pytest: test that we maintain load order unless hook deps require a change.

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

tests/plugins/dep_d.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python3
+from pyln.client import Plugin
+
+"""A simple plugin that must come before dep_e.
+"""
+plugin = Plugin()
+
+
+@plugin.hook('htlc_accepted', before=['dep_e.py'])
+def on_htlc_accepted(htlc, plugin, **kwargs):
+    print("htlc_accepted called")
+    return {'result': 'continue'}
+
+
+plugin.run()

tests/plugins/dep_e.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python3
+from pyln.client import Plugin
+
+"""A simple plugin that registers an htlc_accepted hook..
+"""
+plugin = Plugin()
+
+
+@plugin.hook('htlc_accepted')
+def on_htlc_accepted(htlc, plugin, **kwargs):
+    print("htlc_accepted called")
+    return {'result': 'continue'}
+
+
+plugin.run()

tests/test_plugin.py

@@ -2116,3 +2116,28 @@ def test_hook_dep(node_factory):
     # Complaints about unknown plugin a, but nothing else
     assert l1.daemon.is_in_log("unknown plugin dep_a.py")
     assert not l1.daemon.is_in_log("unknown plugin (?!dep_a.py)")
+
+
+@pytest.mark.xfail(strict=True)
+def test_hook_dep_stable(node_factory):
+    # Load in order A, D, E, B.
+    # A says it has to be before B, D says it has to be before E.
+    # It should load in the order specified.
+
+    dep_a = os.path.join(os.path.dirname(__file__), 'plugins/dep_a.py')
+    dep_b = os.path.join(os.path.dirname(__file__), 'plugins/dep_b.py')
+    dep_d = os.path.join(os.path.dirname(__file__), 'plugins/dep_d.py')
+    dep_e = os.path.join(os.path.dirname(__file__), 'plugins/dep_e.py')
+    l1, l2 = node_factory.line_graph(2, opts=[{},
+                                              {'plugin': [dep_a, dep_d, dep_e, dep_b]}])
+
+    # dep_a mentions deb_c, but nothing else should be unknown.
+    assert l2.daemon.is_in_log("unknown plugin dep_c.py")
+    assert not l2.daemon.is_in_log("unknown plugin (?!|dep_c.py)")
+
+    l1.pay(l2, 100000)
+    # They must be called in this order!
+    l2.daemon.wait_for_log(r"dep_a.py: htlc_accepted called")
+    l2.daemon.wait_for_log(r"dep_d.py: htlc_accepted called")
+    l2.daemon.wait_for_log(r"dep_e.py: htlc_accepted called")
+    l2.daemon.wait_for_log(r"dep_b.py: htlc_accepted called")