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,
+        assert pi.type in [PaymentIdentifierType.SPK, PaymentIdentifierType.MULTILINE,
                            PaymentIdentifierType.BIP21, PaymentIdentifierType.OPENALIAS]
-        assert not self.payto_e.payment_identifier.is_amount_locked()
+
+        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
+
+        amount_valid = is_spk_script or bool(self.amount_e.get_amount())
 
-        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])
+        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:
@@ -551,13 +560,17 @@ 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()):
+            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():

electrum/payment_identifier.py

@@ -120,6 +120,7 @@ class PaymentIdentifier(Logger):
         self.bolt11 = None  # type: Optional[Invoice]
         self.bip21 = None
         self.spk = None
+        self.spk_is_address = False
         #
         self.emaillike = None
         self.domainlike = None
@@ -258,9 +259,11 @@ class PaymentIdentifier(Logger):
                     except InvoiceError as e:
                         self.logger.debug(self._get_error_from_invoiceerror(e))
                 self.set_state(PaymentIdentifierState.AVAILABLE)
-        elif scriptpubkey := self.parse_output(text):
+        elif self.parse_output(text)[0]:
+            scriptpubkey, is_address = self.parse_output(text)
             self._type = PaymentIdentifierType.SPK
             self.spk = scriptpubkey
+            self.spk_is_address = is_address
             self.set_state(PaymentIdentifierState.AVAILABLE)
         elif self.contacts and (contact := self.contacts.by_name(text)):
             if contact['type'] == 'address':
@@ -464,7 +467,8 @@ class PaymentIdentifier(Logger):
             return [PartialTxOutput(scriptpubkey=self.spk, value=amount)]
         elif self.bip21:
             address = self.bip21.get('address')
-            scriptpubkey = self.parse_output(address)
+            scriptpubkey, is_address = self.parse_output(address)
+            assert is_address  # unlikely, but make sure it is an address, not a script
             return [PartialTxOutput(scriptpubkey=scriptpubkey, value=amount)]
         else:
             raise Exception('not onchain')
@@ -499,25 +503,25 @@ class PaymentIdentifier(Logger):
             x, y = line.split(',')
         except ValueError:
             raise Exception("expected two comma-separated values: (address, amount)") from None
-        scriptpubkey = self.parse_output(x)
+        scriptpubkey, is_address = self.parse_output(x)
         if not scriptpubkey:
             raise Exception('Invalid address')
         amount = self.parse_amount(y)
         return PartialTxOutput(scriptpubkey=scriptpubkey, value=amount)
 
-    def parse_output(self, x: str) -> bytes:
+    def parse_output(self, x: str) -> Tuple[bytes, bool]:
         try:
             address = self.parse_address(x)
-            return bytes.fromhex(bitcoin.address_to_script(address))
+            return bytes.fromhex(bitcoin.address_to_script(address)), True
         except Exception as e:
             pass
         try:
             script = self.parse_script(x)
-            return bytes.fromhex(script)
+            return bytes.fromhex(script), False
         except Exception as e:
             pass
 
-        # raise Exception("Invalid address or script.")
+        return None, False
 
     def parse_script(self, x: str):
         script = ''