"Simple" killable (easy-to-breakout) sandbox

local fakeToReal = setmetatable({},{__mode="k"})
local realToFake = setmetatable({},{__mode="v"})
local connections = setmetatable({},{__mode="k"})


local Metafields = {"index","newindex","call","metatable","tostring","len","unm","add","sub","mul","div","mod","pow","concat","eq","lt","le"}

local API,dead = {}

local DeadTrap,Kill = newproxy(true)

local metacount = 0

local function safeprint(...)
    local e = getfenv(0)
    setfenv(0,getfenv())
    print(...) setfenv(0,e)
end

local kill = Instance.new("Folder")
local function killf() kill:Destroy() end
function Kill()
    -- safeprint("metacount:", metacount)
    if metacount > 0 then return DeadTrap end
    spawn(killf) kill.Changed:Wait()
end

do
    local deadMeta = getmetatable(DeadTrap)
    local function deadBringer() return DeadTrap end
    for k,v in pairs(Metafields) do
        deadMeta["__"..v] = deadBringer
    end deadMeta.__call = Kill
end

local sandbox,unsandbox

local function tuple(...)
    return {n=select("#",...),...}
end
local function customConnect(e,f)
    local con = e:Connect(function(...)
        metacount = 0 f(...)
    end)
    connections[con] = true
    return con
end
local function customWait(e)
    local t = tuple(e:Wait())
    metacount = 0
    if dead then Kill() end
    return unpack(t,1,t.n)
end

local instanceMeta = {}
instanceMeta.__metatable = getmetatable(game)
function instanceMeta:__index(key)
    if dead then return DeadTrap end
    local real = fakeToReal[self]
    if key == "connect" or key == "Connect" and typeof(real) == "RBXScriptSignal" then
        return sandbox(customConnect)
    elseif key == "wait" or key == "Wait" and typeof(real) == "RBXScriptSignal" then
        return sandbox(customWait)
    end
    return sandbox(real[key])
end
function instanceMeta:__newindex(key,val)
    if dead then return DeadTrap end
    unsandbox(self)[key] = unsandbox(val)
end
function instanceMeta:__call(...)
    if ... == 123 then return true end -- Kind of an assert for "is this sandboxed?"
    if dead then return Kill() end
    return API.SandboxTuple(unsandbox(self)(API.UnsandboxTuple(...)))
end
function instanceMeta:__tostring()
    if dead then return DeadTrap end
    return "~"..tostring(unsandbox(self)) -- Easier to debug
end

function API.Kill()
    warn("RIP")
    dead = true
    for k in pairs(connections) do
        k:Disconnect()
    end
end

function API.Sandbox(real)
    local fake = realToFake[real]
    if fake then return fake end

    local t = typeof(real)
    if t == "Instance" or t == "RBXScriptSignal" then
        fake = newproxy(true)
        fakeToReal[fake] = real
        realToFake[real] = fake
        local meta = getmetatable(fake)
        for k,v in pairs(instanceMeta) do
            meta[k] = v
        end return fake
    elseif t == "function" then
        local f = function(...)
            if dead then return Kill() end
            return API.SandboxTuple(real(API.UnsandboxTuple(...)))
        end
        fakeToReal[f] = real
        realToFake[real] = f
    elseif t == "table" then
        fake = {}
        for k,v in pairs(real) do
            fake[sandbox(k)] = sandbox(v)
        end return fake
    end

    return real
end sandbox = API.Sandbox
function API.Unsandbox(fake)
    local real = fakeToReal[fake]
    if real then return real end

    local t = typeof(fake)
    if t == "function" then
        local f = function(...)
            if dead then return Kill() end
            return API.UnsandboxTuple(fake(API.SandboxTuple(...)))
        end
        fakeToReal[fake] = f
        realToFake[f] = fake
    elseif t == "table" then
        real = {}
        for k,v in pairs(fake) do
            real[unsandbox(k)] = unsandbox(v)
        end return real
    end

    return fake
end unsandbox = API.Unsandbox

function API.SandboxTuple(...)
    local n,res = select("#",...),{...}
    for i=1,n do
        res[i] = sandbox(res[i])
    end return unpack(res,1,n)
end
function API.UnsandboxTuple(...)
    local n,res = select("#",...),{...}
    for i=1,n do
        res[i] = unsandbox(res[i])
    end return unpack(res,1,n)
end

local Globals,GlobalEnv = {
    tostring = tostring;
    error = error;
    getfenv = getfenv;
    setfenv = setfenv;
    pairs = pairs;
    ipairs = ipairs;
    next = next;
    setmetatable = setmetatable;
    getmetatable = getmetatable;
},getfenv()

function Globals.spawn(f)
    spawn(function(...)
        metacount = 0 f(...)
    end)
end Globals.Spawn = Globals.spawn
function Globals.delay(d,f)
    delay(d,function(...)
        metacount = 0 f(...)
    end)
end Globals.Delay = Globals.delay
function Globals.wait(...)
    if dead then
        metacount = 0
        return Kill()
    end return wait(...)
end Globals.Wait = Globals.wait
function Globals.print(...)
    if dead then return Kill() end
    print(...)
end
function Globals.warn(...)
    if dead then return Kill() end
    warn(...)
end

local metas = setmetatable({},{__mode="k"})
local function proxyMeta(meta)
    local res = {}
    for k,v in pairs(Metafields) do
        v = "__"..v
        res[v] = function(...)
            local m = meta[v]
            if not m then
                error("No metamethod "..v,0)
            end metacount = metacount + 1
            local t = tuple(m(...))
            metacount = metacount - 1
            return unpack(t,1,t.n)
        end
    end
    function res:__index(key)
        local m,t = meta.__index,{}
        metacount = metacount + 1
        if typeof(m) == "function" then
            t = tuple(m(self,key))
        else
            t = {n=1,m[key]}
        end
        metacount = metacount - 1
        return unpack(t,1,t.n)
    end
    function res:__newindex(key,val)
        local m,t = meta.__newindex,{}
        metacount = metacount + 1
        if typeof(m) == "function" then
            m(self,key,val)
        else
            m[key] = val
        end metacount = metacount - 1
    end
    local function e(a) return a end
    function res:__call(...)
        if dead then return Kill() end
        return e(meta.__call)(...)
    end return res
end
function Globals.getmetatable(val)
    local m = metas[val]
    if m and m.__metatable ~= nil then
        return m
    end return m or getmetatable(val)
end
function Globals.setmetatable(val,meta)
    local m = metas[val]
    if m and m.__metatable ~= nil then
        error("cannot change a protected metatable",2)
    end
    setmetatable(val,proxyMeta(meta))
    metas[val] = meta return val
end

function API.Create()
    return setmetatable({},{__index=function(s,k)
        return Globals[k] or dead and DeadTrap or sandbox(GlobalEnv[k])
    end})
end

local unboxedBE = Instance.new("BindableEvent",workspace)
unboxedBE.Name = "SandboxEvent"
spawn(function()
    while wait(0.1) do
        unboxedBE:Fire(game)
    end
end)

spawn(function()
    local env = API.Create()
    delay(0.5,API.Kill)
    setfenv(0,env)
    setfenv(1,env)
    game(123)

    local BE = Instance.new("BindableEvent")
    BE.Event:Connect(warn)
    BE:Fire("hi")

    local eh = setmetatable({},{__index=function(s,k)
        safeprint("eh.__index",k)
        print("Did __index with key",k)
    end})
    local function test(k)
        print(eh[k])
    end
    test()
    spawn(test)
    delay(1,test)

    spawn(function()
        while wait() do
            print("Still running")
        end
    end)

    local BE = workspace.SandboxEvent
    BE.Parent = nil -- otherwise every test leaves one behind
    BE.Event:Connect(function(g) print("fired",g,g(123)) end)
    spawn(function()
        local g = BE.Event:Wait()
        print("waited",g,g(123))
    end)
    delay(1,function()
        local g = BE.Event:Wait()
        print("waited",g,g(123))
    end)
    wait(0.5)
    warn("for realz")
end)

return API