test_lnwallet: unittest trampoline invoice_feature and r_tag

diff --git a/tests/test_lnwallet.py b/tests/test_lnwallet.py
index f50d0d6..1b7637d 100644
--- a/tests/test_lnwallet.py
+++ b/tests/test_lnwallet.py
@@ -1,9 +1,12 @@
 import logging
 import os
 
+import electrum.trampoline
 from . import ElectrumTestCase
+from .test_lnchannel import create_test_channels
 
-from electrum.lnutil import RECEIVED, MIN_FINAL_CLTV_DELTA_ACCEPTED
+from electrum.lnutil import RECEIVED, MIN_FINAL_CLTV_DELTA_ACCEPTED, LnFeatures
+from electrum.lntransport import LNPeerAddr
 from electrum.logging import console_stderr_handler
 from electrum.invoices import LN_EXPIRY_NEVER, PR_UNPAID
 
@@ -52,3 +55,92 @@ class TestLNWallet(ElectrumTestCase):
                 min_final_cltv_delta=min_final_cltv_delta,
                 exp_delay=exp_delay,
             )
+
+    async def test_trampoline_invoice_features_and_routing_hints(self):
+        """
+        When the invoice_features signal trampoline support, routing hints must only
+        contain trampoline nodes. When it does not, all channel can be added as r_tags.
+        We only signal trampoline support in the invoice if all open channels do support trampoline.
+        """
+        wallet = self.lnwallet_anchors
+        self.assertFalse(wallet.uses_trampoline())
+
+        trampoline_peer = self.create_mock_lnwallet(name='trampoline_peer', has_anchors=True)
+        trampoline_pubkey = trampoline_peer.node_keypair.pubkey
+
+        regular_peer = self.create_mock_lnwallet(name='regular_peer', has_anchors=True)
+        regular_pubkey = regular_peer.node_keypair.pubkey
+
+        chan_t, _ = create_test_channels(alice_lnwallet=wallet, bob_lnwallet=trampoline_peer, anchor_outputs=True)
+        chan_r, _ = create_test_channels(alice_lnwallet=wallet, bob_lnwallet=regular_peer, anchor_outputs=True)
+        wallet._add_channel(chan_t)
+        wallet._add_channel(chan_r)
+
+        # only trampoline_peer is a known trampoline forwarder
+        electrum.trampoline._TRAMPOLINE_NODES_UNITTESTS = {
+            'trampoline_peer': LNPeerAddr(
+                host="127.0.0.1",
+                port=9735,
+                pubkey=trampoline_pubkey,
+            ),
+        }
+        self.addCleanup(lambda: electrum.trampoline._TRAMPOLINE_NODES_UNITTESTS.clear())
+
+        amount_msat = 100_000
+
+        # mixed peers: trampoline feature must be stripped, all peers in hints
+        payment_hash = wallet.create_payment_info(amount_msat=amount_msat)
+        pi = wallet.get_payment_info(payment_hash, direction=RECEIVED)
+        self.assertFalse(
+            pi.invoice_features.supports(LnFeatures.OPTION_TRAMPOLINE_ROUTING_OPT_ELECTRUM),
+            "trampoline bit should be stripped when not all peers are trampoline",
+        )
+
+        lnaddr, _ = wallet.get_bolt11_invoice(payment_info=pi, message='test', fallback_address=None)
+        hint_node_ids = {route[0][0] for route in lnaddr.get_routing_info('r')}
+        self.assertEqual(hint_node_ids, {trampoline_pubkey, regular_pubkey})
+
+        # trampoline feature should not be set if we use trampoline but one peer is not a trampoline
+        old_check, wallet.uses_trampoline = wallet.uses_trampoline, lambda: True
+        self.assertTrue(wallet.uses_trampoline())
+
+        payment_hash = wallet.create_payment_info(amount_msat=amount_msat)
+        pi = wallet.get_payment_info(payment_hash, direction=RECEIVED)
+        self.assertFalse(
+            pi.invoice_features.supports(LnFeatures.OPTION_TRAMPOLINE_ROUTING_OPT_ELECTRUM),
+            "trampoline feature should not be set if we use trampoline but one peer is not a trampoline",
+        )
+
+        wallet.clear_invoices_cache()
+        lnaddr, _ = wallet.get_bolt11_invoice(payment_info=pi, message='test', fallback_address=None)
+        hint_node_ids = {route[0][0] for route in lnaddr.get_routing_info('r')}
+        self.assertEqual(hint_node_ids, {trampoline_pubkey, regular_pubkey})
+
+        wallet.uses_trampoline = old_check
+        self.assertFalse(wallet.uses_trampoline())
+
+        # all peers trampoline: we signal trampoline support, even with trampoline disabled
+        electrum.trampoline._TRAMPOLINE_NODES_UNITTESTS['regular_peer'] = LNPeerAddr(
+            host="127.0.0.1",
+            port=9735,
+            pubkey=regular_pubkey,
+        )
+
+        payment_hash2 = wallet.create_payment_info(amount_msat=amount_msat)
+        pi2 = wallet.get_payment_info(payment_hash2, direction=RECEIVED)
+        self.assertTrue(
+            pi2.invoice_features.supports(LnFeatures.OPTION_TRAMPOLINE_ROUTING_OPT_ELECTRUM),
+            "trampoline bit should be present when all peers are trampoline",
+        )
+
+        wallet.clear_invoices_cache()
+        lnaddr2, _ = wallet.get_bolt11_invoice(payment_info=pi2, message='test', fallback_address=None)
+        hint_node_ids2 = {route[0][0] for route in lnaddr2.get_routing_info('r')}
+        self.assertEqual(hint_node_ids2, {trampoline_pubkey, regular_pubkey})
+
+        # assert only trampoline peers are included in r_tags if the invoice_features signal trampoline
+        del electrum.trampoline._TRAMPOLINE_NODES_UNITTESTS['regular_peer']
+        wallet.clear_invoices_cache()
+        lnaddr3, _ = wallet.get_bolt11_invoice(payment_info=pi2, message='test', fallback_address=None)
+        hint_node_ids3 = {route[0][0] for route in lnaddr3.get_routing_info('r')}
+        self.assertEqual(hint_node_ids3, {trampoline_pubkey})