lnpeer.reestablish_chan: enforce order of replaying commitsig/revack

When replaying messages during channel-reestablishment,
previously we first resent all update messages, along with potential commitment_signed messages,
and then we potentially resent a single revoke_and_ack.

This can result in incorrect behaviour in case both a commitment_signed and a revoke_and_ack needs to be resent.
When replaying messages, the relative order of commitment_signed and revoke_and_messages needs to be preserved.
(the order of updates (htlc/fee) in relation to the revack messages does not matter)

implements https://github.com/lightning/bolts/pull/810

The logic here is somewhat based on what c-lightning does:
01e5f1886e/channeld/channeld.c (L3059)

electrum/lnhtlc.py

@@ -28,6 +28,7 @@ class HTLCManager:
             log[LOCAL] = deepcopy(initial)
             log[REMOTE] = deepcopy(initial)
             log[LOCAL]['unacked_updates'] = {}
+            log[LOCAL]['was_revoke_last'] = False
 
         # maybe bootstrap fee_updates if initial_feerate was provided
         if initial_feerate is not None:
@@ -155,6 +156,7 @@ class HTLCManager:
     def send_ctx(self) -> None:
         assert self.ctn_latest(REMOTE) == self.ctn_oldest_unrevoked(REMOTE), (self.ctn_latest(REMOTE), self.ctn_oldest_unrevoked(REMOTE))
         self._set_revack_pending(REMOTE, True)
+        self.log[LOCAL]['was_revoke_last'] = False
 
     @with_lock
     def recv_ctx(self) -> None:
@@ -165,6 +167,7 @@ class HTLCManager:
     def send_rev(self) -> None:
         self.log[LOCAL]['ctn'] += 1
         self._set_revack_pending(LOCAL, False)
+        self.log[LOCAL]['was_revoke_last'] = True
         # htlcs
         for htlc_id in self._maybe_active_htlc_ids[REMOTE]:
             ctns = self.log[REMOTE]['locked_in'][htlc_id]
@@ -287,6 +290,11 @@ class HTLCManager:
         return {ctn: [bfh(msg) for msg in messages]
                 for ctn, messages in self.log[LOCAL]['unacked_updates'].items()}
 
+    @with_lock
+    def was_revoke_last(self) -> bool:
+        """Whether we sent a revoke_and_ack after the last commitment_signed we sent."""
+        return self.log[LOCAL].get('was_revoke_last') or False
+
     ##### Queries re HTLCs:
 
     def get_htlc_by_id(self, htlc_proposer: HTLCOwner, htlc_id: int) -> UpdateAddHtlc:

electrum/lnpeer.py

@@ -115,6 +115,7 @@ class Peer(Logger):
         self._htlc_switch_iterstart_event = asyncio.Event()
         self._htlc_switch_iterdone_event = asyncio.Event()
         self._received_revack_event = asyncio.Event()
+        self.received_commitsig_event = asyncio.Event()
         self.downstream_htlc_resolved_event = asyncio.Event()
 
     def send_message(self, message_name: str, **kwargs):
@@ -1221,25 +1222,28 @@ class Peer(Logger):
         await fut
         we_must_resend_revoke_and_ack, their_next_local_ctn = fut.result()
 
-        # Replay un-acked local updates (including commitment_signed) byte-for-byte.
-        # If we have sent them a commitment signature that they "lost" (due to disconnect),
-        # we need to make sure we replay the same local updates, as otherwise they could
-        # end up with two (or more) signed valid commitment transactions at the same ctn.
-        # Multiple valid ctxs at the same ctn is a major headache for pre-signing spending txns,
-        # e.g. for watchtowers, hence we must ensure these ctxs coincide.
-        # We replay the local updates even if they were not yet committed.
-        unacked = chan.hm.get_unacked_local_updates()
-        n_replayed_msgs = 0
-        for ctn, messages in unacked.items():
-            if ctn < their_next_local_ctn:
-                # They claim to have received these messages and the corresponding
-                # commitment_signed, hence we must not replay them.
-                continue
-            for raw_upd_msg in messages:
-                self.transport.send_bytes(raw_upd_msg)
-                n_replayed_msgs += 1
-        self.logger.info(f'channel_reestablish ({chan.get_id_for_log()}): replayed {n_replayed_msgs} unacked messages')
-        if we_must_resend_revoke_and_ack:
+        def replay_updates_and_commitsig():
+            # Replay un-acked local updates (including commitment_signed) byte-for-byte.
+            # If we have sent them a commitment signature that they "lost" (due to disconnect),
+            # we need to make sure we replay the same local updates, as otherwise they could
+            # end up with two (or more) signed valid commitment transactions at the same ctn.
+            # Multiple valid ctxs at the same ctn is a major headache for pre-signing spending txns,
+            # e.g. for watchtowers, hence we must ensure these ctxs coincide.
+            # We replay the local updates even if they were not yet committed.
+            unacked = chan.hm.get_unacked_local_updates()
+            replayed_msgs = []
+            for ctn, messages in unacked.items():
+                if ctn < their_next_local_ctn:
+                    # They claim to have received these messages and the corresponding
+                    # commitment_signed, hence we must not replay them.
+                    continue
+                for raw_upd_msg in messages:
+                    self.transport.send_bytes(raw_upd_msg)
+                    replayed_msgs.append(raw_upd_msg)
+            self.logger.info(f'channel_reestablish ({chan.get_id_for_log()}): replayed {len(replayed_msgs)} unacked messages. '
+                             f'{[decode_msg(raw_upd_msg)[0] for raw_upd_msg in replayed_msgs]}')
+
+        def resend_revoke_and_ack():
             last_secret, last_point = chan.get_secret_and_point(LOCAL, oldest_unrevoked_local_ctn - 1)
             next_secret, next_point = chan.get_secret_and_point(LOCAL, oldest_unrevoked_local_ctn + 1)
             self.send_message(
@@ -1247,6 +1251,21 @@ class Peer(Logger):
                 channel_id=chan.channel_id,
                 per_commitment_secret=last_secret,
                 next_per_commitment_point=next_point)
+
+        # We need to preserve relative order of last revack and commitsig.
+        # note: it is not possible to recover and reestablish a channel if we are out-of-sync by
+        # more than one ctns, i.e. we will only ever retransmit up to one commitment_signed message.
+        # Hence, if we need to retransmit a revack, without loss of generality, we can either replay
+        # it as the first message or as the last message.
+        was_revoke_last = chan.hm.was_revoke_last()
+        if we_must_resend_revoke_and_ack and not was_revoke_last:
+            self.logger.info(f'channel_reestablish ({chan.get_id_for_log()}): replaying a revoke_and_ack first.')
+            resend_revoke_and_ack()
+        replay_updates_and_commitsig()
+        if we_must_resend_revoke_and_ack and was_revoke_last:
+            self.logger.info(f'channel_reestablish ({chan.get_id_for_log()}): replaying a revoke_and_ack last.')
+            resend_revoke_and_ack()
+
         chan.peer_state = PeerState.GOOD
         if chan.is_funded() and their_next_local_ctn == next_local_ctn == 1:
             self.send_funding_locked(chan)
@@ -1478,6 +1497,8 @@ class Peer(Logger):
         htlc_sigs = list(chunks(data, 64))
         chan.receive_new_commitment(payload["signature"], htlc_sigs)
         self.send_revoke_and_ack(chan)
+        self.received_commitsig_event.set()
+        self.received_commitsig_event.clear()
 
     def on_update_fulfill_htlc(self, chan: Channel, payload):
         preimage = payload["payment_preimage"]

electrum/tests/test_lnpeer.py

@@ -35,7 +35,7 @@ from electrum import lnmsg
 from electrum.logging import console_stderr_handler, Logger
 from electrum.lnworker import PaymentInfo, RECEIVED
 from electrum.lnonion import OnionFailureCode
-from electrum.lnutil import derive_payment_secret_from_payment_preimage
+from electrum.lnutil import derive_payment_secret_from_payment_preimage, UpdateAddHtlc
 from electrum.lnutil import LOCAL, REMOTE
 from electrum.invoices import PR_PAID, PR_UNPAID
 from electrum.interface import GracefulDisconnect
@@ -256,7 +256,7 @@ class MockLNWallet(Logger, NetworkRetryManager[LNPeerAddr]):
 
 class MockTransport:
     def __init__(self, name):
-        self.queue = asyncio.Queue()
+        self.queue = asyncio.Queue()  # incoming messages
         self._name = name
         self.peer_addr = None
 
@@ -265,7 +265,11 @@ class MockTransport:
 
     async def read_messages(self):
         while True:
-            yield await self.queue.get()
+            data = await self.queue.get()
+            if isinstance(data, asyncio.Event):  # to artificially delay messages
+                await data.wait()
+                continue
+            yield data
 
 class NoFeaturesTransport(MockTransport):
     """
@@ -382,8 +386,14 @@ class TestPeer(TestCaseForTestnet):
 
         super().tearDown()
 
-    def prepare_peers(self, alice_channel: Channel, bob_channel: Channel):
-        k1, k2 = keypair(), keypair()
+    def prepare_peers(
+            self, alice_channel: Channel, bob_channel: Channel,
+            *, k1: Keypair = None, k2: Keypair = None,
+    ):
+        if k1 is None:
+            k1 = keypair()
+        if k2 is None:
+            k2 = keypair()
         alice_channel.node_id = k2.pubkey
         bob_channel.node_id = k1.pubkey
         t1, t2 = transport_pair(k1, k2, alice_channel.name, bob_channel.name)
@@ -557,6 +567,130 @@ class TestPeer(TestCaseForTestnet):
         self.assertEqual(alice_channel_0.peer_state, PeerState.BAD)
         self.assertEqual(bob_channel._state, ChannelState.FORCE_CLOSING)
 
+    @staticmethod
+    def _send_fake_htlc(peer: Peer, chan: Channel) -> UpdateAddHtlc:
+        htlc = UpdateAddHtlc(amount_msat=10000, payment_hash=os.urandom(32), cltv_expiry=999, timestamp=1)
+        htlc = chan.add_htlc(htlc)
+        peer.send_message(
+            "update_add_htlc",
+            channel_id=chan.channel_id,
+            id=htlc.htlc_id,
+            cltv_expiry=htlc.cltv_expiry,
+            amount_msat=htlc.amount_msat,
+            payment_hash=htlc.payment_hash,
+            onion_routing_packet=1366 * b"0",
+        )
+        return htlc
+
+    def test_reestablish_replay_messages_rev_then_sig(self):
+        """
+        See https://github.com/lightning/bolts/pull/810#issue-728299277
+
+        Rev then Sig
+        A            B
+         <---add-----
+         ----add---->
+         <---sig-----
+         ----rev----x
+         ----sig----x
+
+        A needs to retransmit:
+        ----rev-->      (note that 'add' can be first too)
+        ----add-->
+        ----sig-->
+        """
+        chan_AB, chan_BA = create_test_channels()
+        k1, k2 = keypair(), keypair()
+        # note: we don't start peer.htlc_switch() so that the fake htlcs are left alone.
+        async def f():
+            p1, p2, w1, w2, _q1, _q2 = self.prepare_peers(chan_AB, chan_BA, k1=k1, k2=k2)
+            async with OldTaskGroup() as group:
+                await group.spawn(p1._message_loop())
+                await group.spawn(p2._message_loop())
+                await p1.initialized
+                await p2.initialized
+                self._send_fake_htlc(p2, chan_BA)
+                self._send_fake_htlc(p1, chan_AB)
+                p2.transport.queue.put_nowait(asyncio.Event())  # break Bob's incoming pipe
+                self.assertTrue(p2.maybe_send_commitment(chan_BA))
+                await p1.received_commitsig_event.wait()
+                await group.cancel_remaining()
+            # simulating disconnection. recreate transports.
+            p1, p2, w1, w2, _q1, _q2 = self.prepare_peers(chan_AB, chan_BA, k1=k1, k2=k2)
+            for chan in (chan_AB, chan_BA):
+                chan.peer_state = PeerState.DISCONNECTED
+            async with OldTaskGroup() as group:
+                await group.spawn(p1._message_loop())
+                await group.spawn(p2._message_loop())
+                with self.assertLogs('electrum', level='INFO') as logs:
+                    async with OldTaskGroup() as group2:
+                        await group2.spawn(p1.reestablish_channel(chan_AB))
+                        await group2.spawn(p2.reestablish_channel(chan_BA))
+                self.assertTrue(any(("alice->bob" in msg and
+                                     "replaying a revoke_and_ack first" in msg) for msg in logs.output))
+                self.assertTrue(any(("alice->bob" in msg and
+                                     "replayed 2 unacked messages. ['update_add_htlc', 'commitment_signed']" in msg) for msg in logs.output))
+                self.assertEqual(chan_AB.peer_state, PeerState.GOOD)
+                self.assertEqual(chan_BA.peer_state, PeerState.GOOD)
+                raise SuccessfulTest()
+        with self.assertRaises(SuccessfulTest):
+            run(f())
+
+    def test_reestablish_replay_messages_sig_then_rev(self):
+        """
+        See https://github.com/lightning/bolts/pull/810#issue-728299277
+
+        Sig then Rev
+        A            B
+         <---add-----
+         ----add---->
+         ----sig----x
+         <---sig-----
+         ----rev----x
+
+        A needs to retransmit:
+        ----add-->
+        ----sig-->
+        ----rev-->
+        """
+        chan_AB, chan_BA = create_test_channels()
+        k1, k2 = keypair(), keypair()
+        # note: we don't start peer.htlc_switch() so that the fake htlcs are left alone.
+        async def f():
+            p1, p2, w1, w2, _q1, _q2 = self.prepare_peers(chan_AB, chan_BA, k1=k1, k2=k2)
+            async with OldTaskGroup() as group:
+                await group.spawn(p1._message_loop())
+                await group.spawn(p2._message_loop())
+                await p1.initialized
+                await p2.initialized
+                self._send_fake_htlc(p2, chan_BA)
+                self._send_fake_htlc(p1, chan_AB)
+                p2.transport.queue.put_nowait(asyncio.Event())  # break Bob's incoming pipe
+                self.assertTrue(p1.maybe_send_commitment(chan_AB))
+                self.assertTrue(p2.maybe_send_commitment(chan_BA))
+                await p1.received_commitsig_event.wait()
+                await group.cancel_remaining()
+            # simulating disconnection. recreate transports.
+            p1, p2, w1, w2, _q1, _q2 = self.prepare_peers(chan_AB, chan_BA, k1=k1, k2=k2)
+            for chan in (chan_AB, chan_BA):
+                chan.peer_state = PeerState.DISCONNECTED
+            async with OldTaskGroup() as group:
+                await group.spawn(p1._message_loop())
+                await group.spawn(p2._message_loop())
+                with self.assertLogs('electrum', level='INFO') as logs:
+                    async with OldTaskGroup() as group2:
+                        await group2.spawn(p1.reestablish_channel(chan_AB))
+                        await group2.spawn(p2.reestablish_channel(chan_BA))
+                self.assertTrue(any(("alice->bob" in msg and
+                                     "replaying a revoke_and_ack last" in msg) for msg in logs.output))
+                self.assertTrue(any(("alice->bob" in msg and
+                                     "replayed 2 unacked messages. ['update_add_htlc', 'commitment_signed']" in msg) for msg in logs.output))
+                self.assertEqual(chan_AB.peer_state, PeerState.GOOD)
+                self.assertEqual(chan_BA.peer_state, PeerState.GOOD)
+                raise SuccessfulTest()
+        with self.assertRaises(SuccessfulTest):
+            run(f())
+
     @needs_test_with_all_chacha20_implementations
     def test_payment(self):
         """Alice pays Bob a single HTLC via direct channel."""