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

@@ -281,6 +281,8 @@ 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;
 
@@ -500,11 +502,13 @@ void SendCoinsDialog::sendButtonClicked([[maybe_unused]] bool checked)
         return;
     }
 
+    // Create PSBT from the prepared transaction
+    CMutableTransaction mtx = CMutableTransaction{*(m_current_transaction->getWtx())};
+    PartiallySignedTransaction psbtx(mtx);
+
     bool send_failure = false;
     if (retval == QMessageBox::Save) {
         // "Create Unsigned" clicked
-        CMutableTransaction mtx = CMutableTransaction{*(m_current_transaction->getWtx())};
-        PartiallySignedTransaction psbtx(mtx);
         bool complete = false;
         // Fill without signing
         const auto err{model->wallet().fillPSBT(std::nullopt, /*sign=*/false, /*bip32derivs=*/true, /*n_signed=*/nullptr, psbtx, complete)};
@@ -518,8 +522,6 @@ void SendCoinsDialog::sendButtonClicked([[maybe_unused]] bool checked)
         assert(!model->wallet().privateKeysDisabled() || model->wallet().hasExternalSigner());
         bool broadcast = true;
         if (model->wallet().hasExternalSigner()) {
-            CMutableTransaction mtx = CMutableTransaction{*(m_current_transaction->getWtx())};
-            PartiallySignedTransaction psbtx(mtx);
             bool complete = false;
             // Always fill without signing first. This prevents an external signer
             // from being called prematurely and is not expensive.
@@ -540,6 +542,22 @@ void SendCoinsDialog::sendButtonClicked([[maybe_unused]] bool checked)
                     presentPSBT(psbtx);
                 }
             }
+        } else {
+            // Sign the transaction now that the user has confirmed they want to send.
+            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

@@ -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);
+        // Transaction is not signed during preparation.
+        const bool should_sign = false;
+        const auto& res = m_wallet->createTransaction(vecSend, coinControl, should_sign, nChangePosRet, nFeeRequired);
         newTx = res ? *res : nullptr;
         transaction.setTransactionFee(nFeeRequired);
         if (fSubtractFeeFromAmount && newTx)