wallet: anchor reserve: handle wallet.use_change being off

fixes https://github.com/spesmilo/electrum/issues/10231

electrum/wallet.py

@@ -1850,6 +1850,7 @@ class Abstract_Wallet(ABC, Logger, EventListener):
     def get_change_addresses_for_new_transaction(
             self, preferred_change_addr=None, *, allow_reusing_used_change_addrs: bool = True,
     ) -> List[str]:
+        """note: might return an empty list! (e.g. if use_change is disabled, or allow_reuse is False)"""
         change_addrs = []
         if preferred_change_addr:
             if isinstance(preferred_change_addr, (list, tuple)):
@@ -2111,7 +2112,8 @@ class Abstract_Wallet(ABC, Logger, EventListener):
                     to_distribute -= reserve_sized_input.value_sats()
                 else:
                     self.logger.info(f'Adding change output to meet utxo reserve requirements')
-                    change_addr = self.get_change_addresses_for_new_transaction(change_addr)[0]
+                    change_addrs = self.get_change_addresses_for_new_transaction(change_addr)
+                    change_addr = change_addrs[0] if change_addrs else tx_inputs[0].address
                     change = PartialTxOutput.from_address_and_value(change_addr, self.config.LN_UTXO_RESERVE)
                     change.is_utxo_reserve = True  # for GUI
                     outputs.append(change)

tests/test_wallet_vertical.py

@@ -2107,6 +2107,27 @@ class TestWalletSending(ElectrumTestCase):
         tx = make_tx(to_self_address)
         self.assertEqual(1, len(tx.outputs()))
 
+    async def test_ln_reserve__usechange_off(self):
+        """Send all the coins using 'max', with wallet.use_change being off.
+        This will create a reserve UTXO, reusing an input address.
+        """
+        wallet, outgoing_address, to_self_address = self._create_cause_carbon_wallet()
+        wallet.use_change = False
+        def make_tx():
+            outputs = [PartialTxOutput.from_address_and_value(outgoing_address, '!')]
+            wallet.lnworker = mock.Mock()
+            wallet.lnworker.has_anchor_channels.return_value = True
+            return wallet.make_unsigned_transaction(
+                outputs = outputs,
+                fee_policy = FixedFeePolicy(100),
+            )
+        tx = make_tx()
+        self.assertEqual(1, len(tx.inputs()))
+        self.assertEqual(2, len(tx.outputs()))
+        outputs = {txout.address: txout.value for txout in tx.outputs()}
+        assert outgoing_address in outputs
+        assert outputs[tx.inputs()[0].address] == self.config.LN_UTXO_RESERVE  # address-reuse
+
     async def test_ln_reserve_keep_existing_reserve(self):
         """
         tests if make_unsigned_transaction keeps the existing reserve utxo