Table Freezing

Table freezing irreversibly and permanently forbids any future modifications to a given table.

Note that table freezing is an optional feature that has to be compiled in by defining the PLUTO_ENABLE_TABLE_FREEZING macro in luaconf.h or your build config.

When enabled, it adds the following C APIs:

• lua_freezetable(L, idx)
• lua_istablefrozen(L, idx)
• lua_erriffrozen(L, idx)

and table library functions:

• table.isfrozen
• table.freeze

Disallowing any edits to the global environment table
pluto
table.freeze(_G)
_G.string = {} -- attempt to modify frozen table.

Creating a constant local that's associated with a frozen table
pluto
local Frozen <const> = table.freeze({ 1, 2, 3 })
Frozen = {} -- attempt to reassign constant.
Frozen[1] = "new value" -- attempt to modify frozen table.
rawset(Frozen, "key", "value") -- attempt to modify frozen table.

Trying to swap the value with the debug library
pluto
local Frozen <const> = table.freeze({ 1, 2, 3 })
for i = 1, 249 do
  local name, value = debug.getlocal(1, i)
  if name == "Frozen" then
    debug.setlocal(1, i, { ["key"] = "hello world" }) -- attempt to modify local variable with a frozen table.
  end
end

What will this prevent?

All modifications to the table from within the Lua environment will be prevented, including those from the debug library.

caution

If you're going to use this for a sandbox, ensure you call table.freeze before any users can access the Lua environment, otherwise they can replace that function.