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Try something more like the following:

	local scope = makeScope()

	scope:register( finalizer )

	local result = bar( scope )


	return result

 Here scope plays the role of your U object. The need to invoke close on the scope after the call to bar keeps it alive.

If capturing result seems like a pain, then wrap the pattern in a standard method on scope:

	scope:executeAndClose( bar, scope )

Which could be defined as:

	local function pcall2call( success, ... )
		if not success then
			error( ( ... ) )	-- Note: Play with level offset for better messages
			return ...

	local function _executeAndClose_finish( scope, ... )
		return pcall2call( ... )

	function Scope:executeAndClose( ... )
		return _executeAndClose_finish( self, pcall( ... ) )

[ Side note: Per the other current LuaJIT thread, this is also an example of my usage of varargs in generic code. ]


P.S. Other interesting things to do include having the register function return a function that can be used to execute the finalizer early or cancel it or whatever though that results in more data creation and collection traffic.