Re: 5.2 _ENV garbage collection question

[Gaspard Bucher <gaspard@...>]

On Mon, Jun 27, 2011 at 3:02 PM, Peter Cawley <lua@corsix.org> wrote:

On Mon, Jun 27, 2011 at 1:57 PM, Gaspard Bucher <gaspard@teti.ch> wrote:
> This means that the required script gets the original global _ENV even
> though _LOADED._G has been changed. It can also read the first "foo".
>
> It seems that removing _LOADED._G is not enough to purge "foo"...

That would be because when a chunk is loaded, it is assigned as its
environment the value require"debug".getregistry()[2]. The environment
for your first chunk was from here, so unless changed, your second
chunk gets the same thing from here.

Thanks Moonlit master ! From the sources, index 2 in the registry is:

#define LUA_RIDX_GLOBALS 2

It's nice to know that the environment for loaded chunks comes from there. With this knew knowledge, I tried different scenarios to purge the global environment, and the only solution I found is either to set _ENV before setting "foo" and then removing the environment or to nil "foo":

===================== test4.lua

local print, collectgarbage = print, collectgarbage

local req = require "debug".getregistry()

local gc_mt = {}

function gc_mt.__gc(tbl)

  print('gc', tbl.name)

end

foo = {name='foo'}

setmetatable(foo,gc_mt)

_ENV = {}

req[2].foo = nil --- we cannot completely nil the global environment (error in __gc tag method (attempt to call a nil value)...

collectgarbage()  --- correctly garbage collects "foo"

print('end')

=====================

Gaspard