cpfp: include fee already paid by parent tx in calculation

fixes #5244

electrum/gui/qt/main_window.py

@@ -733,6 +733,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
         return text
 
     def format_fee_rate(self, fee_rate):
+        # fee_rate is in sat/kB
         return format_fee_satoshis(fee_rate/1000, num_zeros=self.num_zeros) + ' sat/byte'
 
     def get_decimal_point(self):
@@ -3273,6 +3274,10 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
 
     def cpfp(self, parent_tx, new_tx):
         total_size = parent_tx.estimated_size() + new_tx.estimated_size()
+        parent_fee = self.wallet.get_tx_fee(parent_tx)
+        if parent_fee is None:
+            self.show_error(_("Can't CPFP: unknown fee for parent transaction."))
+            return
         d = WindowModalDialog(self, _('Child Pays for Parent'))
         vbox = QVBoxLayout(d)
         msg = (
@@ -3297,21 +3302,42 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
         grid.addWidget(output_amount, 2, 1)
         fee_e = BTCAmountEdit(self.get_decimal_point)
         # FIXME with dyn fees, without estimates, there are all kinds of crashes here
-        def f(x):
-            a = max_fee - fee_e.get_amount()
-            output_amount.setText((self.format_amount(a) + ' ' + self.base_unit()) if a else '')
-        fee_e.textChanged.connect(f)
-        fee = self.config.fee_per_kb() * total_size / 1000
+        combined_fee = QLabel('')
+        combined_feerate = QLabel('')
+        def on_fee_edit(x):
+            out_amt = max_fee - fee_e.get_amount()
+            out_amt_str = (self.format_amount(out_amt) + ' ' + self.base_unit()) if out_amt else ''
+            output_amount.setText(out_amt_str)
+            comb_fee = parent_fee + fee_e.get_amount()
+            comb_fee_str = (self.format_amount(comb_fee) + ' ' + self.base_unit()) if comb_fee else ''
+            combined_fee.setText(comb_fee_str)
+            comb_feerate = comb_fee / total_size * 1000
+            comb_feerate_str = self.format_fee_rate(comb_feerate) if comb_feerate else ''
+            combined_feerate.setText(comb_feerate_str)
+        fee_e.textChanged.connect(on_fee_edit)
+        def get_child_fee_from_total_feerate(fee_per_kb):
+            fee = fee_per_kb * total_size / 1000 - parent_fee
+            fee = min(max_fee, fee)
+            fee = max(total_size, fee)  # pay at least 1 sat/byte for combined size
+            return fee
+        suggested_feerate = self.config.fee_per_kb()
+        if suggested_feerate is None:
+            self.show_error(f'''{_("Can't CPFP'")}: {_('Dynamic fee estimates not available')}''')
+            return
+        fee = get_child_fee_from_total_feerate(suggested_feerate)
         fee_e.setAmount(fee)
-        grid.addWidget(QLabel(_('Fee' + ':')), 3, 0)
+        grid.addWidget(QLabel(_('Fee for child') + ':'), 3, 0)
         grid.addWidget(fee_e, 3, 1)
         def on_rate(dyn, pos, fee_rate):
-            fee = fee_rate * total_size / 1000
-            fee = min(max_fee, fee)
+            fee = get_child_fee_from_total_feerate(fee_rate)
             fee_e.setAmount(fee)
         fee_slider = FeeSlider(self, self.config, on_rate)
         fee_slider.update()
         grid.addWidget(fee_slider, 4, 1)
+        grid.addWidget(QLabel(_('Total fee') + ':'), 5, 0)
+        grid.addWidget(combined_fee, 5, 1)
+        grid.addWidget(QLabel(_('Total feerate') + ':'), 6, 0)
+        grid.addWidget(combined_feerate, 6, 1)
         vbox.addLayout(grid)
         vbox.addLayout(Buttons(CancelButton(d), OkButton(d)))
         if not d.exec_():
@@ -3325,7 +3351,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
         self.show_transaction(new_tx)
 
     def bump_fee_dialog(self, tx):
-        is_relevant, is_mine, v, fee = self.wallet.get_wallet_delta(tx)
+        fee = self.wallet.get_tx_fee(tx)
         if fee is None:
             self.show_error(_("Can't bump fee: unknown fee for original transaction."))
             return