pytest: Add test for failcode conversion

We added a conversion of failcodes that do not have sufficient information in
faac4b2. That means that a failcode that'd require additional information
in order to be a correct error to return in an onion is mapped to a generic
one since we can't backfill the information.

This tests that the mapping is performed correctly and replicates the
situation in #4070

tests/plugins/htlc_accepted-failcode.py

@@ -0,0 +1,38 @@
+#!/usr/bin/env python3
+"""A simply plugin that fails HTLCs with a configurable failcode.
+
+"""
+from pyln.client import Plugin
+
+
+plugin = Plugin()
+
+
+@plugin.hook('htlc_accepted')
+def on_htlc_accepted(htlc, onion, plugin, **kwargs):
+    res = {"result": "fail"}
+
+    if plugin.failmsg is not None:
+        res['failure_message'] = plugin.failmsg
+
+    if plugin.failcode is not None:
+        res['failure_code'] = plugin.failcode
+
+    return res
+
+
+@plugin.method('setfailcode')
+def setfailcode(plugin, code=None, msg=None):
+    """Sets the failcode to return when receiving an incoming HTLC.
+    """
+    plugin.failcode = code
+    plugin.failmsg = msg
+
+
+@plugin.init()
+def on_init(**kwargs):
+    plugin.failcode = None
+    plugin.failmsg = None
+
+
+plugin.run()

tests/test_plugin.py

@@ -1889,3 +1889,42 @@ def test_htlc_accepted_hook_crash(node_factory, executor):
 
     with pytest.raises(RpcError, match=r'failed: WIRE_TEMPORARY_NODE_FAILURE'):
         f.result(10)
+
+
+def test_htlc_accepted_hook_failcodes(node_factory):
+    plugin = os.path.join(os.path.dirname(__file__), 'plugins/htlc_accepted-failcode.py')
+    l1, l2 = node_factory.line_graph(2, opts=[{}, {'plugin': plugin}])
+
+    # First let's test the newer failure_message, which should get passed
+    # through without being mapped.
+    tests = {
+        '2002': 'WIRE_TEMPORARY_NODE_FAILURE',
+        '400F' + 12 * '00': 'WIRE_INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS',
+        '4009': 'WIRE_REQUIRED_CHANNEL_FEATURE_MISSING',
+        '4016' + 3 * '00': 'WIRE_INVALID_ONION_PAYLOAD',
+    }
+
+    for failmsg, expected in tests.items():
+        l2.rpc.setfailcode(msg=failmsg)
+        inv = l2.rpc.invoice(42, 'failmsg{}'.format(failmsg), '')['bolt11']
+        with pytest.raises(RpcError, match=r'failcodename.: .{}.'.format(expected)):
+            l1.rpc.pay(inv)
+
+    # And now test the older failcode return value. This is deprecated and can
+    # be removed once we have removed the failcode correction code in
+    # peer_htlcs.c. The following ones get remapped
+    tests.update({
+        '400F': 'WIRE_TEMPORARY_NODE_FAILURE',
+        '4009': 'WIRE_TEMPORARY_NODE_FAILURE',
+        '4016': 'WIRE_TEMPORARY_NODE_FAILURE',
+    })
+
+    for failcode, expected in tests.items():
+        # Do not attempt with full messages
+        if len(failcode) > 4:
+            continue
+
+        l2.rpc.setfailcode(code=failcode)
+        inv = l2.rpc.invoice(42, 'failcode{}'.format(failcode), '')['bolt11']
+        with pytest.raises(RpcError, match=r'failcodename.: .{}.'.format(expected)):
+            l1.rpc.pay(inv)