c_void causes internal error

[nowakf]

Creating a userdata method with a LightUserData as an argument, then passing a void pointer to that method, causes the following error:

thread '<unnamed>' panicked at 'mlua internal error: LUA_TNONE in pop_value (this is a bug, please file an issue)', /home/username/.cargo/registry/src/github.com-1ecc6299db9ec823/mlua-0.7.3/src/lua.rs:1991:18

minimal reproduction: main.lua:

ffi = require 'ffi'
pwd = io.popen('pwd'):read('*l')
print(pwd)
test = assert(package.loadlib(pwd .. '/target/debug/libminimal_tnone.so', 'luaopen_test'))()
test:buggy(ffi.cast('void *', ffi.new('int[1]')))

lib.rs:

use mlua::prelude::*;

struct Test;

#[mlua::lua_module]
fn test(lua: &Lua) -> LuaResult<Test> {
    Ok(Test)
}

impl mlua::UserData for Test {
    fn add_methods<'lua, M: mlua::UserDataMethods<'lua, Self>>(methods: &mut M) {
        methods.add_method("buggy", |_, _, ptr: mlua::LightUserData| Ok("kaboom") );
    }
}

Cargo.toml:

[package]
name = "minimal_tnone"
version = "0.1.0"
edition = "2021"

[dependencies]
mlua = {version = "0.7.3", features = ["luajit", "vendored", "module"]}

[lib]
crate-type = ["cdylib"]

This is on linux, x86. It's kinda annoying because a void * is a very useful thing to be able to pass from lua to rust.

[nowakf]

aptarshi Guha wrote:
> I hope i haven't missed this, but does  the C function lua_type return 10
> for cdata objects? Is there #define for this (e.g. like LUA_TCDATA)

Only internally. But, yes, it's guaranteed to be 10 for LuaJIT 2.0.

> Or is there another better way to check whether a lua object is a CDATA.

No, because the classic Lua/C API and the FFI shouldn't be mixed.
Deal with cdata on the Lua side only. Call C functions that need
to be passed cdata only via the FFI.

--Mike

https://www.freelists.org/post/luajit/Type-of-CDATA-as-returned-by-lua-type,1

And it's the same problem (int lua_type() returns 10).

So I guess this is all my bad (or perhaps the error message should be updated to inform the user that mixing luajit's ffi capabilities and mlua is not supported?)

[khvzak]

Yeah, LUA_TCDATA is an internal LuaJIT type not supported by mlua (as well as LUA_TPROTO). I'll update the error message, thanks! Maybe you can do a trick and pass a pointer as a number: tonumber(ffi.cast('uint64_t', ffi.new('int[1]'))) but it's highly unsafe.