qt send: use bolt11 fallback addr even if LN is disabled

Given a wallet with LN disabled,
and a bolt11 invoice (or a bip21 uri that only contains bolt11 but lacks a top-level address),
if the bolt11 invoice includes a fallback address,
we would previously just error "Lightning is disabled".

Now we offer the user to pay on-chain using the fallback address.

closes https://github.com/spesmilo/electrum/issues/8047

electrum/gui/qt/send_tab.py

@@ -483,7 +483,7 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
         self.amount_e.textEdited.emit("")
         self.window.show_send_tab()
 
-    def read_invoice(self):
+    def read_invoice(self) -> Optional[Invoice]:
         if self.check_send_tab_payto_line_and_show_errors():
             return
         try:
@@ -491,9 +491,6 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
                 invoice_str = self.payto_e.lightning_invoice
                 if not invoice_str:
                     return
-                if not self.wallet.has_lightning():
-                    self.show_error(_('Lightning is disabled'))
-                    return
                 invoice = Invoice.from_bech32(invoice_str)
                 if invoice.amount_msat is None:
                     amount_sat = self.amount_e.get_amount()
@@ -502,6 +499,9 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
                     else:
                         self.show_error(_('No amount'))
                         return
+                if not self.wallet.has_lightning() and not invoice.can_be_paid_onchain():
+                    self.show_error(_('Lightning is disabled'))
+                    return
                 return invoice
             else:
                 outputs = self.read_outputs()
@@ -646,19 +646,22 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
 
     def pay_lightning_invoice(self, invoice: Invoice):
         amount_sat = invoice.get_amount_sat()
-        key = invoice.get_id()
         if amount_sat is None:
             raise Exception("missing amount for LN invoice")
-        if not self.wallet.lnworker.can_pay_invoice(invoice):
-            num_sats_can_send = int(self.wallet.lnworker.num_sats_can_send())
-            lightning_needed = amount_sat - num_sats_can_send
-            lightning_needed += (lightning_needed // 20) # operational safety margin
+        # note: lnworker might be None if LN is disabled,
+        #       in which case we should still offer the user to pay onchain.
+        lnworker = self.wallet.lnworker
+        if lnworker is None or not lnworker.can_pay_invoice(invoice):
             coins = self.window.get_coins(nonlocal_only=True)
-            can_pay_onchain = invoice.get_address() and self.wallet.can_pay_onchain(invoice.get_outputs(), coins=coins)
-            can_pay_with_new_channel = self.wallet.lnworker.suggest_funding_amount(amount_sat, coins=coins)
-            can_pay_with_swap = self.wallet.lnworker.suggest_swap_to_send(amount_sat, coins=coins)
-            rebalance_suggestion = self.wallet.lnworker.suggest_rebalance_to_send(amount_sat)
-            can_rebalance = bool(rebalance_suggestion) and self.window.num_tasks() == 0
+            can_pay_onchain = invoice.can_be_paid_onchain() and self.wallet.can_pay_onchain(invoice.get_outputs(), coins=coins)
+            can_pay_with_new_channel = False
+            can_pay_with_swap = False
+            can_rebalance = False
+            if lnworker:
+                can_pay_with_new_channel = lnworker.suggest_funding_amount(amount_sat, coins=coins)
+                can_pay_with_swap = lnworker.suggest_swap_to_send(amount_sat, coins=coins)
+                rebalance_suggestion = lnworker.suggest_rebalance_to_send(amount_sat)
+                can_rebalance = bool(rebalance_suggestion) and self.window.num_tasks() == 0
             choices = {}
             if can_rebalance:
                 msg = ''.join([
@@ -685,7 +688,8 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
                 ])
                 choices[3] = msg
             msg = _('You cannot pay that invoice using Lightning.')
-            if self.wallet.lnworker.channels:
+            if lnworker and lnworker.channels:
+                num_sats_can_send = int(lnworker.num_sats_can_send())
                 msg += '\n' + _('Your channels can send {}.').format(self.format_amount(num_sats_can_send) + ' ' + self.base_unit())
             if not choices:
                 self.window.show_error(msg)
@@ -706,13 +710,14 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
                     self.pay_onchain_dialog(coins, invoice.get_outputs())
             return
 
+        assert lnworker is not None
         # FIXME this is currently lying to user as we truncate to satoshis
         amount_msat = invoice.get_amount_msat()
         msg = _("Pay lightning invoice?") + '\n\n' + _("This will send {}?").format(self.format_amount_and_units(Decimal(amount_msat)/1000))
         if not self.question(msg):
             return
         self.save_pending_invoice()
-        coro = self.wallet.lnworker.pay_invoice(invoice.lightning_invoice, amount_msat=amount_msat)
+        coro = lnworker.pay_invoice(invoice.lightning_invoice, amount_msat=amount_msat)
         self.window.run_coroutine_from_thread(coro, _('Sending payment'))
 
     def broadcast_transaction(self, tx: Transaction):

electrum/invoices.py

@@ -136,6 +136,12 @@ class Invoice(StoredObject):
             outputs = self.outputs
         return outputs
 
+    def can_be_paid_onchain(self) -> bool:
+        if self.is_lightning():
+            return bool(self._lnaddr.get_fallback_address())
+        else:
+            return True
+
     def get_expiration_date(self):
         # 0 means never
         return self.exp + self.time if self.exp else 0