pytest: test the reply functionality (via blinded path) using a plugin.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/plugins/onionmessage-reply.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python3
+"""
+This plugin is used to test the `onion_message` hook.
+"""
+from lightning import Plugin
+
+plugin = Plugin()
+
+
+@plugin.hook("onion_message")
+def on_onion_message(plugin, onion_message, **kwargs):
+    if 'reply_path' not in onion_message:
+        plugin.log("no reply path")
+        return
+
+    plugin.rpc.call('sendonionmessage', [onion_message['reply_path']])
+    plugin.log("Sent reply via {}".format(onion_message['reply_path']))
+    return {"result": "continue"}
+
+
+plugin.run()

tests/test_misc.py

@@ -2217,6 +2217,36 @@ def test_sendonionmessage(node_factory):
     assert l3.daemon.wait_for_log('Got onionmsg')
 
 
+@unittest.skipIf(not EXPERIMENTAL_FEATURES, "Needs sendonionmessage")
+def test_sendonionmessage_reply(node_factory):
+    blindedpathtool = os.path.join(os.path.dirname(__file__), "..", "devtools", "blindedpath")
+
+    plugin = os.path.join(os.path.dirname(__file__), "plugins", "onionmessage-reply.py")
+    l1, l2, l3 = node_factory.line_graph(3, opts={'plugin': plugin})
+
+    # Make reply path
+    output = subprocess.check_output(
+        [blindedpathtool, '--simple-output', 'create', l2.info['id'], l1.info['id']]
+    ).decode('ASCII').strip()
+
+    # First line is blinding, then <peerid> then <encblob>.
+    blinding, p1, p1enc, p2 = output.split('\n')
+    # First hop can't be blinded!
+    assert p1 == l2.info['id']
+
+    l1.rpc.call('sendonionmessage',
+                {'hops':
+                 [{'id': l2.info['id']},
+                  {'id': l3.info['id']}],
+                 'reply_path':
+                 {'blinding': blinding,
+                  'path': [{'id': p1, 'enctlv': p1enc}, {'id': p2}]}})
+
+    assert l3.daemon.wait_for_log('Got onionmsg reply_blinding reply_path')
+    assert l3.daemon.wait_for_log('Sent reply via')
+    assert l1.daemon.wait_for_log('Got onionmsg')
+
+
 @unittest.skipIf(not DEVELOPER, "needs --dev-force-privkey")
 def test_getsharedsecret(node_factory):
     """