 +95,7 @@ class WalletModel : public QObject
     };
 
     // prepare transaction for getting txfee before sending coins
-    SendCoinsReturn prepareTransaction(WalletModelTransaction &transaction, const wallet::CCoinControl& coinControl);
+    SendCoinsReturn prepareTransaction(WalletModelTransaction &transaction, const wallet::CCoinControl& coinControl, bool sign = false);
 
     // Send coins to a list of recipients
     void sendCoins(WalletModelTransaction& transaction);