[qt] Display more helpful message when adding a send address has failed

Addresses bitcoin#12796.

When we're unable to add a sending address to the address book because it
already exists as a receiving address, display a message indicating as much.
This should help avoid confusion about an address supposedly already in the
book but which isn't currently visible in the interface.

src/qt/addresstablemodel.cpp

@@ -407,21 +407,31 @@ bool AddressTableModel::removeRows(int row, int count, const QModelIndex &parent
     return true;
 }
 
-/* Look up label for address in address book, if not found return empty string.
- */
 QString AddressTableModel::labelForAddress(const QString &address) const
 {
-    {
-        CTxDestination destination = DecodeDestination(address.toStdString());
-        std::string name;
-        if (walletModel->wallet().getAddress(destination, &name))
-        {
-            return QString::fromStdString(name);
-        }
+    std::string name;
+    if (getAddressData(address, &name, /* purpose= */ nullptr)) {
+        return QString::fromStdString(name);
+    }
+    return QString();
+}
+
+QString AddressTableModel::purposeForAddress(const QString &address) const
+{
+    std::string purpose;
+    if (getAddressData(address, /* name= */ nullptr, &purpose)) {
+        return QString::fromStdString(purpose);
     }
     return QString();
 }
 
+bool AddressTableModel::getAddressData(const QString &address,
+        std::string* name,
+        std::string* purpose) const {
+    CTxDestination destination = DecodeDestination(address.toStdString());
+    return walletModel->wallet().getAddress(destination, name, /* is_mine= */ nullptr, purpose);
+}
+
 int AddressTableModel::lookupAddress(const QString &address) const
 {
     QModelIndexList lst = match(index(0, Address, QModelIndex()),

src/qt/addresstablemodel.h

@@ -67,10 +67,12 @@ class AddressTableModel : public QAbstractTableModel
      */
     QString addRow(const QString &type, const QString &label, const QString &address, const OutputType address_type);
 
-    /* Look up label for address in address book, if not found return empty string.
-     */
+    /** Look up label for address in address book, if not found return empty string. */
     QString labelForAddress(const QString &address) const;
 
+    /** Look up purpose for address in address book, if not found return empty string. */
+    QString purposeForAddress(const QString &address) const;
+
     /* Look up row index of an address in the model.
        Return -1 if not found.
      */
@@ -86,6 +88,9 @@ class AddressTableModel : public QAbstractTableModel
     QStringList columns;
     EditStatus editStatus;
 
+    /** Look up address book data given an address string. */
+    bool getAddressData(const QString &address, std::string* name, std::string* purpose) const;
+
     /** Notify listeners that data changed. */
     void emitDataChanged(int index);
 

src/qt/editaddressdialog.cpp

@@ -109,7 +109,7 @@ void EditAddressDialog::accept()
             break;
         case AddressTableModel::DUPLICATE_ADDRESS:
             QMessageBox::warning(this, windowTitle(),
-                tr("The entered address \"%1\" is already in the address book.").arg(ui->addressEdit->text()),
+                getDuplicateAddressWarning(),
                 QMessageBox::Ok, QMessageBox::Ok);
             break;
         case AddressTableModel::WALLET_UNLOCK_FAILURE:
@@ -129,6 +129,25 @@ void EditAddressDialog::accept()
     QDialog::accept();
 }
 
+QString EditAddressDialog::getDuplicateAddressWarning() const
+{
+    QString dup_address = ui->addressEdit->text();
+    QString existing_label = model->labelForAddress(dup_address);
+    QString existing_purpose = model->purposeForAddress(dup_address);
+
+    if (existing_purpose == "receive" &&
+            (mode == NewSendingAddress || mode == EditSendingAddress)) {
+        return tr(
+            "Address \"%1\" already exists as a receiving address with label "
+            "\"%2\" and so cannot be added as a sending address."
+            ).arg(dup_address).arg(existing_label);
+    }
+    return tr(
+        "The entered address \"%1\" is already in the address book with "
+        "label \"%2\"."
+        ).arg(dup_address).arg(existing_label);
+}
+
 QString EditAddressDialog::getAddress() const
 {
     return address;

src/qt/editaddressdialog.h

@@ -45,6 +45,9 @@ public Q_SLOTS:
 private:
     bool saveCurrentRow();
 
+    /** Return a descriptive string when adding an already-existing address fails. */
+    QString getDuplicateAddressWarning() const;
+
     Ui::EditAddressDialog *ui;
     QDataWidgetMapper *mapper;
     Mode mode;