someta_final.lua

-- Problem: it appears to be impossible to protect a metatable from being unset
-- while also allowing Lua-side code to read/write metamethods. The solution is
-- to use a proxy. 

-- The idea is, we use __metatable so that getmetatable() returns a proxy
-- object which inherits from the real metatable. User code can read/write
-- fields to the real metatable through this proxy.

-- Note: Keep __metatable intact on the proxy to (1) protect user code from
-- changing the behavior of the proxy and (2) protect user code from getting
-- access to the real metatable via getmetatable(proxy).__index


-- READ ONLY FORM
do
  local function throw_write_error(self,k)
    k = tostring(k)
    error("attempt to alter protected metatable (with key '"..k.."')",2)
  end
  
  function setprotectedmt(tbl,meta)
    local meta_proxy = setmetatable({},{
      __index = meta,
      __newindex = throw_write_error,
      __metatable = false,
    })
    meta.__metatable = meta_proxy
    return setmetatable(tbl,meta)
  end
end

-- Test
local test = {}
local mt = { __newindex = print }
setprotectedmt(test,mt)

assert(getmetatable(test).__newindex == print)
assert(not pcall(function() getmetatable(test).__newindex = function() end end))
assert(not pcall(setmetatable,test,nil))
test.a = 1  -- prints <table: n>, 1


-- WRITE NEW ONLY FORM
-- This form protects any fields present at the time the metatable was set. All
-- other fields can be written to.
do
  local function throw_write_error(k)
    k = tostring(k)
    error("attempt to alter protected metamethod '"..k.."'",3)
  end
  
  function setprotectedmt(tbl,meta)
    if type(meta) ~= 'table' then
      error("bad argument #1 to 'setprotectedmt' (table expected)",2)
    end
    
    local protected = {}
    for k in next,meta do
      protected[k] = true
    end
    local meta_proxy = setmetatable({},{
      __index = meta,
      __newindex = function(self,k,v)
        if protected[k] then throw_write_error(k) end
        meta[k] = v
      end,
      __metatable = false,
    })
    meta.__metatable = meta_proxy
    return setmetatable(tbl,meta)
  end
end

-- Test
local test = {}
local mt = { __newindex = print }
setprotectedmt(test,mt)

assert(getmetatable(test).__newindex == print)
assert(not pcall(function() getmetatable(test).__newindex = function() end end))
assert(pcall(function() getmetatable(test).__custom = true end))  -- can write custom field?
assert(not pcall(setmetatable,test,nil))
test.a = 1  -- prints <table: n>, 1


-- __protected FORM
-- This form expects a custom __protected on the *real* metatable. If present,
-- any field in that table will be protected from writes. All other fields can
-- be safely written.
do
  local function throw_write_error(k)
    k = tostring(k)
    error("attempt to alter protected metamethod '"..k.."'",3)
  end
  
  function setprotectedmt(tbl,meta)
    local meta_proxy = setmetatable({},{
      __index = meta,
      __newindex = function(self,k,v)
        if k == '__protected' or (meta.__protected and meta.__protected[k]) then
          throw_write_error(k)
        end
        meta[k] = v
      end,
      __metatable = false,
    })
    meta.__metatable = meta_proxy
    return setmetatable(tbl,meta)
  end
end

-- Test
local test = {}
local mt = { __newindex = print, __protected = { __newindex = true } }
setprotectedmt(test,mt)

assert(getmetatable(test).__newindex == print)
assert(not pcall(function() getmetatable(test).__newindex = function() end end))
assert(pcall(function() getmetatable(test).__custom = true end))  -- can write custom field?
assert(not pcall(setmetatable,test,nil))
test.a = 1  -- prints <table: n>, 1