Workaround to make __gc work on tables

[davidor]

In LuaJIT and Lua 5.1, the __gc metamethod does not work with tables, it only works with "userdata". This PR introduces a module that implements a workaround to make it work with tables.

[codeclimate]

variable 'table_with_gc' is never accessed

[davidor]

It's OK. We need to define it so it's garbage collected later.

[davidor]

I approved this one manually in codeclimate.

[mikz]

I think it would be nice to also hide the this metatable by:

mt.__metatable = getmetatable(table)

Then getmetatable(proxy) will return the metatable returned by getmetatable(table).

This would not propagate if someone would change the metatable of table directly, but that can be avoided by changing the API a bit.

[mikz]

We could rename this library to setmetatable_gc(table, mt).

It could make more sense when using the __metatable key.
All changes to GC'd objects metatables should be done with this method.

[mikz]

We should avoid allocating functions for each object.

These can be shared by (even when using __metatable):

local rawgetmetatable = debug.getmetatable
local getmetatable = getmetatable

local function __len(t)
  return #(rawgetmetatable(t).__table)
end

[davidor]

I didn't know about debug.getmetatable 👍

[mikz]

I think we should defined this as:

getmetatable(proxy).__table(...)

Because now it basically defines all tables callable.

Unfortunately that can yield rather cryptic error message: attempt to call field '__table' (a nil value).

Unfortunately Lua tries to be helpful and the error message depends on how you call the table:

• attempt to call local 'varname' (a table value)
• attempt to call a table value

So replicating those messages might not be possible.

Based on how the function is stored (or if it is a field).
The only workaround I can imagine is using pcall like:

local function __call(self, ...)
  local mt = rawgetmetatable(self)
  local ret = { pcall(mt.__table, ...) }

  local ok = table.remove(ret, 1)

  if ok then
	  return unpack(ret)
  else
	  error(ret[1], 2)
  end
end

That would throw error: ERROR: gc.lua:66: attempt to call a table value (even with correct line pointing to the original caller).

[mikz]

We could avoid that CC error by assert(table_with_gc) right ?

[codeclimate]

value assigned to variable 'table_with_gc' is unused

[codeclimate]

shadowing upvalue 'table' on line 11

[davidor]

@mikz I addressed your comments. In some cases, I adopted a slightly different solution. Can you take a look?

[mikz]

Why not calling tostring(original_table(proxy)) ?

[davidor]

This was needed because I wasn't calling setmetatable(t, metatable) at the beginning of set_metatable_gc. I needed to do that, otherwise, __index would not work.
I fixed this in a new commit. Please check it. I'll squash it after review.

[mikz]

👍 This is great!