tests: lnpeer: test_dont_settle_htlc_receiver_and_forwarder

diff --git a/tests/test_lnpeer.py b/tests/test_lnpeer.py
index 941a4a1..344c268 100644
--- a/tests/test_lnpeer.py
+++ b/tests/test_lnpeer.py
@@ -33,7 +33,7 @@ from electrum.util import NetworkRetryManager, bfh, OldTaskGroup, EventListener,
 from electrum.lnpeer import Peer
 from electrum.lntransport import LNPeerAddr
 from electrum.crypto import privkey_to_pubkey
-from electrum.lnutil import Keypair, PaymentFailure, LnFeatures, HTLCOwner, PaymentFeeBudget
+from electrum.lnutil import Keypair, PaymentFailure, LnFeatures, HTLCOwner, PaymentFeeBudget, RECEIVED
 from electrum.lnchannel import ChannelState, PeerState, Channel
 from electrum.lnrouter import LNPathFinder, PathEdge, LNPathInconsistent
 from electrum.channel_db import ChannelDB
@@ -41,7 +41,7 @@ from electrum.lnworker import LNWallet, NoPathFound, SentHtlcInfo, PaySession
 from electrum.lnmsg import encode_msg, decode_msg
 from electrum import lnmsg
 from electrum.logging import console_stderr_handler, Logger
-from electrum.lnworker import PaymentInfo, RECEIVED
+from electrum.lnworker import PaymentInfo
 from electrum.lnonion import OnionFailureCode, OnionRoutingFailure, OnionHopsDataSingle, OnionPacket
 from electrum.lnutil import LOCAL, REMOTE, UpdateAddHtlc, RecvMPPResolution
 from electrum.invoices import PR_PAID, PR_UNPAID, Invoice, LN_EXPIRY_NEVER
@@ -2949,6 +2949,79 @@ class TestPeerForwarding(TestPeer):
                 any('bob->carol' in msg and 'on_update_fail_malformed_htlc' in msg for msg in logs.output)
             )
 
+    async def test_dont_settle_htlcs_receiver_and_forwarder(self):
+        """
+        Test that the receiver and forwarder doesn't settle htlcs once they get the preimage if the payment
+        hash is in LNWallet.dont_settle_htlcs. E.g. the forwarder could be a just-in-time channel provider.
+        Alice -> Bob -> Carol. Carol and Bob shouldn't release the preimage.
+        """
+        async def run_test(test_trampoline):
+            graph = self.prepare_chans_and_peers_in_graph(self.GRAPH_DEFINITIONS['line_graph'])
+            peers = graph.peers.values()
+
+            if test_trampoline:
+                electrum.trampoline._TRAMPOLINE_NODES_UNITTESTS = {
+                    graph.workers['bob'].name: LNPeerAddr(host="127.0.0.1", port=9735, pubkey=graph.workers['bob'].node_keypair.pubkey),
+                }
+                await self._activate_trampoline(graph.workers['carol'])
+                await self._activate_trampoline(graph.workers['alice'])
+
+            lnaddr, pay_req = self.prepare_invoice(graph.workers['carol'], include_routing_hints=True)
+            # test both receiver (carol) and forwarder (bob)
+            graph.workers['bob'].dont_settle_htlcs[lnaddr.paymenthash.hex()] = None
+            graph.workers['carol'].dont_settle_htlcs[lnaddr.paymenthash.hex()] = None
+
+            payment_successful = asyncio.Event()
+            async def pay():
+                self.assertEqual(PR_UNPAID, graph.workers['carol'].get_payment_status(lnaddr.paymenthash, direction=RECEIVED))
+                result, log = await graph.workers['alice'].pay_invoice(pay_req)
+                self.assertEqual(PR_PAID, graph.workers['carol'].get_payment_status(lnaddr.paymenthash, direction=RECEIVED))
+                self.assertTrue(result)
+                payment_successful.set()
+
+            async def check_doesnt_settle():
+                while not graph.workers['carol'].received_mpp_htlcs:
+                    await asyncio.sleep(0.1)  # wait until carol received the htlcs
+
+                await asyncio.sleep(0.2)  # give carol time to accidentally release the preimage
+                self.assertEqual(PR_UNPAID, graph.workers['carol'].get_payment_status(lnaddr.paymenthash, direction=RECEIVED))
+                self.assertIsNone(graph.workers['bob'].get_preimage(lnaddr.paymenthash), "bob got preimage from carol")
+                # now allow carol to release the preimage to bob
+                del graph.workers['carol'].dont_settle_htlcs[lnaddr.paymenthash.hex()]
+
+                # wait for carol to release the preimage to bob
+                while not graph.workers['bob'].get_preimage(lnaddr.paymenthash):
+                    await asyncio.sleep(0.1)
+
+                # give bob some time to settle the htlcs to alice (this would complete the payment)
+                await asyncio.sleep(0.2)
+                self.assertIsNone(graph.workers['alice'].get_preimage(lnaddr.paymenthash), "alice got preimage from bob")
+                self.assertFalse(payment_successful.is_set(), "bob released preimage")
+
+                # now allow bob to settle the htlcs
+                del graph.workers['bob'].dont_settle_htlcs[lnaddr.paymenthash.hex()]
+                await payment_successful.wait()
+                raise PaymentDone()
+
+            async def f():
+                async with OldTaskGroup() as group:
+                    for peer in peers:
+                        await group.spawn(peer._message_loop())
+                        await group.spawn(peer.htlc_switch())
+                    for peer in peers:
+                        await peer.initialized
+
+                    await group.spawn(pay())
+                    await group.spawn(check_doesnt_settle())
+                    # stop the taskgroup if anything takes too long
+                    await group.spawn(asyncio.wait_for(asyncio.sleep(4), timeout=3))
+
+            await f()
+
+        for trampoline in (False, True):
+            with self.assertRaises(PaymentDone):
+                await run_test(trampoline)
+
 
 class TestPeerDirectAnchors(TestPeerDirect):
     TEST_ANCHOR_CHANNELS = True