register distinction between address and script for SPK type payment identifiers and allow zero amount for

script destinations.

This is mainly to support OP_RETURN outputs, which typically have a zero amount output value,
but as we don't special case OP_RETURN, this is currently done for all non-address scripts

Also, it's probably good to add a warning popup for OP_RETURN outputs with a non-zero output value, but this
would also need special casing for OP_RETURN.

Saving of script output payment identifiers is disabled for now, as reading the script from the stored invoice
back into human-readable form is currently not implemented, and currently only lightning invoices or address output
is supported.

electrum/gui/qt/send_tab.py

@@ -205,9 +205,14 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
 
     def on_amount_changed(self, text):
         # FIXME: implement full valid amount check to enable/disable Pay button
-        pi_valid = self.payto_e.payment_identifier.is_valid() if self.payto_e.payment_identifier else False
-        pi_error = self.payto_e.payment_identifier.is_error() if pi_valid else False
-        self.send_button.setEnabled(bool(self.amount_e.get_amount()) and pi_valid and not pi_error)
+        pi = self.payto_e.payment_identifier
+        if not pi:
+            self.send_button.setEnabled(False)
+            return
+        pi_error = pi.is_error() if pi.is_valid() else False
+        is_spk_script = pi.type == PaymentIdentifierType.SPK and not pi.spk_is_address
+        valid_amount = is_spk_script or bool(self.amount_e.get_amount())
+        self.send_button.setEnabled(pi.is_valid() and not pi_error and valid_amount)
 
     def do_paste(self):
         self.logger.debug('do_paste')
@@ -224,16 +229,20 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
             self.show_error(_('Invalid payment identifier'))
 
     def spend_max(self):
-        if self.payto_e.payment_identifier is None:
+        pi = self.payto_e.payment_identifier
+
+        if pi is None or pi.type == PaymentIdentifierType.UNKNOWN:
             return
 
-        assert self.payto_e.payment_identifier.type in [PaymentIdentifierType.SPK, PaymentIdentifierType.MULTILINE,
-                                                        PaymentIdentifierType.BIP21, PaymentIdentifierType.OPENALIAS]
-        assert not self.payto_e.payment_identifier.is_amount_locked()
+        assert pi.type in [PaymentIdentifierType.SPK, PaymentIdentifierType.MULTILINE,
+                           PaymentIdentifierType.BIP21, PaymentIdentifierType.OPENALIAS]
+
+        if pi.type == PaymentIdentifierType.BIP21:
+            assert 'amount' not in pi.bip21
 
         if run_hook('abort_send', self):
             return
-        outputs = self.payto_e.payment_identifier.get_onchain_outputs('!')
+        outputs = pi.get_onchain_outputs('!')
         if not outputs:
             return
         make_tx = lambda fee_est, *, confirmed_only=False: self.wallet.make_unsigned_transaction(
@@ -446,10 +455,13 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
             self.spend_max()
 
         pi_unusable = pi.is_error() or (not self.wallet.has_lightning() and not pi.is_onchain())
+        is_spk_script = pi.type == PaymentIdentifierType.SPK and not pi.spk_is_address
 
-        self.send_button.setEnabled(not pi_unusable and bool(self.amount_e.get_amount()) and not pi.has_expired())
-        self.save_button.setEnabled(not pi_unusable and pi.type not in [PaymentIdentifierType.LNURLP,
-                                                                        PaymentIdentifierType.LNADDR])
+        amount_valid = is_spk_script or bool(self.amount_e.get_amount())
+
+        self.send_button.setEnabled(not pi_unusable and amount_valid and not pi.has_expired())
+        self.save_button.setEnabled(not pi_unusable and not is_spk_script and \
+                                    pi.type not in [PaymentIdentifierType.LNURLP, PaymentIdentifierType.LNADDR])
 
     def _handle_payment_identifier(self):
         self.update_fields()
@@ -479,11 +491,8 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
     def read_invoice(self) -> Optional[Invoice]:
         if self.check_payto_line_and_show_errors():
             return
-        amount_sat = self.read_amount()
-        if not amount_sat:
-            self.show_error(_('No amount'))
-            return
 
+        amount_sat = self.read_amount()
         invoice = invoice_from_payment_identifier(
             self.payto_e.payment_identifier, self.wallet, amount_sat, self.get_message())
         if not invoice:
@@ -551,15 +560,19 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
     def do_edit_invoice(self, invoice: 'Invoice'):  # FIXME broken
         assert not bool(invoice.get_amount_sat())
         text = invoice.lightning_invoice if invoice.is_lightning() else invoice.get_address()
-        self.payto_e._on_input_btn(text)
+        self.set_payment_identifier(text)
         self.amount_e.setFocus()
         # disable save button, because it would create a new invoice
         self.save_button.setEnabled(False)
 
     def do_pay_invoice(self, invoice: 'Invoice'):
         if not bool(invoice.get_amount_sat()):
-            self.show_error(_('No amount'))
-            return
+            pi = self.payto_e.payment_identifier
+            if pi.type == PaymentIdentifierType.SPK and not pi.spk_is_address:
+                pass
+            else:
+                self.show_error(_('No amount'))
+                return
         if invoice.is_lightning():
             self.pay_lightning_invoice(invoice)
         else: