gui: enable Send for external signer

Before this change the send confirmation dialog would keep the Send option disabled. The Create Unsigned choice would actually send. This is potentially confusing.

With this change the Create Unsigned button will not attempt to sign and always produce a PSBT. The Send button will attempt to sign, and only return a PSBT if more signatures are needed.

When using an external signer, the Create Unsigned option only appears when PSBT controls are enabled in the wallet settings.

Closes #551

src/qt/sendcoinsdialog.cpp

@@ -490,7 +490,7 @@ void SendCoinsDialog::sendButtonClicked([[maybe_unused]] bool checked)
     assert(m_current_transaction);
 
     const QString confirmation = tr("Confirm send coins");
-    auto confirmationDialog = new SendConfirmationDialog(confirmation, question_string, informative_text, detailed_text, SEND_CONFIRM_DELAY, !model->wallet().privateKeysDisabled(), model->getOptionsModel()->getEnablePSBTControls(), this);
+    auto confirmationDialog = new SendConfirmationDialog(confirmation, question_string, informative_text, detailed_text, SEND_CONFIRM_DELAY, /*enable_send=*/!model->wallet().privateKeysDisabled() || model->wallet().hasExternalSigner(), /*always_show_unsigned=*/model->getOptionsModel()->getEnablePSBTControls(), this);
     confirmationDialog->setAttribute(Qt::WA_DeleteOnClose);
     // TODO: Replace QDialog::exec() with safer QDialog::show().
     const auto retval = static_cast<QMessageBox::StandardButton>(confirmationDialog->exec());
@@ -502,19 +502,32 @@ void SendCoinsDialog::sendButtonClicked([[maybe_unused]] bool checked)
     }
 
     bool send_failure = false;
-    if (retval == QMessageBox::Save) {
+    if (retval == QMessageBox::Save) { // Create Unsigned clicked
         CMutableTransaction mtx = CMutableTransaction{*(m_current_transaction->getWtx())};
         PartiallySignedTransaction psbtx(mtx);
+        // Copy PSBT to clipboard and offer to save
+        presentPSBT(psbtx);
+    } else { // Send clicked
+        assert(!model->wallet().privateKeysDisabled() || model->wallet().hasExternalSigner());
         bool complete = false;
         if (model->wallet().hasExternalSigner()) {
+            CMutableTransaction mtx = CMutableTransaction{*(m_current_transaction->getWtx())};
+            PartiallySignedTransaction psbtx(mtx);
             send_failure = !signWithExternalSigner(psbtx, mtx, complete);
+            // A transaction signed with an external signer is not always complete,
+            // e.g. in a multisig wallet.
+            if (complete) {
+                // Prepare transaction for broadcast transaction if complete
+                const CTransactionRef tx = MakeTransactionRef(mtx);
+                m_current_transaction->setWtx(tx);
+            } else if (!send_failure) {
+                presentPSBT(psbtx);
+            }
         }
 
-        // Broadcast transaction if complete (even with an external signer this
-        // is not always the case, e.g. in a multisig wallet).
-        if (complete) {
-            const CTransactionRef tx = MakeTransactionRef(mtx);
-            m_current_transaction->setWtx(tx);
+        // Broadcast the transaction, unless an external was used and it needs more signatures.
+        if (!model->wallet().hasExternalSigner() || complete) {
+            // now send the prepared transaction
             WalletModel::SendCoinsReturn sendStatus = model->sendCoins(*m_current_transaction);
             // process sendStatus and on error generate message shown to user
             processSendCoinsReturn(sendStatus);
@@ -524,23 +537,6 @@ void SendCoinsDialog::sendButtonClicked([[maybe_unused]] bool checked)
             } else {
                 send_failure = true;
             }
-            return;
-        }
-
-        // Copy PSBT to clipboard and offer to save
-        assert(!complete);
-        presentPSBT(psbtx);
-    } else {
-        assert(!model->wallet().privateKeysDisabled());
-        // now send the prepared transaction
-        WalletModel::SendCoinsReturn sendStatus = model->sendCoins(*m_current_transaction);
-        // process sendStatus and on error generate message shown to user
-        processSendCoinsReturn(sendStatus);
-
-        if (sendStatus.status == WalletModel::OK) {
-            Q_EMIT coinsSent(m_current_transaction->getWtx()->GetHash());
-        } else {
-            send_failure = true;
         }
     }
     if (!send_failure) {

src/qt/sendcoinsdialog.h

@@ -128,6 +128,8 @@ class SendConfirmationDialog : public QMessageBox
 
 public:
     SendConfirmationDialog(const QString& title, const QString& text, const QString& informative_text = "", const QString& detailed_text = "", int secDelay = SEND_CONFIRM_DELAY, bool enable_send = true, bool always_show_unsigned = true, QWidget* parent = nullptr);
+    /* Returns QMessageBox::Cancel, QMessageBox::Yes when send is
+       clicked and QMessageBox::Save when Create Unsigned is clicked. */
     int exec() override;
 
 private Q_SLOTS: