Re: [5.2+] Can a function access/modify/replace its caller's _ENV?

[Sean Conner <sean@...>]

It was thus said that the Great Jonathan Goble once stated:
> On Fri, Dec 21, 2018 at 10:47 PM Jonathan Goble <jcgoble3@gmail.com> wrote:
> 
> > Can a function (Lua or C) in Lua 5.2+ gain access to its caller's _ENV,
> > and if so, can it modify that table and/or replace that table with a new
> > one? I know access and modification are trivial if you require passing the
> > _ENV table as an argument, and replacement is trivial if you simply return
> > a table that documentation specifies must be assigned to _ENV. I'm not
> > interested in those simple "tricks".
> >
> > What I'm looking for is an approach that allows all three (access, modify,
> > and replace) without writing "_ENV" (or the name of any variable that
> > refers to _ENV) anywhere in the statement that calls the function. I'm open
> > to any solution, including complicated debug library hacks and/or the C
> > API, as long as it would work consistently. Bonus points for a single
> > approach that works on 5.2, 5.3, and the 5.4work git HEAD with minimal or
> > no branching to deal with differences between versions.
> >
> > Any thoughts on how to do this? Or is it not possible?
> >
> 
> I played around in the REPL and think I've figured it out. debug.getupvalue
> and debug.setupvalue (and their C equivalents) give the ability to access
> and replace the upvalues of an arbitrary function, but need the function
> object itself rather than a stack level. To get the calling function,
> debug.getinfo with "uf" for the "what" parameter and 2 for the stack level
> gives me the caller and its number of upvalues. Then I pass the function to
> debug.getupvalue with each index up to the number of upvalues until I find
> the one I want. Finally, I can directly mutate the table, or replace it
> with another using debug.setupvalue with the same index. I haven't tried
> changing _ENV yet, but this has worked with other upvalues, so I assume it
> will work with _ENV as well.
> 
> *puts on mad scientist hat* Might attempt an interesting project during the
> break between semesters.

  You might want to check out the thread starting here:

	http://lua-users.org/lists/lua-l/2018-11/msg00386.html

  -spc