[PATCH] lua: fix decimal comparison with box.NULL

[Vladimir Davydov]

On Tue, Aug 27, 2019 at 02:22:09PM +0300, Serge Petrenko wrote:
> This problem is similar to the one fixed in commit
> 3c6c1cc96a1a510493c21c472565d4063e403ba2 (lua:fix decimal comparison
> with nil)
> We should handle box.NULL the same way.
> 
> Closes #4454
> ---
> https://github.com/tarantool/tarantool/issues/4454
> https://github.com/tarantool/tarantool/tree/sp/gh-4454-decimal-cmp-null
> 
>  src/lua/decimal.c         |  3 ++-
>  test/app/decimal.result   | 30 ++++++++++++++++++++++++++++++
>  test/app/decimal.test.lua |  8 ++++++++
>  3 files changed, 40 insertions(+), 1 deletion(-)
> 
> diff --git a/src/lua/decimal.c b/src/lua/decimal.c
> index 23e50ba68..8905a0b9d 100644
> --- a/src/lua/decimal.c
> +++ b/src/lua/decimal.c
> @@ -235,7 +235,8 @@ static int
>  ldecimal_eq(struct lua_State *L)
>  {
>  	assert(lua_gettop(L) == 2);
> -	if (lua_isnil(L, 1) || lua_isnil(L, 2)) {
> +	if (lua_isnil(L, 1) || lua_isnil(L, 2) ||
> +	    luaL_isnull(L, 1) || luaL_isnull(L, 2)) {
>  		lua_pushboolean(L, false);
>  		return 1;
>  	}
> diff --git a/test/app/decimal.result b/test/app/decimal.result
> index 8251e13d8..90d0984b0 100644
> --- a/test/app/decimal.result
> +++ b/test/app/decimal.result
> @@ -248,6 +248,36 @@ a <= nil
>   | ---
>   | - error: '[string "return a <= nil "]:1: attempt to compare decimal with nil'
>   | ...
> +-- and with box.NULL
> +--
> +a == box.NULL
> + | ---
> + | - false
> + | ...
> +a ~= box.NULL
> + | ---
> + | - true
> + | ...
> +a > box.NULL
> + | ---
> + | - error: '[string "return a > box.NULL "]:1: expected decimal, number or string as
> + |     1 argument'
> + | ...

1 argument?

> +a < box.NULL
> + | ---
> + | - error: '[string "return a < box.NULL "]:1: expected decimal, number or string as
> + |     2 argument'
> + | ...

2 argument?

Why does the error message differ for the two seemingly equivalent
cases? What's "1 argument"?

Note, in case of comparison with nil the error message is different:

 | - error: '[string "return a > nil "]:1: attempt to compare decimal with nil'
 | - error: '[string "return a < nil "]:1: attempt to compare decimal with nil'