Modify Lua Chunk Environment: Lua 5.2 - lua

Question

It is my understanding that in Lua 5.2 that environments are stored in upvalues named _ENV. This has made it really confusing for me to modify the environment of a chunk before running it, but after loading it.
I would like to load a file with some functions and use the chunk to inject those functions into various environments. Example:
chunk = loadfile( "file" )
-- Inject chunk's definitions
chunk._ENV = someTable -- imaginary syntax
chunk( )
chunk._ENV = someOtherTable
chunk( )
Is this possible from within Lua? The only examples I can find of modifying this upvalue are with the C api (another example from C api), but I am trying to do this from within Lua. Is this possible?
Edit: I'm unsure of accepting answers using the debug library. The docs state that the functions may be slow. I'm doing this for efficiency so that entire chunks don't have to be parsed from strings (or a file, even worse) just to inject variable definitions into various environments.
Edit: Looks like this is impossible: Recreating setfenv() in Lua 5.2
Edit: I suppose the best way for me to do this is to bind a C function that can modify the environment. Though this is a much more annoying way of going about it.
Edit: I believe a more natural way to do this would be to load all chunks into separate environments. These can be "inherited" by any other environment by setting a metatable that refers to a global copy of a chunk. This does not require any upvalue modification post-load, but still allows for multiple environments with those function definitions.

Answer 1

The simplest way to allow a chunk to be run in different environments is to make this explicit and have it receive an environment. Adding this line at the top of the chunk achieves this:
_ENV=...
Now you can call chunk(env1) and later chunk(env2) at your pleasure.
There, no debug magic with upvalues.
Although it will be clear if your chunk contains that line, you can add it at load time, by writing a suitable reader function that first sends that line and then the contents of the file.

Answer 2

I do not understand why you want to avoid using the debug library, while you are happy to use a C function (neither is possible in a sandbox.)
It can be done using debug.upvaluejoin:
function newEnvForChunk(chunk, index)
local newEnv = {}
local function source() return newEnv end
debug.upvaluejoin(chunk, 1, source, 1)
if index then setmetatable(newEnv, {__index=index}) end
return newEnv
end
Now load any chunk like this:
local myChunk = load "print(x)"
It will initially inherit the enclosing _ENV. Now give it a new one:
local newEnv = newEnvForChunk(myChunk, _ENV)
and insert a value for 'x':
newEnv.x = 99
Now when you run the chunk, it should see the value for x:
myChunk()
=> 99

Answer 3

If you don't want to modify your chunk (per LHF's great answer) here are two alternatives:
Set up a blank environment, then dynamically change its environment to yours
function compile(code)
local meta = {}
local env = setmetatable({},meta)
return {meta=meta, f=load('return '..code, nil, nil, env)}
end
function eval(block, scope)
block.meta.__index=scope
return block.f()
end
local block = compile('a + b * c')
print(eval(block, {a=1, b=2, c=3})) --> 7
print(eval(block, {a=2, b=3, c=4})) --> 14
Set up a blank environment, and re-set its values with your own each time
function compile(code)
local env = {}
return {env=env, f=load('return '..code, nil, nil, env)}
end
function eval(block, scope)
for k,_ in pairs(block.env) do block.env[k]=nil end
for k,v in pairs(scope) do block.env[k]=v end
return block.f()
end
local block = compile('a + b * c')
print(eval(block, {a=1, b=2, c=3})) --> 7
print(eval(block, {a=2, b=3, c=4})) --> 14
Note that if micro-optimizations matter, the first option is about 2✕ as slow as the _ENV=... answer, while the second options is about 8–9✕ as slow.