drivers: on_task_change gets a NULL task status on ttl expiration

[bigbes]

In the "ttl tasks" loop of the fifottl and utubettl driver fibers the loop variable shadows the state module:

queue/queue/abstract/driver/fifottl.lua

Lines 112 to 117 in 03cece7

	for _, state in pairs(ttl_states) do
	task = self.space.index.watch:min{ state }
	if task ~= nil and task[i_status] == state then
	if now >= task[i_next_event] then
	task = self:delete(task[i_id]):transform(2, 1, state.DONE)
	self:on_task_change(task, 'ttl')

queue/queue/abstract/driver/utubettl.lua

Lines 239 to 244 in 03cece7

	for _, state in pairs(ttl_states) do
	task = self.space.index.watch:min{ state }
	if task ~= nil and task[i_status] == state then
	if now >= task[i_next_event] then
	task = self:delete(task[i_id]):transform(2, 1, state.DONE)
	self:on_task_change(task, 'ttl')

Inside the loop state is a status string, so state.DONE indexes a string and evaluates to nil. transform(2, 1, nil) then writes NULL into the status field instead of DONE:

tarantool> box.tuple.new{10, 'r', 111}:transform(2, 1, nil)
---
- [10, null, 111]
...

So the task passed to the user's on_task_change callback with the ttl event is malformed. Internal bookkeeping is unaffected: the READY/TAKEN branches in abstract.lua are skipped for a deleted task anyway and the statistics are still counted.

Steps to reproduce

local tube = queue.create_tube('t', 'fifottl', {ttl = 0.1,
 on_task_change = function(task, event)
 if event == 'ttl' then require('log').info('status: %s', tostring(task[2])) end
 end})
tube:put('expired')

Actual behavior

status: cdata<void *>: NULL

Expected behavior

status: - (state.DONE)

Renaming the loop variable is enough.