Lua strange behaviour with _ENV

Question

Can anybody explain why lua 5.3.5 acts this way?

a="global"
local b="local"

function problem(_ENV)
    a="fn_a"
    b="fn_b"
end

problem{}
print(_VERSION)
print("a",a)
print("b",b)

--[[ https://www.lua.org/cgi-bin/demo output:
Lua 5.3
a       global
b       fn_b
]]

Why local variable can be changed after _ENV changed? Is it bug or feature?

Roughly speaking, locals have priority over upvalues which have priority over globals. — Egor Skriptunoff, Nov 24 '18 at 15:21
Locals are not in the environment; They are in the chunk (compilation unit). So, it's a feature. — Tom Blodget, Nov 24 '18 at 21:51
If you are attempting to consume user-provided code, do research [Lua sandboxing](https://stackoverflow.com/a/1225373/2226988). — Tom Blodget, Nov 24 '18 at 21:55
Sandbox has the same problem local b a="global" b="local" function fn() a="fn_a" b="fn_b" end print(a,b) run_sandbox(sandbox_env,fn) print(a,b) — Sergey Kovalev, Nov 27 '18 at 11:48

score 1 · Answer 1 · answered Nov 24 '18 at 21:14

1

Q: How to eliminate all upvalues?
A:

a="global"
local b="local"

-- load() actually eliminates all upvalues :-)
no_problem = assert(load[[
    return function (_ENV)
        a="fn_a"
        b="fn_b"
    end
]])()

no_problem{}
print(_VERSION)
print("a",a)  -->  a    global
print("b",b)  -->  b    local

answered Nov 24 '18 at 21:14

Egor Skriptunoff

23,359
2
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It illiminates upvalues only inside load block but if I call some other function from load block the behaviour will be the same. It can modify parents state. So only io.popen or special c function that create different lua context can truly separate code. – Sergey Kovalev Nov 27 '18 at 12:09
@SergeyKovalev - You can use `load` block inside another `load` block ;-) – Egor Skriptunoff Nov 27 '18 at 14:00

score 0 · Answer 2 · answered Nov 24 '18 at 15:18

0

You are creating upvalue with the following code:

local b="local"
function problem(_ENV)
    b="fn_b"
end

During parsing of the function interpreter detects, that function refers to local variable from one of encompassing scopes that are visible from within the function and thus links it directly. Access to local variables precedes lookup of globals in the _ENV. If you put local b="local" after the function definition, it will not be altered by function calls.

answered Nov 24 '18 at 15:18

Dimitry

2,204
1
16
24

How to eliminate all upvalues? – Sergey Kovalev Nov 24 '18 at 16:50
@SergeyKovalev Upvalues cannot be "eliminated". Simply don't reference them. – IS4 Jan 16 '19 at 22:21

Lua strange behaviour with _ENV

2 Answers2