Merge pull request #6003 from spesmilo/htlc_switch

Htlc switch

electrum/lnchannel.py

@@ -408,7 +408,7 @@ class Channel(Logger):
         self.logger.info("add_htlc")
         return htlc
 
-    def receive_htlc(self, htlc: UpdateAddHtlc) -> UpdateAddHtlc:
+    def receive_htlc(self, htlc: UpdateAddHtlc, onion_packet:bytes = None) -> UpdateAddHtlc:
         """
         ReceiveHTLC adds an HTLC to the state machine's remote update log. This
         method should be called in response to receiving a new HTLC from the remote
@@ -427,6 +427,11 @@ class Channel(Logger):
                     f' HTLC amount: {htlc.amount_msat}')
         with self.db_lock:
             self.hm.recv_htlc(htlc)
+            local_ctn = self.get_latest_ctn(LOCAL)
+            remote_ctn = self.get_latest_ctn(REMOTE)
+            if onion_packet:
+                self.hm.log['unfulfilled_htlcs'][htlc.htlc_id] = local_ctn, remote_ctn, onion_packet.hex(), False
+
         self.logger.info("receive_htlc")
         return htlc
 

electrum/lnhtlc.py

@@ -25,6 +25,7 @@ class HTLCManager:
             log[LOCAL] = deepcopy(initial)
             log[REMOTE] = deepcopy(initial)
             log['unacked_local_updates2'] = {}
+            log['unfulfilled_htlcs'] = {}  # htlc_id -> onion_packet
 
         # maybe bootstrap fee_updates if initial_feerate was provided
         if initial_feerate is not None:

electrum/lnpeer.py

@@ -249,6 +249,7 @@ class Peer(Logger):
     async def main_loop(self):
         async with self.taskgroup as group:
             await group.spawn(self._message_loop())
+            await group.spawn(self.htlc_switch())
             await group.spawn(self.query_gossip())
             await group.spawn(self.process_gossip())
 
@@ -1131,195 +1132,137 @@ class Peer(Logger):
         self.logger.info(f"on_update_add_htlc. chan {chan.short_channel_id}. htlc_id {htlc_id}")
         cltv_expiry = int.from_bytes(payload["cltv_expiry"], 'big')
         amount_msat_htlc = int.from_bytes(payload["amount_msat"], 'big')
-        onion_packet = OnionPacket.from_bytes(payload["onion_routing_packet"])
+        onion_packet = payload["onion_routing_packet"]
-        processed_onion = process_onion_packet(onion_packet, associated_data=payment_hash, our_onion_private_key=self.privkey)
         if chan.get_state() != channel_states.OPEN:
             raise RemoteMisbehaving(f"received update_add_htlc while chan.get_state() != OPEN. state was {chan.get_state()}")
         if cltv_expiry > bitcoin.NLOCKTIME_BLOCKHEIGHT_MAX:
             asyncio.ensure_future(self.lnworker.force_close_channel(channel_id))
             raise RemoteMisbehaving(f"received update_add_htlc with cltv_expiry > BLOCKHEIGHT_MAX. value was {cltv_expiry}")
         # add htlc
-        htlc = UpdateAddHtlc(amount_msat=amount_msat_htlc,
+        htlc = UpdateAddHtlc(
-                             payment_hash=payment_hash,
+            amount_msat=amount_msat_htlc,
-                             cltv_expiry=cltv_expiry,
+            payment_hash=payment_hash,
-                             timestamp=int(time.time()),
+            cltv_expiry=cltv_expiry,
-                             htlc_id=htlc_id)
+            timestamp=int(time.time()),
-        htlc = chan.receive_htlc(htlc)
+            htlc_id=htlc_id)
-        # TODO: fulfilling/failing/forwarding of htlcs should be robust to going offline.
+        chan.receive_htlc(htlc, onion_packet)
-        #       instead of storing state implicitly in coroutines, we could decouple it from receiving the htlc.
+
-        #       maybe persist the required details, and have a long-running task that makes these decisions.
+    def maybe_forward_htlc(self, chan: Channel, htlc: UpdateAddHtlc, *,
-        local_ctn = chan.get_latest_ctn(LOCAL)
+                           onion_packet: OnionPacket, processed_onion: ProcessedOnionPacket):
-        remote_ctn = chan.get_latest_ctn(REMOTE)
-        if processed_onion.are_we_final:
-            asyncio.ensure_future(self._maybe_fulfill_htlc(chan=chan,
-                                                           htlc=htlc,
-                                                           local_ctn=local_ctn,
-                                                           remote_ctn=remote_ctn,
-                                                           onion_packet=onion_packet,
-                                                           processed_onion=processed_onion))
-        else:
-            asyncio.ensure_future(self._maybe_forward_htlc(chan=chan,
-                                                           htlc=htlc,
-                                                           local_ctn=local_ctn,
-                                                           remote_ctn=remote_ctn,
-                                                           onion_packet=onion_packet,
-                                                           processed_onion=processed_onion))
-
-    @log_exceptions
-    async def _maybe_forward_htlc(self, chan: Channel, htlc: UpdateAddHtlc, *, local_ctn: int, remote_ctn: int,
-                                  onion_packet: OnionPacket, processed_onion: ProcessedOnionPacket):
-        await self.await_local(chan, local_ctn)
-        await self.await_remote(chan, remote_ctn)
         # Forward HTLC
-        # FIXME: this is not robust to us going offline before payment is fulfilled
         # FIXME: there are critical safety checks MISSING here
         forwarding_enabled = self.network.config.get('lightning_forward_payments', False)
         if not forwarding_enabled:
             self.logger.info(f"forwarding is disabled. failing htlc.")
-            reason = OnionRoutingFailureMessage(code=OnionFailureCode.PERMANENT_CHANNEL_FAILURE, data=b'')
+            return OnionRoutingFailureMessage(code=OnionFailureCode.PERMANENT_CHANNEL_FAILURE, data=b'')
-            await self.fail_htlc(chan, htlc.htlc_id, onion_packet, reason)
-            return
         dph = processed_onion.hop_data.per_hop
         next_chan = self.lnworker.get_channel_by_short_id(dph.short_channel_id)
         next_chan_scid = dph.short_channel_id
-        next_peer = self.lnworker.peers[next_chan.node_id]
         local_height = self.network.get_local_height()
         if next_chan is None:
             self.logger.info(f"cannot forward htlc. cannot find next_chan {next_chan_scid}")
-            reason = OnionRoutingFailureMessage(code=OnionFailureCode.UNKNOWN_NEXT_PEER, data=b'')
+            return OnionRoutingFailureMessage(code=OnionFailureCode.UNKNOWN_NEXT_PEER, data=b'')
-            await self.fail_htlc(chan, htlc.htlc_id, onion_packet, reason)
-            return
         outgoing_chan_upd = next_chan.get_outgoing_gossip_channel_update()[2:]
         outgoing_chan_upd_len = len(outgoing_chan_upd).to_bytes(2, byteorder="big")
         if not next_chan.can_send_update_add_htlc():
             self.logger.info(f"cannot forward htlc. next_chan {next_chan_scid} cannot send ctx updates. "
                              f"chan state {next_chan.get_state()}, peer state: {next_chan.peer_state}")
-            reason = OnionRoutingFailureMessage(code=OnionFailureCode.TEMPORARY_CHANNEL_FAILURE,
+            data = outgoing_chan_upd_len + outgoing_chan_upd
-                                                data=outgoing_chan_upd_len+outgoing_chan_upd)
+            return OnionRoutingFailureMessage(code=OnionFailureCode.TEMPORARY_CHANNEL_FAILURE, data=data)
-            await self.fail_htlc(chan, htlc.htlc_id, onion_packet, reason)
-            return
         next_cltv_expiry = int.from_bytes(dph.outgoing_cltv_value, 'big')
         if htlc.cltv_expiry - next_cltv_expiry < NBLOCK_OUR_CLTV_EXPIRY_DELTA:
-            reason = OnionRoutingFailureMessage(code=OnionFailureCode.INCORRECT_CLTV_EXPIRY,
+            data = htlc.cltv_expiry.to_bytes(4, byteorder="big") + outgoing_chan_upd_len + outgoing_chan_upd
-                                                data=(htlc.cltv_expiry.to_bytes(4, byteorder="big")
+            return OnionRoutingFailureMessage(code=OnionFailureCode.INCORRECT_CLTV_EXPIRY, data=data)
-                                                      + outgoing_chan_upd_len + outgoing_chan_upd))
-            await self.fail_htlc(chan, htlc.htlc_id, onion_packet, reason)
-            return
         if htlc.cltv_expiry - lnutil.NBLOCK_DEADLINE_BEFORE_EXPIRY_FOR_RECEIVED_HTLCS <= local_height \
                 or next_cltv_expiry <= local_height:
-            reason = OnionRoutingFailureMessage(code=OnionFailureCode.EXPIRY_TOO_SOON,
+            data = outgoing_chan_upd_len + outgoing_chan_upd
-                                                data=outgoing_chan_upd_len+outgoing_chan_upd)
+            return OnionRoutingFailureMessage(code=OnionFailureCode.EXPIRY_TOO_SOON, data=data)
-            await self.fail_htlc(chan, htlc.htlc_id, onion_packet, reason)
-            return
         if max(htlc.cltv_expiry, next_cltv_expiry) > local_height + lnutil.NBLOCK_CLTV_EXPIRY_TOO_FAR_INTO_FUTURE:
-            reason = OnionRoutingFailureMessage(code=OnionFailureCode.EXPIRY_TOO_FAR, data=b'')
+            return OnionRoutingFailureMessage(code=OnionFailureCode.EXPIRY_TOO_FAR, data=b'')
-            await self.fail_htlc(chan, htlc.htlc_id, onion_packet, reason)
-            return
         next_amount_msat_htlc = int.from_bytes(dph.amt_to_forward, 'big')
-        forwarding_fees = fee_for_edge_msat(forwarded_amount_msat=next_amount_msat_htlc,
+        forwarding_fees = fee_for_edge_msat(
-                                            fee_base_msat=lnutil.OUR_FEE_BASE_MSAT,
+            forwarded_amount_msat=next_amount_msat_htlc,
-                                            fee_proportional_millionths=lnutil.OUR_FEE_PROPORTIONAL_MILLIONTHS)
+            fee_base_msat=lnutil.OUR_FEE_BASE_MSAT,
+            fee_proportional_millionths=lnutil.OUR_FEE_PROPORTIONAL_MILLIONTHS)
         if htlc.amount_msat - next_amount_msat_htlc < forwarding_fees:
-            reason = OnionRoutingFailureMessage(code=OnionFailureCode.FEE_INSUFFICIENT,
+            data = next_amount_msat_htlc.to_bytes(8, byteorder="big") + outgoing_chan_upd_len + outgoing_chan_upd
-                                                data=(next_amount_msat_htlc.to_bytes(8, byteorder="big")
+            return OnionRoutingFailureMessage(code=OnionFailureCode.FEE_INSUFFICIENT, data=data)
-                                                      + outgoing_chan_upd_len + outgoing_chan_upd))
-            await self.fail_htlc(chan, htlc.htlc_id, onion_packet, reason)
-            return
-
         self.logger.info(f'forwarding htlc to {next_chan.node_id}')
-        next_htlc = UpdateAddHtlc(amount_msat=next_amount_msat_htlc, payment_hash=htlc.payment_hash, cltv_expiry=next_cltv_expiry, timestamp=int(time.time()))
+        next_htlc = UpdateAddHtlc(
+            amount_msat=next_amount_msat_htlc,
+            payment_hash=htlc.payment_hash,
+            cltv_expiry=next_cltv_expiry,
+            timestamp=int(time.time()))
         next_htlc = next_chan.add_htlc(next_htlc)
-        next_remote_ctn = next_chan.get_latest_ctn(REMOTE)
+        next_peer = self.lnworker.peers[next_chan.node_id]
-        next_peer.send_message(
+        try:
-            "update_add_htlc",
+            next_peer.send_message(
-            channel_id=next_chan.channel_id,
+                "update_add_htlc",
-            id=next_htlc.htlc_id,
+                channel_id=next_chan.channel_id,
-            cltv_expiry=dph.outgoing_cltv_value,
+                id=next_htlc.htlc_id,
-            amount_msat=dph.amt_to_forward,
+                cltv_expiry=dph.outgoing_cltv_value,
-            payment_hash=next_htlc.payment_hash,
+                amount_msat=dph.amt_to_forward,
-            onion_routing_packet=processed_onion.next_packet.to_bytes()
+                payment_hash=next_htlc.payment_hash,
-        )
+                onion_routing_packet=processed_onion.next_packet.to_bytes()
-        await next_peer.await_remote(next_chan, next_remote_ctn)
+            )
-        success, preimage, reason = await self.lnworker.await_payment(next_htlc.payment_hash)
+        except BaseException as e:
-        if success:
+            self.logger.info(f"failed to forward htlc: error sending message. {e}")
-            await self._fulfill_htlc(chan, htlc.htlc_id, preimage)
+            data = outgoing_chan_upd_len + outgoing_chan_upd
-            self.logger.info("htlc forwarded successfully")
+            return OnionRoutingFailureMessage(code=OnionFailureCode.TEMPORARY_CHANNEL_FAILURE, data=data)
-        else:
+        return None
-            # TODO: test this
+
-            self.logger.info(f"forwarded htlc has failed, {reason}")
+    def maybe_fulfill_htlc(self, chan: Channel, htlc: UpdateAddHtlc, *,
-            await self.fail_htlc(chan, htlc.htlc_id, onion_packet, reason)
+                          onion_packet: OnionPacket, processed_onion: ProcessedOnionPacket):
-
-    @log_exceptions
-    async def _maybe_fulfill_htlc(self, chan: Channel, htlc: UpdateAddHtlc, *, local_ctn: int, remote_ctn: int,
-                                  onion_packet: OnionPacket, processed_onion: ProcessedOnionPacket):
-        await self.await_local(chan, local_ctn)
-        await self.await_remote(chan, remote_ctn)
         try:
             info = self.lnworker.get_payment_info(htlc.payment_hash)
             preimage = self.lnworker.get_preimage(htlc.payment_hash)
         except UnknownPaymentHash:
             reason = OnionRoutingFailureMessage(code=OnionFailureCode.INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS, data=b'')
-            await self.fail_htlc(chan, htlc.htlc_id, onion_packet, reason)
+            return False, reason
-            return
         expected_received_msat = int(info.amount * 1000) if info.amount is not None else None
         if expected_received_msat is not None and \
                 not (expected_received_msat <= htlc.amount_msat <= 2 * expected_received_msat):
             reason = OnionRoutingFailureMessage(code=OnionFailureCode.INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS, data=b'')
-            await self.fail_htlc(chan, htlc.htlc_id, onion_packet, reason)
+            return False, reason
-            return
         local_height = self.network.get_local_height()
         if local_height + MIN_FINAL_CLTV_EXPIRY_ACCEPTED > htlc.cltv_expiry:
             reason = OnionRoutingFailureMessage(code=OnionFailureCode.FINAL_EXPIRY_TOO_SOON, data=b'')
-            await self.fail_htlc(chan, htlc.htlc_id, onion_packet, reason)
+            return False, reason
-            return
         cltv_from_onion = int.from_bytes(processed_onion.hop_data.per_hop.outgoing_cltv_value, byteorder="big")
         if cltv_from_onion != htlc.cltv_expiry:
             reason = OnionRoutingFailureMessage(code=OnionFailureCode.FINAL_INCORRECT_CLTV_EXPIRY,
                                                 data=htlc.cltv_expiry.to_bytes(4, byteorder="big"))
-            await self.fail_htlc(chan, htlc.htlc_id, onion_packet, reason)
+            return False, reason
-            return
         amount_from_onion = int.from_bytes(processed_onion.hop_data.per_hop.amt_to_forward, byteorder="big")
         if amount_from_onion > htlc.amount_msat:
             reason = OnionRoutingFailureMessage(code=OnionFailureCode.FINAL_INCORRECT_HTLC_AMOUNT,
                                                 data=htlc.amount_msat.to_bytes(8, byteorder="big"))
-            await self.fail_htlc(chan, htlc.htlc_id, onion_packet, reason)
+            return False, reason
-            return
+        # all good
-        #self.network.trigger_callback('htlc_added', htlc, invoice, RECEIVED)
+        return preimage, None
-        await self.lnworker.enable_htlc_settle.wait()
-        await self._fulfill_htlc(chan, htlc.htlc_id, preimage)
 
-    async def _fulfill_htlc(self, chan: Channel, htlc_id: int, preimage: bytes):
+    def fulfill_htlc(self, chan: Channel, htlc_id: int, preimage: bytes):
         self.logger.info(f"_fulfill_htlc. chan {chan.short_channel_id}. htlc_id {htlc_id}")
-        if not chan.can_send_ctx_updates():
+        assert chan.can_send_ctx_updates(), f"cannot send updates: {chan.short_channel_id}"
-            self.logger.info(f"dropping chan update (fulfill htlc {htlc_id}) for {chan.short_channel_id}. "
-                             f"cannot send updates")
-            return
         chan.settle_htlc(preimage, htlc_id)
         payment_hash = sha256(preimage)
         self.lnworker.payment_received(payment_hash)
-        remote_ctn = chan.get_latest_ctn(REMOTE)
         self.send_message("update_fulfill_htlc",
                           channel_id=chan.channel_id,
                           id=htlc_id,
                           payment_preimage=preimage)
-        await self.await_remote(chan, remote_ctn)
 
-    async def fail_htlc(self, chan: Channel, htlc_id: int, onion_packet: OnionPacket,
+    def fail_htlc(self, chan: Channel, htlc_id: int, onion_packet: OnionPacket,
-                        reason: OnionRoutingFailureMessage):
+                  reason: OnionRoutingFailureMessage):
         self.logger.info(f"fail_htlc. chan {chan.short_channel_id}. htlc_id {htlc_id}. reason: {reason}")
-        if not chan.can_send_ctx_updates():
+        assert chan.can_send_ctx_updates(), f"cannot send updates: {chan.short_channel_id}"
-            self.logger.info(f"dropping chan update (fail htlc {htlc_id}) for {chan.short_channel_id}. "
-                             f"cannot send updates")
-            return
         chan.fail_htlc(htlc_id)
-        remote_ctn = chan.get_latest_ctn(REMOTE)
         error_packet = construct_onion_error(reason, onion_packet, our_onion_private_key=self.privkey)
         self.send_message("update_fail_htlc",
                           channel_id=chan.channel_id,
                           id=htlc_id,
                           len=len(error_packet),
                           reason=error_packet)
-        await self.await_remote(chan, remote_ctn)
 
     def on_revoke_and_ack(self, payload):
         channel_id = payload["channel_id"]
@@ -1484,3 +1427,52 @@ class Peer(Logger):
         # broadcast
         await self.network.try_broadcasting(closing_tx, 'closing')
         return closing_tx.txid()
+
+    async def htlc_switch(self):
+        while True:
+            await asyncio.sleep(0.1)
+            for chan_id, chan in self.channels.items():
+                if not chan.can_send_ctx_updates():
+                    continue
+                self.maybe_send_commitment(chan)
+                done = set()
+                unfulfilled = chan.hm.log.get('unfulfilled_htlcs', {})
+                for htlc_id, (local_ctn, remote_ctn, onion_packet_hex, forwarded) in unfulfilled.items():
+                    if chan.get_oldest_unrevoked_ctn(LOCAL) <= local_ctn:
+                        continue
+                    if chan.get_oldest_unrevoked_ctn(REMOTE) <= remote_ctn:
+                        continue
+                    chan.logger.info(f'found unfulfilled htlc: {htlc_id}')
+                    onion_packet = OnionPacket.from_bytes(bytes.fromhex(onion_packet_hex))
+                    htlc = chan.hm.log[REMOTE]['adds'][htlc_id]
+                    payment_hash = htlc.payment_hash
+                    processed_onion = process_onion_packet(onion_packet, associated_data=payment_hash, our_onion_private_key=self.privkey)
+                    preimage, error = None, None
+                    if processed_onion.are_we_final:
+                        preimage, error = self.maybe_fulfill_htlc(
+                            chan=chan,
+                            htlc=htlc,
+                            onion_packet=onion_packet,
+                            processed_onion=processed_onion)
+                    elif not forwarded:
+                        error = self.maybe_forward_htlc(
+                            chan=chan,
+                            htlc=htlc,
+                            onion_packet=onion_packet,
+                            processed_onion=processed_onion)
+                        if not error:
+                            unfulfilled[htlc_id] = local_ctn, remote_ctn, onion_packet_hex, True
+                    else:
+                        f = self.lnworker.pending_payments[payment_hash]
+                        if f.done():
+                            success, preimage, error = f.result()
+                    if preimage:
+                        await self.lnworker.enable_htlc_settle.wait()
+                        self.fulfill_htlc(chan, htlc.htlc_id, preimage)
+                        done.add(htlc_id)
+                    if error:
+                        self.fail_htlc(chan, htlc.htlc_id, onion_packet, error)
+                        done.add(htlc_id)
+                # cleanup
+                for htlc_id in done:
+                    unfulfilled.pop(htlc_id)

electrum/lnworker.py

@@ -55,7 +55,7 @@ from .lnutil import (Outpoint, LNPeerAddr,
                      ShortChannelID, PaymentAttemptLog, PaymentAttemptFailureDetails)
 from .lnutil import ln_dummy_address, ln_compare_features
 from .transaction import PartialTxOutput, PartialTransaction, PartialTxInput
-from .lnonion import OnionFailureCode
+from .lnonion import OnionFailureCode, process_onion_packet, OnionPacket
 from .lnmsg import decode_msg
 from .i18n import _
 from .lnrouter import RouteEdge, LNPaymentRoute, is_route_sane_to_use

electrum/tests/test_lnpeer.py

@@ -238,7 +238,7 @@ class TestPeer(ElectrumTestCase):
             self.assertEqual(alice_channel.peer_state, peer_states.GOOD)
             self.assertEqual(bob_channel.peer_state, peer_states.GOOD)
             gath.cancel()
-        gath = asyncio.gather(reestablish(), p1._message_loop(), p2._message_loop())
+        gath = asyncio.gather(reestablish(), p1._message_loop(), p2._message_loop(), p1.htlc_switch(), p1.htlc_switch())
         async def f():
             await gath
         with self.assertRaises(concurrent.futures.CancelledError):
@@ -253,7 +253,7 @@ class TestPeer(ElectrumTestCase):
             result = await LNWallet._pay(w1, pay_req)
             self.assertEqual(result, True)
             gath.cancel()
-        gath = asyncio.gather(pay(), p1._message_loop(), p2._message_loop())
+        gath = asyncio.gather(pay(), p1._message_loop(), p2._message_loop(), p1.htlc_switch(), p2.htlc_switch())
         async def f():
             await gath
         with self.assertRaises(concurrent.futures.CancelledError):
@@ -271,7 +271,7 @@ class TestPeer(ElectrumTestCase):
             # wait so that pending messages are processed
             #await asyncio.sleep(1)
             gath.cancel()
-        gath = asyncio.gather(reestablish(), p1._message_loop(), p2._message_loop())
+        gath = asyncio.gather(reestablish(), p1._message_loop(), p2._message_loop(), p1.htlc_switch(), p2.htlc_switch())
         async def f():
             await gath
         with self.assertRaises(concurrent.futures.CancelledError):
@@ -285,7 +285,7 @@ class TestPeer(ElectrumTestCase):
             result = await LNWallet._pay(w1, pay_req)
             self.assertTrue(result)
             gath.cancel()
-        gath = asyncio.gather(pay(), p1._message_loop(), p2._message_loop())
+        gath = asyncio.gather(pay(), p1._message_loop(), p2._message_loop(), p1.htlc_switch(), p2.htlc_switch())
         async def f():
             await gath
         with self.assertRaises(concurrent.futures.CancelledError):
@@ -313,7 +313,7 @@ class TestPeer(ElectrumTestCase):
         async def set_settle():
             await asyncio.sleep(0.1)
             w2.enable_htlc_settle.set()
-        gath = asyncio.gather(pay(), set_settle(), p1._message_loop(), p2._message_loop())
+        gath = asyncio.gather(pay(), set_settle(), p1._message_loop(), p2._message_loop(), p1.htlc_switch(), p2.htlc_switch())
         async def f():
             await gath
         with self.assertRaises(concurrent.futures.CancelledError):
@@ -338,7 +338,7 @@ class TestPeer(ElectrumTestCase):
         # AssertionError is ok since we shouldn't use old routes, and the
         # route finding should fail when channel is closed
         async def f():
-            await asyncio.gather(w1._pay_to_route(route, addr), p1._message_loop(), p2._message_loop())
+            await asyncio.gather(w1._pay_to_route(route, addr), p1._message_loop(), p2._message_loop(), p1.htlc_switch(), p2.htlc_switch())
         with self.assertRaises(PaymentFailure):
             run(f())