plugins: simplify htlc_accepted hook payload-setting API.

As discussed with Christian, prepending the length to the payload returned
is awkward, but it's the only way to set a legacy payload.  As this will
be soon deprecated, simplify the external API.

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

doc/PLUGINS.md

@@ -804,8 +804,10 @@ if we're the recipient, or attempt to forward it otherwise. Notice that the
 usual checks such as sufficient fees and CLTV deltas are still enforced.
 
 It can also replace the `onion.payload` by specifying a `payload` in
-the response.  This will be re-parsed; it's useful for removing onion
-fields which a plugin doesn't want lightningd to consider.
+the response.  Note that this is always a TLV-style payload, so unlike
+`onion.payload` there is no length prefix (and it must be at least 4
+hex digits long).  This will be re-parsed; it's useful for removing
+onion fields which a plugin doesn't want lightningd to consider.
 
 
 ```json

plugins/keysend.c

@@ -18,17 +18,13 @@ static struct command_result *
 htlc_accepted_continue(struct command *cmd, struct tlv_tlv_payload *payload)
 {
 	struct json_stream *response;
-	u8 *binpayload, *rawpayload;
 	response = jsonrpc_stream_success(cmd);
 
 	json_add_string(response, "result", "continue");
 	if (payload) {
-		binpayload = tal_arr(cmd, u8, 0);
+		u8 *binpayload = tal_arr(cmd, u8, 0);
 		towire_tlvstream_raw(&binpayload, payload->fields);
-		rawpayload = tal_arr(cmd, u8, 0);
-		towire_bigsize(&rawpayload, tal_bytelen(binpayload));
-		towire(&rawpayload, binpayload, tal_bytelen(binpayload));
-		json_add_string(response, "payload", tal_hex(cmd, rawpayload));
+		json_add_string(response, "payload", tal_hex(cmd, binpayload));
 	}
 	return command_finished(cmd, response);
 }

tests/plugins/replace_payload.py

@@ -13,11 +13,16 @@
 def on_htlc_accepted(htlc, onion, plugin, **kwargs):
     # eg. '2902017b04016d0821fff5b6bd5018c8731aa0496c3698ef49f132ef9a3000c94436f4957e79a2f8827b'
     # (but values change depending on pay's randomness!)
+    print("payload was:{}".format(onion['payload']))
+    assert onion['payload'][0:2] == '29'
+
     if plugin.replace_payload == 'corrupt_secret':
+        # Note: we don't include length prefix in returned payload, since it doesn't
+        # support the pre-TLV legacy form.
         if onion['payload'][18] == '0':
-            newpayload = onion['payload'][:18] + '1' + onion['payload'][19:]
+            newpayload = onion['payload'][2:18] + '1' + onion['payload'][19:]
         else:
-            newpayload = onion['payload'][:18] + '0' + onion['payload'][19:]
+            newpayload = onion['payload'][2:18] + '0' + onion['payload'][19:]
     else:
         newpayload = plugin.replace_payload
     print("payload was:{}".format(onion['payload']))