lightningd: allow htlc_accepted hook to replace onion payload.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-added: `htlc_accepted` hook can now offer a replacement onion `payload`.

tests/plugins/replace_payload.py

@@ -0,0 +1,35 @@
+#!/usr/bin/env python3
+"""Plugin that replaces HTLC payloads.
+
+This feature is important if we want to accept an HTLC tlv field not
+accepted by lightningd.
+"""
+from pyln.client import Plugin
+
+plugin = Plugin()
+
+
+@plugin.hook("htlc_accepted")
+def on_htlc_accepted(htlc, onion, plugin, **kwargs):
+    # eg. '2902017b04016d0821fff5b6bd5018c8731aa0496c3698ef49f132ef9a3000c94436f4957e79a2f8827b'
+    # (but values change depending on pay's randomness!)
+    if plugin.replace_payload == 'corrupt_secret':
+        if onion['payload'][18] == '0':
+            newpayload = onion['payload'][:18] + '1' + onion['payload'][19:]
+        else:
+            newpayload = onion['payload'][:18] + '0' + onion['payload'][19:]
+    else:
+        newpayload = plugin.replace_payload
+    print("payload was:{}".format(onion['payload']))
+    print("payload now:{}".format(newpayload))
+
+    return {'result': 'continue', 'payload': newpayload}
+
+
+@plugin.method('setpayload')
+def setpayload(plugin, payload: bool):
+    plugin.replace_payload = payload
+    return {}
+
+
+plugin.run()