Re: Unpicking require

[Xmilia Hermit <xmilia.hermit@...>]

Roberto Ierusalimschy wrote:

> dlclose is called when you close the Lua state, calling lua_close.
> The package loadlib creates a table in the registry, where it puts
> all C packages it loads. We can see this table with this code:
>
>    print(debug.getregistry()._CLIBS)    -- for Lua 5.4
>
> This table has a metamethod __gc that dlclose's all libraries in that
> table. As the table is in the registry, it is never collected, so that
> it is called only when we close the Lua state.
>
> Because this table is created and marked before we can load any C
> module, it will be collected after the collection of any object
> created by any C module.

This seems not to be true when closing a state and a finalizer creates a new object with a finalizer and calls os.exit(0, true). The os.exit call will append the new finalizer to the list tobefnz which already contains the finalizer for the _CLIBS table. With this the finalizer will run with all the c libs closed. The following code will crash with a Segmentation fault due to the closed c lib.

local tl = require "testlib"

setmetatable({},{__gc=function()
    -- Called when state is closed
    setmetatable({},{__gc=function()
        -- Called as last finalizer during os.exit(0, true)
        -- Here _CLIBS finalizer was already called
        tl.test()
    end})
    os.exit(0, true)
end})

where testlib can be replaced with any c lib but here is the version used:

#include "lua.h"
#include "lauxlib.h"

static int test(lua_State* L) {
    return 0;
}

static const struct luaL_Reg functions [] = {
    {"test", test},
    {NULL, NULL}
};

int luaopen_testlib(lua_State *L) {
    luaL_newlib(L, functions);
    return 1;
}

Regards,
Xmilia