qt: Defer transaction signing until user clicks Send

Fixes #30070

When creating an unsigned PSBT from the GUI, the transaction was already signed during preparation, causing legacy inputs to have non-empty scriptSig fields. The PSBT parser then rejects them.

This defers signing until the user clicks "Send" instead of signing during preparation. Fee calculation still works since transactions can be created without signing.

Follows the approach suggested by @achow101 in the issue comments.

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

@@ -147,7 +147,7 @@ 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)
 {
     CAmount total = 0;
     bool fSubtractFeeFromAmount = false;
@@ -203,7 +203,9 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
         int nChangePosRet = -1;
 
         auto& newTx = transaction.getWtx();
-        const auto& res = m_wallet->createTransaction(vecSend, coinControl, /*sign=*/!wallet().privateKeysDisabled(), nChangePosRet, nFeeRequired);
+        // Only sign if explicitly requested via the sign parameter.
+        const bool should_sign = sign && !wallet().privateKeysDisabled();
+        const auto& res = m_wallet->createTransaction(vecSend, coinControl, should_sign, nChangePosRet, nFeeRequired);
         newTx = res ? *res : nullptr;
         transaction.setTransactionFee(nFeeRequired);
         if (fSubtractFeeFromAmount && newTx)

src/qt/walletmodel.h

@@ -96,7 +96,7 @@ 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);