submarine_swaps: fix bugs and create method for max_amount_forward_swap

electrum/gui/qt/swap_dialog.py

@@ -138,10 +138,11 @@ class SwapDialog(WindowModalDialog):
         self._update_tx('!')
         if self.tx:
             amount = self.tx.output_value_for_address(ln_dummy_address())
-            max_swap_amt = self.swap_manager.get_max_amount()
-            max_recv_amt_ln = int(self.swap_manager.num_sats_can_receive())
-            max_recv_amt_oc = self.swap_manager.get_send_amount(max_recv_amt_ln, is_reverse=False) or float('inf')
-            max_amt = int(min(max_swap_amt, max_recv_amt_oc))
+            max_amt = self.swap_manager.max_amount_forward_swap()
+            if max_amt is None:
+                self.send_amount_e.setAmount(None)
+                self.max_button.setChecked(False)
+                return
             if amount > max_amt:
                 amount = max_amt
                 self._update_tx(amount)

electrum/submarine_swaps.py

@@ -639,3 +639,12 @@ class SwapManager(Logger):
         sig = bytes.fromhex(tx.sign_txin(0, swap.privkey))
         witness = [sig, preimage, witness_script]
         txin.witness = bytes.fromhex(construct_witness(witness))
+
+    def max_amount_forward_swap(self) -> Optional[int]:
+        """ returns None if we cannot swap """
+        max_swap_amt_ln = self.get_max_amount()
+        max_recv_amt_ln = int(self.num_sats_can_receive())
+        max_amt_ln = int(min(max_swap_amt_ln, max_recv_amt_ln))
+        max_amt_oc = self.get_send_amount(max_amt_ln, is_reverse=False) or 0
+        min_amt_oc = self.get_send_amount(self.min_amount, is_reverse=False) or 0
+        return max_amt_oc if max_amt_oc >= min_amt_oc else None