qt swap_dialog: "max" button now respects max htlc value

electrum/gui/qt/swap_dialog.py

@@ -1,10 +1,11 @@
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Optional
 
 from PyQt5.QtWidgets import QLabel, QVBoxLayout, QGridLayout, QPushButton
 
 from electrum.i18n import _
 from electrum.lnutil import ln_dummy_address
-from electrum.transaction import PartialTxOutput
+from electrum.transaction import PartialTxOutput, PartialTransaction
+from electrum.submarine_swaps import SWAP_MAX_VALUE_SAT
 
 from .util import (WindowModalDialog, Buttons, OkButton, CancelButton,
                    EnterButton, ColorScheme, WWLabel, read_QIcon)
@@ -24,6 +25,8 @@ Do you want to continue?
 
 class SwapDialog(WindowModalDialog):
 
+    tx: Optional[PartialTransaction]
+
     def __init__(self, window: 'ElectrumWindow'):
         WindowModalDialog.__init__(self, window, _('Submarine Swap'))
         self.window = window
@@ -31,6 +34,7 @@ class SwapDialog(WindowModalDialog):
         self.lnworker = self.window.wallet.lnworker
         self.swap_manager = self.lnworker.swap_manager
         self.network = window.network
+        self.tx = None
         vbox = QVBoxLayout(self)
         vbox.addWidget(WWLabel('Swap lightning funds for on-chain funds if you need to increase your receiving capacity. This service is powered by the Boltz backend.'))
         self.send_amount_e = BTCAmountEdit(self.window.get_decimal_point)
@@ -99,15 +103,24 @@ class SwapDialog(WindowModalDialog):
         if self.is_reverse:
             return
         if self.max_button.isChecked():
-            self.update_tx('!')
+            self._spend_max_forward_swap()
-            if self.tx:
-                txo = self.tx.outputs()[0]
-                self.send_amount_e.setAmount(txo.value)
         else:
             self.tx = None
             self.send_amount_e.setAmount(None)
             self.update_fee()
 
+    def _spend_max_forward_swap(self):
+        self.update_tx('!')
+        if self.tx:
+            amount = self.tx.output_value_for_address(ln_dummy_address())
+            if amount > SWAP_MAX_VALUE_SAT:
+                amount = SWAP_MAX_VALUE_SAT
+                self.update_tx(amount)
+            if self.tx:
+                amount = self.tx.output_value_for_address(ln_dummy_address())
+                assert amount <= SWAP_MAX_VALUE_SAT
+                self.send_amount_e.setAmount(amount)
+
     def on_send_edited(self):
         if self.send_amount_e.follows:
             return
@@ -154,12 +167,12 @@ class SwapDialog(WindowModalDialog):
             fee = sm.get_claim_fee()
         else:
             is_max = self.max_button.isChecked()
-            onchain_amount = '!' if is_max else self.send_amount_e.get_amount()
+            if is_max:
+                self._spend_max_forward_swap()
+            else:
+                onchain_amount = self.send_amount_e.get_amount()
                 self.update_tx(onchain_amount)
             fee = self.tx.get_fee() if self.tx else None
-            if is_max and self.tx:
-                txo = self.tx.outputs()[0]
-                self.send_amount_e.setAmount(txo.value)
         fee_text = self.window.format_amount(fee) + ' ' + self.window.base_unit() if fee else ''
         self.fee_label.setText(fee_text)
 

electrum/submarine_swaps.py

@@ -11,7 +11,7 @@ from .bitcoin import address_to_script, script_to_p2wsh, redeem_script_to_addres
 from .transaction import TxOutpoint, PartialTxInput, PartialTxOutput, PartialTransaction, construct_witness
 from .transaction import script_GetOp, match_script_against_template, OPPushDataGeneric, OPPushDataPubkey
 from .util import log_exceptions
-from .lnutil import REDEEM_AFTER_DOUBLE_SPENT_DELAY, ln_dummy_address
+from .lnutil import REDEEM_AFTER_DOUBLE_SPENT_DELAY, ln_dummy_address, LN_MAX_HTLC_VALUE_MSAT
 from .bitcoin import dust_threshold
 from .logging import Logger
 from .lnutil import hex_to_bytes
@@ -25,6 +25,9 @@ if TYPE_CHECKING:
 API_URL = 'https://lightning.electrum.org/api'
 
 
+SWAP_MAX_VALUE_SAT = LN_MAX_HTLC_VALUE_MSAT // 1000
+
+
 WITNESS_TEMPLATE_SWAP = [
     opcodes.OP_HASH160,
     OPPushDataGeneric(lambda x: x == 20),