keysend: enforce BOLT11 description length limit

The keysend plugin previously used `> 1023` as the cutoff for
description length when inserting an invoice. This was
inconsistent with invoice.c, which enforces the BOLT11 description
field limit defined in `common/bolt11.h`.

This patch switches to using `BOLT11_FIELD_BYTE_LIMIT` directly.
As a result, keysend no longer fails on descriptions between
641–1023 bytes, which previously caused unexpected failures.

A new regression test (`test_keysend_description_size_limit`)
exercises boundary cases just below, at, and above the limit.

Changelog-Fixed: Protocol: `keysend` with descriptions of length 640-1023 bytes fixed.
Signed-off-by: Wes Payne <[email protected]>

Author: noblepayne

plugins/keysend.c

@@ -561,7 +561,7 @@ static struct command_result *htlc_accepted_call(struct command *cmd,
 					   (const char *)desc_field->value);
 		json_add_string(req->js, "description", desc);
 		/* Don't exceed max possible desc length! */
-		if (strlen(desc) > 1023)
+		if (strlen(desc) >= BOLT11_FIELD_BYTE_LIMIT)
 			json_add_bool(req->js, "deschashonly", true);
 	} else {
 		json_add_string(req->js, "description", "keysend");

tests/test_pay.py

@@ -11,6 +11,7 @@
     tu64_encode
 )
 import copy
+import json
 import os
 import pytest
 import random
@@ -3697,6 +3698,30 @@ def test_keysend_maxfee(node_factory):
     assert len(l3.rpc.listinvoices()['invoices']) == 1
 
 
+@pytest.mark.parametrize("tlv_payload_length", [638, 639, 640, 641, 1022, 1023, 1024])
+def test_keysend_description_size_limit(node_factory, tlv_payload_length):
+    """
+    Test keysend description handling near BOLT11 field size limits.
+
+    Checks boundary conditions where the payload length is just below,
+    exactly at, and just above the maximum allowed tagged-field size.
+
+    See common/bolt11.h: BOLT11_FIELD_BYTE_LIMIT.
+    """
+    l1, l2 = node_factory.line_graph(2, wait_for_announce=True)
+    amt = 10000
+    prefix = 'keysend: {"message": ""}'
+    base_len = len(prefix)
+
+    # Prep TLV payload with test length
+    body_len = tlv_payload_length - base_len
+    tlv_payload = json.dumps({"message": "a" * body_len}).encode().hex()
+
+    # Send keysend payment with test payload
+    l1.rpc.keysend(l2.info["id"], amt, extratlvs={7629169: tlv_payload})
+    assert len(l2.rpc.listinvoices()["invoices"]) == 1
+
+
 def test_invalid_onion_channel_update(node_factory):
     '''
     Some onion failures "should" send a `channel_update`.