Lua 5.4 <close> variable attribute and error propagation behavior

In Lua 5.4, the <close> variable attribute provides deterministic resource cleanup. When a variable with this attribute goes out of scope, the __close metamethod is automatically invoked. This behavior remains consistent even during stack unwinding via errors.

Execution Frequency

Does the __close metamethod execute exactly once if an error is raised within the block?

Yes. The virtual machine ensures that the __close metamethod is called exactly once when the variable becomes unreachable. This holds true whether the scope is exited via a normal return, a break, or an error. The mechanism is designed to guarantee that resources are not leaked even if the execution flow fails unexpectedly.

Error Precedence and Propagation

If the __close metamethod itself raises an error during a scope exit already triggered by a previous error, which error takes precedence?

The original error takes precedence. If an error is already being propagated (the stack is currently unwinding) and the __close metamethod throws a new error, the new error is discarded. This prevents cleanup logic from masking the root cause of the failure, which is a critical design choice for robust error handling.

Verification Steps

To verify this behavior in a Lua 5.4 environment, you can use the following test pattern:

local res = setmetatable({}, {
  __close = function(v)
    print("Cleanup running...")
    -- To test error precedence, uncomment the line below:
    -- error("Cleanup failed!")\n  end
})

local success, err = pcall(function()
  local x = res [<close>]
  x = x
  error("Main block error!")
end)

printf("Success: %s\n", tostring(success))
printf("Error: %s\n", tostring(err))

Assumptions and Constraints

• This behavior assumes a standard Lua 5.4 implementation. Compatibility shims for Lua 5.3 may not respect the <close> attribute.
• The __close metamethod is only triggered when the variable goes out of scope; reassessing the variable within the same scope will trigger the cleanup for the old value immediately.