[Tarantool-patches] [PATCH] lua: panic on lua_gettop() negative return value

[Ilya Kosarev <i.kosarev@tarantool.org>]

According to gh-4649 report it seems to be possible that we are getting
segfault on empty diag in iproto_reply_error() due to negative count of
dumped entries returned from port_lua_do_dump() in tx_process_call().
It can only happen due to lua_gettop() returning negative value in
encode_lua_call(). This should not happen at all, so it is the reason
to panic.

Closes #4649
---
Branch: https://github.com/tarantool/tarantool/tree/i.kosarev/gh-4649-empty-diag-from-tx_process_call
Issue: https://github.com/tarantool/tarantool/issues/4649

@ChangeLog:
 * Panic in case of critical problem: lua_gettop() returning negative
 value (gh-4649).

 src/box/lua/call.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/box/lua/call.c b/src/box/lua/call.c
index ca871e077..82ca47cbe 100644
--- a/src/box/lua/call.c
+++ b/src/box/lua/call.c
@@ -361,6 +361,8 @@ encode_lua_call(lua_State *L)
 
 	struct luaL_serializer *cfg = luaL_msgpack_default;
 	int size = lua_gettop(port->L);
+	if (size < 0)
+		panic("lua_gettop() returned negative value");
 	for (int i = 1; i <= size; ++i)
 		luamp_encode(port->L, cfg, &stream, i);
 	port->size = size;
-- 
2.17.1