UnBIOS: Load custom BIOSes in ComputerCraft without modifying ROM

unbios.lua

-- UnBIOS by JackMacWindows
-- This will undo most of the changes/additions made in the BIOS, but some things may remain wrapped if `debug` is unavailable
-- To use, just place a `bios.lua` in the root of the drive, and run this program
-- Here's a list of things that are irreversibly changed:
-- * both `bit` and `bit32` are kept for compatibility
-- * string metatable blocking (on old versions of CC)
-- In addition, if `debug` is not available these things are also irreversibly changed:
-- * old Lua 5.1 `load` function (for loading from a function)
-- * `loadstring` prefixing (before CC:T 1.96.0)
-- * `http.request`
-- * `os.shutdown` and `os.reboot`
-- * `peripheral`
-- * `turtle.equip[Left|Right]`
-- Licensed under the MIT license
if _HOST:find("UnBIOS") then return end
local keptAPIs = {bit32 = true, bit = true, ccemux = true, config = true, coroutine = true, debug = true, fs = true, http = true, mounter = true, os = true, periphemu = true, peripheral = true, redstone = true, rs = true, term = true, utf8 = true, _HOST = true, _CC_DEFAULT_SETTINGS = true, _CC_DISABLE_LUA51_FEATURES = true, _VERSION = true, assert = true, collectgarbage = true, error = true, gcinfo = true, getfenv = true, getmetatable = true, ipairs = true, __inext = true,load = true, loadstring = true, math = true, newproxy = true, next = true, pairs = true, pcall = true, rawequal = true, rawget = true, rawlen = true, rawset = true, select = true, setfenv = true, setmetatable = true, string = true, table = true, tonumber = true, tostring = true, type = true, unpack = true, xpcall = true, turtle = true, pocket = true, commands = true, _G = true}
local t = {}
for k in pairs(_G) do if not keptAPIs[k] then table.insert(t, k) end end
for _,k in ipairs(t) do _G[k] = nil end
local native = _G.term.native()
for _, method in ipairs {"nativePaletteColor", "nativePaletteColour", "screenshot"} do native[method] = _G.term[method] end
_G.term = native
if _G.http then
    _G.http.checkURL = _G.http.checkURLAsync
    _G.http.websocket = _G.http.websocketAsync
end
if _G.commands then _G.commands = _G.commands.native end
if _G.turtle then _G.turtle.native, _G.turtle.craft = nil end
local delete = {os = {"version", "pullEventRaw", "pullEvent", "run", "loadAPI", "unloadAPI", "sleep"}, http = _G.http and {"get", "post", "put", "delete", "patch", "options", "head", "trace", "listen", "checkURLAsync", "websocketAsync"}, fs = {"complete", "isDriveRoot"}}
for k,v in pairs(delete) do for _,a in ipairs(v) do _G[k][a] = nil end end
_G._HOST = _G._HOST .. " (UnBIOS)"
-- Set up TLCO
-- This functions by crashing `rednet.run` by removing `os.pullEventRaw`. Normally
-- this would cause `parallel` to throw an error, but we replace `error` with an
-- empty placeholder to let it continue and return without throwing. This results
-- in the `pcall` returning successfully, preventing the error-displaying code
-- from running - essentially making it so that `os.shutdown` is called immediately
-- after the new BIOS exits.
--
-- From there, the setup code is placed in `term.native` since it's the first
-- thing called after `parallel` exits. This loads the new BIOS and prepares it
-- for execution. Finally, it overwrites `os.shutdown` with the new function to
-- allow it to be the last function called in the original BIOS, and returns.
-- From there execution continues, calling the `term.redirect` dummy, skipping
-- over the error-handling code (since `pcall` returned ok), and calling
-- `os.shutdown()`. The real `os.shutdown` is re-added, and the new BIOS is tail
-- called, which effectively makes it run as the main chunk.
local olderror = error
_G.error = function() end
_G.term.redirect = function() end
function _G.term.native()
    _G.term.native = nil
    _G.term.redirect = nil
    _G.error = olderror
    term.setBackgroundColor(32768)
    term.setTextColor(1)
    term.setCursorPos(1, 1)
    term.setCursorBlink(true)
    term.clear()
    local file = fs.open("/bios.lua", "r")
    if file == nil then
        term.setCursorBlink(false)
        term.setTextColor(16384)
        term.write("Could not find /bios.lua. UnBIOS cannot continue.")
        term.setCursorPos(1, 2)
        term.write("Press any key to continue")
        coroutine.yield("key")
        os.shutdown()
    end
    local fn, err = loadstring(file.readAll(), "@bios.lua")
    file.close()
    if fn == nil then
        term.setCursorBlink(false)
        term.setTextColor(16384)
        term.write("Could not load /bios.lua. UnBIOS cannot continue.")
        term.setCursorPos(1, 2)
        term.write(err)
        term.setCursorPos(1, 3)
        term.write("Press any key to continue")
        coroutine.yield("key")
        os.shutdown()
    end
    setfenv(fn, _G)
    local oldshutdown = os.shutdown
    os.shutdown = function()
        os.shutdown = oldshutdown
        return fn()
    end
end
if debug then
    -- Restore functions that were overwritten in the BIOS
    -- Apparently this has to be done *after* redefining term.native
    local function restoreValue(tab, idx, name, hint)
        local i, key, value = 1, debug.getupvalue(tab[idx], hint)
        while key ~= name and key ~= nil do
            key, value = debug.getupvalue(tab[idx], i)
            i=i+1
        end
        tab[idx] = value or tab[idx]
    end
    restoreValue(_G, "loadstring", "nativeloadstring", 1)
    restoreValue(_G, "load", "nativeload", 5)
    if http then restoreValue(http, "request", "nativeHTTPRequest", 3) end
    restoreValue(os, "shutdown", "nativeShutdown", 1)
    restoreValue(os, "reboot", "nativeReboot", 1)
    if turtle then
        restoreValue(turtle, "equipLeft", "v", 1)
        restoreValue(turtle, "equipRight", "v", 1)
    end
    do
        local i, key, value = 1, debug.getupvalue(peripheral.isPresent, 2)
        while key ~= "native" and key ~= nil do
            key, value = debug.getupvalue(peripheral.isPresent, i)
            i=i+1
        end
        _G.peripheral = value or peripheral
    end
end
coroutine.yield()

suggestion: restore native peripheral API
suggestion 2: restore native turtle and commands APIs
suggestion 3: add peripheral API to the list of things irreversibly changed by debug unavailability
suggestion 4: add __inext to keptAPIs
(edit: it looks like the above suggestions were accepted!)