payment_identifier: return onchain invoice when GUI requests MAX amount (fixes #8900)

electrum/gui/qt/send_tab.py

@@ -3,7 +3,7 @@
 # file LICENCE or http://www.opensource.org/licenses/mit-license.php
 
 from decimal import Decimal
-from typing import Optional, TYPE_CHECKING, Sequence, List, Callable
+from typing import Optional, TYPE_CHECKING, Sequence, List, Callable, Union
 from PyQt5.QtCore import pyqtSignal, QPoint, QSize, Qt
 from PyQt5.QtWidgets import (QLabel, QVBoxLayout, QGridLayout, QHBoxLayout,
                              QWidget, QToolTip, QPushButton, QApplication)
@@ -600,7 +600,7 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
         else:
             self.pay_onchain_dialog(invoice.outputs, invoice=invoice)
 
-    def read_amount(self) -> List[PartialTxOutput]:
+    def read_amount(self) -> Union[int, str]:
         amount = '!' if self.max_button.isChecked() else self.get_amount()
         return amount
 

electrum/payment_identifier.py

@@ -675,11 +675,13 @@ class PaymentIdentifier(Logger):
 def invoice_from_payment_identifier(
     pi: 'PaymentIdentifier',
     wallet: 'Abstract_Wallet',
-    amount_sat: int,
+    amount_sat: Union[int, str],
     message: str = None
 ) -> Optional[Invoice]:
     assert pi.state in [PaymentIdentifierState.AVAILABLE, PaymentIdentifierState.MERCHANT_NOTIFY]
-    if pi.is_lightning():
+    assert pi.is_onchain() if amount_sat == '!' else True  # MAX should only be allowed if pi has onchain destination
+
+    if pi.is_lightning() and not amount_sat == '!':
         invoice = pi.bolt11
         if not invoice:
             return