Routehints get ignored if invoice >10,000 sats

plugins/pay.c

@@ -1881,9 +1881,25 @@ struct payment_modifier *paymod_mods[] = {
 	&local_channel_hints_pay_mod,
 	&exemptfee_pay_mod,
 	&directpay_pay_mod,
-	&presplit_pay_mod,
 	&shadowroute_pay_mod,
+	/* NOTE: The order in which these two paymods are executed is
+	 * significant!
+	 * routehints *must* execute first before presplit.
+	 *
+	 * FIXME: Giving an ordered list of paymods to the paymod
+	 * system is the wrong interface, given that the order in
+	 * which paymods execute is significant.
+	 * (This is typical of Entity-Component-System pattern.)
+	 * What should be done is that libplugin-pay should have a
+	 * canonical list of paymods in the order they execute
+	 * correctly, and whether they are default-enabled/default-disabled,
+	 * and then clients like `pay` and `keysend` will disable/enable
+	 * paymods that do not help them, instead of the current interface
+	 * where clients provide an *ordered* list of paymods they want to
+	 * use.
+	 */
 	&routehints_pay_mod,
+	&presplit_pay_mod,
 	&waitblockheight_pay_mod,
 	&retry_pay_mod,
 	&adaptive_splitter_pay_mod,

tests/test_pay.py

@@ -3221,3 +3221,31 @@ def test_bolt11_null_after_pay(node_factory, bitcoind):
     pays = l2.rpc.listpays()["pays"]
     assert(pays[0]["bolt11"] == invl1)
     assert('amount_msat' in pays[0] and pays[0]['amount_msat'] == amt)
+
+
+def test_mpp_presplit_routehint_conflict(node_factory, bitcoind):
+    '''
+    We had a bug where pre-splitting the payment prevents *any*
+    routehints from being taken.
+    We tickle that bug here by building l1->l2->l3, but with
+    l2->l3 as an unpublished channel.
+    If the payment is large enough to trigger pre-splitting, the
+    routehints are not applied in any of the splits.
+    '''
+    l1, l2, l3 = node_factory.get_nodes(3)
+
+    l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
+    l1l2 = l1.fund_channel(l2, 10**7, announce_channel=True)
+    l2.rpc.connect(l3.info['id'], 'localhost', l3.port)
+    l2.fund_channel(l3, 10**7, announce_channel=False)
+
+    bitcoind.generate_block(6)
+    sync_blockheight(bitcoind, [l1, l2, l3])
+
+    # Wait for l3 to learn about l1->l2, otherwise it will think
+    # l2 is a deadend and not add it to the routehint.
+    wait_for(lambda: len(l3.rpc.listchannels(l1l2)['channels']) >= 2)
+
+    inv = l3.rpc.invoice(Millisatoshi(2 * 10000 * 1000), 'i', 'i', exposeprivatechannels=True)['bolt11']
+
+    l1.rpc.pay(inv)