Potential stack overflow in InnoDB fulltext search

With a complex MATCH...AGAINST string, the call stack inside the server could look like this:

fts_ast_visit_sub_exp

fts_query_visitor

fts_ast_visit

fts_ast_visit

...

fts_ast_visit_sub_exp

fts_query_visitor

fts_ast_visit

fts_ast_visit

fts_ast_visit_sub_exp

fts_query_visitor

fts_ast_visit

fts_query

ha_innobase::ft_init_ext

Item_func_match::init_search

init_ftfuncs

JOIN::optimize_inner

JOIN::optimize

mysql_select

On my platform (GCC 10.2.0 -Og AMD64 debug build), the stack usage is 464 bytes for each fts_ast_visit_sub_exp(), and further 9,248 bytes from Item_func_match::init_search() to the first fts_ast_visit_sub_exp().

To prevent stack overflow, we should return an error if too deep recursion is encountered.