optional "failure_onion" in reply to htlc_accepted hook.

Changelog-Added: `htlc_accepted` hook can now return custom `failure_onion`.

doc/PLUGINS.md

@@ -1053,7 +1053,25 @@ onion fields which a plugin doesn't want lightningd to consider.
 ```
 
 `fail` will tell `lightningd` to fail the HTLC with a given hex-encoded
-`failure_message` (please refer to the [spec][bolt4-failure-messages] for details: `incorrect_or_unknown_payment_details` is the most common).
+`failure_message` (please refer to the [spec][bolt4-failure-messages] for
+details: `incorrect_or_unknown_payment_details` is the most common).
+
+
+```json
+{
+  "result": "fail",
+  "failure_onion": "[serialized error packet]"
+}
+```
+
+Instead of `failure_message` the response can contain a hex-encoded
+`failure_onion` that will be used instead (please refer to the
+[spec][bolt4-failure-onion] for details). This can be used, for example,
+if you're writing a bridge between two Lightning Networks. Note that
+`lightningd` will apply the obfuscation step to the value returned here
+with its own shared secret (and key type `ammag`) before returning it to
+the previous hop.
+
 
 ```json
 {
@@ -1263,6 +1281,7 @@ The plugin must broadcast it and respond with the following fields:
 [jsonrpc-notification-spec]: https://www.jsonrpc.org/specification#notification
 [bolt4]: https://github.com/lightningnetwork/lightning-rfc/blob/master/04-onion-routing.md
 [bolt4-failure-messages]: https://github.com/lightningnetwork/lightning-rfc/blob/master/04-onion-routing.md#failure-messages
+[bolt4-failure-onion]: https://github.com/lightningnetwork/lightning-rfc/blob/master/04-onion-routing.md#returning-errors
 [bolt2-open-channel]: https://github.com/lightningnetwork/lightning-rfc/blob/master/02-peer-protocol.md#the-open_channel-message
 [sendcustommsg]: lightning-dev-sendcustommsg.7.html
 [oddok]: https://github.com/lightningnetwork/lightning-rfc/blob/master/00-introduction.md#its-ok-to-be-odd

lightningd/peer_htlcs.c

@@ -888,8 +888,8 @@ static bool htlc_accepted_hook_deserialize(struct htlc_accepted_hook_payload *re
 	struct htlc_in *hin = request->hin;
 	struct lightningd *ld = request->ld;
 	struct preimage payment_preimage;
-	const jsmntok_t *resulttok, *paykeytok, *payloadtok;
-	u8 *payload;
+	const jsmntok_t *resulttok, *paykeytok, *payloadtok, *failoniontok;
+	u8 *payload, *failonion;
 
 	if (!toks || !buffer)
 		return true;
@@ -940,6 +940,16 @@ static bool htlc_accepted_hook_deserialize(struct htlc_accepted_hook_payload *re
 				      " hook: %.*s",
 				      failmsgtok->end - failmsgtok->start,
 				      buffer + failmsgtok->start);
+		} else if ((failoniontok = json_get_member(buffer, toks,
+							       "failure_onion"))) {
+			failonion = json_tok_bin_from_hex(NULL, buffer, failoniontok);
+			if (!failonion)
+				fatal("Bad failure_onion for htlc_accepted"
+				      " hook: %.*s",
+				      failoniontok->end -  failoniontok->start,
+				      buffer + failoniontok->start);
+			fail_in_htlc(hin, take(new_onionreply(tmpctx, failonion)));
+			return false;
 		} else if (deprecated_apis
 			   && (failcodetok = json_get_member(buffer, toks,
 							     "failure_code"))) {