lightningd: Restore forwarding of legacy onions.

Partial revert of 43a833e405
"lightningd: remove support for legacy onion format."; we restore the
ability to decode legacy onions for forwarding, but not to generate them.
(We don't accept them properly since making payment_secret compulsory
anyway, so no real change there!)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Removed: Protocol: ... but we still forward legacy HTLC onions for now.

common/onion.c

@@ -109,10 +109,11 @@ u8 *onion_final_hop(const tal_t *ctx,
 	return make_tlv_hop(ctx, tlv);
 }
 
-/* Returns true if valid, and fills in len. */
+/* Returns true if valid, and fills in type. */
 static bool pull_payload_length(const u8 **cursor,
 				size_t *max,
 				bool has_realm,
+				enum onion_payload_type *type,
 				size_t *len)
 {
 	/* *len will incorporate bytes we read from cursor */
@@ -127,6 +128,19 @@ static bool pull_payload_length(const u8 **cursor,
 	if (!cursor)
 		return false;
 
+	/* BOLT #4:
+	 * - Legacy `hop_data` format, identified by a single `0x00` byte for
+	 *   length. In this case the `hop_payload_length` is defined to be 32
+	 *   bytes.
+	 */
+	if (has_realm && *len == 0) {
+		if (type)
+			*type = ONION_V0_PAYLOAD;
+		assert(*cursor - start == 1);
+		*len = 1 + 32;
+		return true;
+	}
+
 	/* BOLT #4:
 	 * - `tlv_payload` format, identified by any length over `1`. In this
 	 *   case the `hop_payload_length` is equal to the numeric value of
@@ -142,6 +156,8 @@ static bool pull_payload_length(const u8 **cursor,
 				return false;
 		}
 
+		if (type)
+			*type = ONION_TLV_PAYLOAD;
 		*len += (*cursor - start);
 		return true;
 	}
@@ -150,10 +166,11 @@ static bool pull_payload_length(const u8 **cursor,
 }
 
 size_t onion_payload_length(const u8 *raw_payload, size_t len, bool has_realm,
-			    bool *valid)
+			    bool *valid,
+			    enum onion_payload_type *type)
 {
 	size_t max = len, payload_len;
-	*valid = pull_payload_length(&raw_payload, &max, has_realm, &payload_len);
+	*valid = pull_payload_length(&raw_payload, &max, has_realm, type, &payload_len);
 
 	/* If it's not valid, copy the entire thing. */
 	if (!*valid)
@@ -210,12 +227,31 @@ struct onion_payload *onion_decode(const tal_t *ctx,
 	size_t max = tal_bytelen(cursor), len;
 	struct tlv_tlv_payload *tlv;
 
-	if (!pull_payload_length(&cursor, &max, true, &len)) {
+	if (!pull_payload_length(&cursor, &max, true, &p->type, &len)) {
 		*failtlvtype = 0;
 		*failtlvpos = tal_bytelen(rs->raw_payload);
 		goto fail_no_tlv;
 	}
 
+	/* Very limited legacy handling: forward only. */
+	if (p->type == ONION_V0_PAYLOAD && rs->nextcase == ONION_FORWARD) {
+		p->forward_channel = tal(p, struct short_channel_id);
+		fromwire_short_channel_id(&cursor, &max, p->forward_channel);
+		p->total_msat = NULL;
+		p->amt_to_forward = fromwire_amount_msat(&cursor, &max);
+		p->outgoing_cltv = fromwire_u32(&cursor, &max);
+		p->payment_secret = NULL;
+		p->blinding = NULL;
+		/* We can't handle blinding with a legacy payload */
+		if (blinding)
+			return tal_free(p);
+		/* If they somehow got an invalid onion this far, fail. */
+		if (!cursor)
+			return tal_free(p);
+		p->tlv = NULL;
+		return p;
+	}
+
 	/* We do this manually so we can accept extra types, and get
 	 * error off and type. */
 	tlv = tlv_tlv_payload_new(p);

common/onion.h

@@ -6,7 +6,14 @@
 
 struct route_step;
 
+enum onion_payload_type {
+	ONION_V0_PAYLOAD = 0,
+	ONION_TLV_PAYLOAD = 1,
+};
+
 struct onion_payload {
+	enum onion_payload_type type;
+
 	struct amount_msat amt_to_forward;
 	u32 outgoing_cltv;
 	struct amount_msat *total_msat;
@@ -43,6 +50,7 @@ u8 *onion_final_hop(const tal_t *ctx,
  * @len: length of @raw_payload in bytes.
  * @has_realm: used for HTLCs, where first byte 0 is magical.
  * @valid: set to true if it is valid, false otherwise.
+ * @type: if non-NULL, set to type of payload if *@valid is true.
  *
  * If @valid is set, there is room for the HMAC immediately following,
  * as the return value is <= ROUTING_INFO_SIZE - HMAC_SIZE.  Otherwise,
@@ -50,7 +58,8 @@ u8 *onion_final_hop(const tal_t *ctx,
  */
 size_t onion_payload_length(const u8 *raw_payload, size_t len,
 			    bool has_realm,
-			    bool *valid);
+			    bool *valid,
+			    enum onion_payload_type *type);
 
 /**
  * onion_decode: decode payload from a decrypted onion.

common/sphinx.c

@@ -627,7 +627,7 @@ struct route_step *process_onionpacket(
 	payload_size = onion_payload_length(paddedheader,
 					    tal_bytelen(msg->routinginfo),
 					    has_realm,
-					    &valid);
+					    &valid, NULL);
 
 	/* Can't decode?  Treat it as terminal. */
 	if (!valid) {

common/test/run-gossmap_local.c

@@ -25,10 +25,6 @@ void towire_bigsize(u8 **pptr UNNEEDED, const bigsize_t val UNNEEDED)
 /* Generated stub for towire_channel_id */
 void towire_channel_id(u8 **pptr UNNEEDED, const struct channel_id *channel_id UNNEEDED)
 { fprintf(stderr, "towire_channel_id called!\n"); abort(); }
-/* Generated stub for type_to_string_ */
-const char *type_to_string_(const tal_t *ctx UNNEEDED, const char *typename UNNEEDED,
-			    union printable_types u UNNEEDED)
-{ fprintf(stderr, "type_to_string_ called!\n"); abort(); }
 /* AUTOGENERATED MOCKS END */
 
 /* Canned gossmap, taken from tests/test_gossip.py's

common/test/run-route-specific.c

@@ -51,10 +51,6 @@ void towire_tlv(u8 **pptr UNNEEDED,
 /* Generated stub for towire_wireaddr */
 void towire_wireaddr(u8 **pptr UNNEEDED, const struct wireaddr *addr UNNEEDED)
 { fprintf(stderr, "towire_wireaddr called!\n"); abort(); }
-/* Generated stub for type_to_string_ */
-const char *type_to_string_(const tal_t *ctx UNNEEDED, const char *typename UNNEEDED,
-			    union printable_types u UNNEEDED)
-{ fprintf(stderr, "type_to_string_ called!\n"); abort(); }
 /* AUTOGENERATED MOCKS END */
 
 static void write_to_store(int store_fd, const u8 *msg)

common/test/run-route.c

@@ -44,10 +44,6 @@ void towire_tlv(u8 **pptr UNNEEDED,
 /* Generated stub for towire_wireaddr */
 void towire_wireaddr(u8 **pptr UNNEEDED, const struct wireaddr *addr UNNEEDED)
 { fprintf(stderr, "towire_wireaddr called!\n"); abort(); }
-/* Generated stub for type_to_string_ */
-const char *type_to_string_(const tal_t *ctx UNNEEDED, const char *typename UNNEEDED,
-			    union printable_types u UNNEEDED)
-{ fprintf(stderr, "type_to_string_ called!\n"); abort(); }
 /* AUTOGENERATED MOCKS END */
 
 static void write_to_store(int store_fd, const u8 *msg)

common/test/run-sphinx-xor_cipher_stream.c

@@ -91,7 +91,8 @@ struct onionreply *new_onionreply(const tal_t *ctx UNNEEDED, const u8 *contents
 /* Generated stub for onion_payload_length */
 size_t onion_payload_length(const u8 *raw_payload UNNEEDED, size_t len UNNEEDED,
 			    bool has_realm UNNEEDED,
-			    bool *valid UNNEEDED)
+			    bool *valid UNNEEDED,
+			    enum onion_payload_type *type UNNEEDED)
 { fprintf(stderr, "onion_payload_length called!\n"); abort(); }
 /* Generated stub for pubkey_from_node_id */
 bool pubkey_from_node_id(struct pubkey *key UNNEEDED, const struct node_id *id UNNEEDED)

devtools/onion.c

@@ -280,7 +280,7 @@ static void runtest(const char *filename)
 			errx(1, "Error serializing message.");
 		onion_payload_length(step->raw_payload,
 				     tal_bytelen(step->raw_payload),
-				     true, &valid);
+				     true, &valid, NULL);
 		assert(valid);
 		printf("  Payload: %s\n", tal_hex(ctx, step->raw_payload));
 		printf("  Next onion: %s\n", tal_hex(ctx, serialized));

lightningd/peer_htlcs.c

@@ -993,7 +993,15 @@ static void htlc_accepted_hook_serialize(struct htlc_accepted_hook_payload *p,
 
 	json_add_hex_talarr(s, "payload", rs->raw_payload);
 	if (p->payload) {
-		json_add_string(s, "type", "tlv");
+		switch (p->payload->type) {
+		case ONION_V0_PAYLOAD:
+			json_add_string(s, "type", "legacy");
+			break;
+
+		case ONION_TLV_PAYLOAD:
+			json_add_string(s, "type", "tlv");
+			break;
+		}
 
 		if (p->payload->forward_channel)
 			json_add_short_channel_id(s, "short_channel_id",

tests/test_pay.py

@@ -5169,7 +5169,6 @@ def test_sendpay_grouping(node_factory, bitcoind):
     assert([p['status'] for p in pays] == ['failed', 'failed', 'complete'])
 
 
-@pytest.mark.xfail("needs lecacy onion support")
 def test_legacyonion(node_factory, bitcoind):
     # We have to replicate the topology we created onion with, exactly.
     l1, l2, l3 = node_factory.line_graph(3, fundchannel=False)