require.lua

--- usage: 
-- require = require"require".require
-- :o)

local error, ipairs, type = error, ipairs, type

local t_concat = table.concat

--- Helpers

local function _tostring(s) return ""..s end

local function checkstring(s)
    local success, res = pcall(_tostring, s)
    if success then return res end
    error("bad argument #1 to 'require' (string expected, got "..type(s)..")", 3)
end

local function callable(f)
    if type(f) == "function" then return true end
    local mt = getmetatable(f) or false
    return mt and type(mt.__call) == "function"
end

--- for Lua 5.1

local package, p_loaded = package, package.loaded

local sentinel = {}
-- sentinel = function()end

local function require51 (name)
    name = checkstring(name)
    print ("sentinel", sentinel, p_loaded[name])
    if p_loaded[name] == sentinel then
        error("loop or previous error loading module '"..name.."'", 2)
    elseif not p_loaded[name] then
        local msg = {}
        local loader
        for _, searcher in ipairs(package.loaders) do
            loader = searcher(name)
            if callable(loader) then break end
            if type(loader) == "string" then
                -- `loader` is actually an error message
                msg[#msg + 1] = loader
            end
            loader = nil
        end
        if loader == nil then
            error("module '" .. name .. "' not found: "..t_concat(msg), 2)
        end
        p_loaded[name] = sentinel
        print("before load", sentinel)
        local res = loader(name)
        print("res", res)
        if res ~= nil then
            p_loaded[name] = res
        else
            p_loaded[name] = true
        end
        print("after load", sentinel)
    end
    return p_loaded[name]
end

--- for Lua 5.2

local function require52 (name)
    name = checkstring(name)
    if not p_loaded[name] then
        local msg = {}
        local loader, param
        for _, searcher in ipairs(package.searchers) do
            loader, param = searcher(name)
            if callable(loader) then break end
            if type(loader) == "string" then
                -- `loader` is actually an error message
                msg[#msg + 1] = loader
            end
            loader = nil
        end
        if loader == nil then
            error("module '" .. name .. "' not found: "..table.concat(msg), 2)
        end
        local res = loader(name, param)
        if res ~= nil then
            p_loaded[name] = res
        else
            p_loaded[name] = true
        end
    end
    return p_loaded[name]
end

local module = {require51 = require51, require52 = require52}

if _VERSION == "Lua 5.1" then module.require = require51 end
if _VERSION == "Lua 5.2" then module.require = require52 end

--- rerequire :o)

for _, o in ipairs{
    {"rerequiredefault", require},
    {"rerequire", module.require},
    {"rerequire51", require51},
    {"rerequire52", require52}
} do
    local rereq, req = o[1], o[2]
    module[rereq] = function(name)
        p_loaded[name] = nil
        return req(name)
    end
end

return module

On line #23 you should use debug.getmetatable (if available) because of __metatable...

indeed, thanks