pytest: test for lack of nagle delay.

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

Author: rustyrussell

tests/test_connection.py

@@ -4822,6 +4822,31 @@ def test_private_channel_no_reconnect(node_factory):
     assert only_one(l1.rpc.listpeers()['peers'])['connected'] is False
 
 
+@pytest.mark.xfail(strict=True)
+@unittest.skipIf(VALGRIND, "We assume machine is reasonably fast")
+def test_no_delay(node_factory):
+    """Is our Nagle disabling for critical messages working?"""
+    l1, l2 = node_factory.line_graph(2)
+
+    scid = only_one(l1.rpc.listpeerchannels()['channels'])['short_channel_id']
+    routestep = {
+        'amount_msat': 100,
+        'id': l2.info['id'],
+        'delay': 5,
+        'channel': scid
+    }
+    start = time.time()
+    # If we were stupid enough to leave Nagle enabled, this would add 200ms
+    # seconds delays each way!
+    for _ in range(100):
+        phash = random.randbytes(32).hex()
+        l1.rpc.sendpay([routestep], phash)
+        with pytest.raises(RpcError, match="WIRE_INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS"):
+            l1.rpc.waitsendpay(phash)
+    end = time.time()
+    assert end < start + 100 * 0.5
+
+
 def test_listpeerchannels_by_scid(node_factory):
     l1, l2, l3 = node_factory.line_graph(3, announce_channels=False)