lnwallet: don't blacklist htlc failures without channel update

diff --git a/electrum/lnworker.py b/electrum/lnworker.py
index cb1efff..8ce68f4 100644
--- a/electrum/lnworker.py
+++ b/electrum/lnworker.py
@@ -2194,13 +2194,18 @@ class LNWallet(Logger):
             raise PaymentFailure(f'payment destination reported error: {failure_msg.code_name()}') from None
 
         # TODO: handle unknown next peer?
-        # handle failure codes that include a channel update
+        # handle failure codes that may include a channel update
         if code in failure_codes:
             offset = failure_codes[code]
             channel_update_len = int.from_bytes(data[offset:offset+2], byteorder="big")
             channel_update_as_received = data[offset+2: offset+2+channel_update_len]
-            payload = self._decode_channel_update_msg(channel_update_as_received)
-            if payload is None:
+            if channel_update_len == 0:
+                # the channel_update became optional
+                # https://github.com/lightning/bolts/blob/93b7ee031b50acd59967a105f1326176a37628f9/04-onion-routing.md?plain=1#L1384-L1389
+                # without an update we cannot correct our local policy for the channel, so we avoid (blacklist) it,
+                # except for liquidity failures, where the liquidity hint suffices to retry
+                blacklist = code != OnionFailureCode.TEMPORARY_CHANNEL_FAILURE
+            elif (payload := self._decode_channel_update_msg(channel_update_as_received)) is None:
                 self.logger.info(f'could not decode channel_update for failed htlc: '
                                  f'{channel_update_as_received.hex()}')
                 blacklist = True
@@ -2211,17 +2216,15 @@ class LNWallet(Logger):
                 # apply the channel update or get blacklisted
                 blacklist, handled = self._handle_chanupd_from_failed_htlc(
                     payload, route=route, sender_idx=sender_idx, failure_msg=failure_msg)
-                # we interpret a temporary channel failure as a liquidity issue
-                # in the channel and update our liquidity hints accordingly
-                if code == OnionFailureCode.TEMPORARY_CHANNEL_FAILURE:
-                    self.network.path_finder.update_liquidity_hints(
-                        route,
-                        amount_msat,
-                        failing_channel=ShortChannelID(failing_channel))
-                # if we can't decide on some action, we are stuck
-                if not (blacklist or handled):
-                    raise PaymentFailure(failure_msg.code_name())
-        # for errors that do not include a channel update
+                assert blacklist or handled, "some action has to be taken on a failure with channel update"
+            # we interpret a temporary channel failure as a liquidity issue
+            # in the channel and update our liquidity hints accordingly
+            if code == OnionFailureCode.TEMPORARY_CHANNEL_FAILURE:
+                self.network.path_finder.update_liquidity_hints(
+                    route,
+                    amount_msat,
+                    failing_channel=ShortChannelID(failing_channel))
+        # for errors that never include a channel update
         else:
             blacklist = True
         if blacklist:
@@ -2271,6 +2274,8 @@ class LNWallet(Logger):
                 handled = True
             else:
                 blacklist = True
+        else:
+            raise Exception(f"unexpected chan upd UpdateStatus: {r}")
         return blacklist, handled
 
     @classmethod
diff --git a/tests/test_lnwallet.py b/tests/test_lnwallet.py
index b08bcf0..1fd7382 100644
--- a/tests/test_lnwallet.py
+++ b/tests/test_lnwallet.py
@@ -555,3 +555,32 @@ class TestLNWallet(ElectrumTestCase):
             route=two_hop_route('carol'), sender_idx=0, amount_msat=amount_msat,
             failure_msg=OnionRoutingFailure(code=OnionFailureCode.FEE_INSUFFICIENT, data=failure_data))
         self.assertTrue(path_finder._is_edge_blacklisted(chan_cd.short_channel_id, now=now))
+
+    async def test_missing_channel_update_from_failed_htlc(self):
+        # the channel_update in UPDATE-type failure messages is optional, nodes
+        # omitting it set the channel_update len field to zero:
+        # https://github.com/lightning/bolts/blob/93b7ee031b50acd59967a105f1326176a37628f9/04-onion-routing.md?plain=1#L1384-L1389
+        # a TEMPORARY_CHANNEL_FAILURE without channel update is a liquidity issue:
+        # we record a liquidity hint and must not blacklist the channel
+        graph = lnhelpers.prepare_chans_and_peers_in_graph(self, lnhelpers._GRAPH_DEFINITIONS['square_graph'])
+        alice_w = graph.workers['alice']
+        path_finder = alice_w.network.path_finder
+        amount_msat = 100_000_000
+        route = [
+            RouteEdge(
+                start_node=graph.workers[a].node_keypair.pubkey,
+                end_node=graph.workers[b].node_keypair.pubkey,
+                short_channel_id=graph.channels[(a, b)][0].short_channel_id,
+                fee_base_msat=0, fee_proportional_millionths=0, cltv_delta=10, node_features=0,
+            ) for a, b in [('alice', 'bob'), ('bob', 'dave')]
+        ]
+        failure_data = (0).to_bytes(2, 'big')  # channel_update len field set to zero
+        alice_w.handle_error_code_from_failed_htlc(
+            route=route, sender_idx=0, amount_msat=amount_msat,
+            failure_msg=OnionRoutingFailure(code=OnionFailureCode.TEMPORARY_CHANNEL_FAILURE, data=failure_data))
+        chan_bd = graph.channels[('bob', 'dave')][0]
+        self.assertFalse(path_finder._is_edge_blacklisted(chan_bd.short_channel_id, now=int(time.time())))
+        hint_bd = path_finder.liquidity_hints.get_hint(chan_bd.short_channel_id)
+        pubkey_b = graph.workers['bob'].node_keypair.pubkey
+        pubkey_d = graph.workers['dave'].node_keypair.pubkey
+        self.assertEqual(amount_msat, hint_bd.cannot_send(pubkey_b < pubkey_d))