pytest: Test that we don't try to pay too many htlcs at once through an unknown channel.

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

tests/test_xpay.py

@@ -1020,6 +1020,53 @@ def test_xpay_bip353(node_factory):
     l2.rpc.xpay('[email protected]', 100)
 
 
+@pytest.mark.xfail(strict=True)
+def test_xpay_limited_max_accepted_htlcs(node_factory):
+    """xpay should try to reduce flows to 6 if there is an unannounced channel, and only try more if that fails"""
+    CHANNEL_SIZE_SATS = 10**6
+    l1, l2 = node_factory.line_graph(2,
+                                     fundamount=CHANNEL_SIZE_SATS * 20,
+                                     opts=[{}, {'max-concurrent-htlcs': 6}],
+                                     announce_channels=False)
+
+    # We want 10 paths between l3 and l1.
+    l3 = node_factory.get_node()
+    nodes = node_factory.get_nodes(10)
+    for n in nodes:
+        node_factory.join_nodes([l3, n, l1], fundamount=CHANNEL_SIZE_SATS)
+
+    # We don't want to use up capacity, so we make payment fail.
+    inv1 = l1.rpc.invoice(f"{CHANNEL_SIZE_SATS * 5}sat",
+                          'test_xpay_limited_max_accepted_htlcs',
+                          'test_xpay_limited_max_accepted_htlcs')['bolt11']
+    l1.rpc.delinvoice('test_xpay_limited_max_accepted_htlcs', 'unpaid')
+
+    with pytest.raises(RpcError, match="Destination said it doesn't know invoice"):
+        l3.rpc.xpay(inv1)
+
+    # 7 flows.
+    l3.daemon.wait_for_log('Final answer has 7 flows')
+
+    # If we have a routehint, it will squeeze into 6.
+    inv2 = l2.rpc.invoice(f"{CHANNEL_SIZE_SATS * 5}sat",
+                          'test_xpay_limited_max_accepted_htlcs',
+                          'test_xpay_limited_max_accepted_htlcs')['bolt11']
+    l2.rpc.delinvoice('test_xpay_limited_max_accepted_htlcs', 'unpaid')
+    with pytest.raises(RpcError, match="Destination said it doesn't know invoice"):
+        l3.rpc.xpay(inv2)
+
+    # 6 flows.
+    l3.daemon.wait_for_log('Final answer has 6 flows')
+
+    # If we force it, it will use more flows.
+    inv2 = l2.rpc.invoice(f"{CHANNEL_SIZE_SATS * 6}sat",
+                          'test_xpay_limited_max_accepted_htlcs2',
+                          'test_xpay_limited_max_accepted_htlcs2')['bolt11']
+    l2.rpc.delinvoice('test_xpay_limited_max_accepted_htlcs2', 'unpaid')
+    with pytest.raises(RpcError, match="We got temporary_channel_failure"):
+        l3.rpc.xpay(inv2)
+
+
 def test_xpay_blockheight_mismatch(node_factory, bitcoind, executor):
     """We should wait a (reasonable) amount if the final node gives us a blockheight that would explain our failure."""
     l1, l2, l3 = node_factory.line_graph(3, wait_for_announce=True)