Add ability to pass args to start_key function

[ArtDu]

expirationd/expirationd.lua

Lines 691 to 698 in c3b86c8

	-- check start_key
	if options.start_key ~= nil or options.start_key == box.NULL then
	if type(options.start_key) == "function" then
	task.start_key = function() return options.start_key() end
	else
	task.start_key = function() return options.start_key end
	end
	end

expirationd/expirationd.lua

Lines 382 to 390 in c3b86c8

	-- default iterate_with function
	local function default_iterate_with(task)
	return task.index:pairs(task.start_key(), { iterator = task.iterator_type })
	:take_while(
	function()
	return task:process_while()
	end
	)
	end

Should be rewritten to:

-- check start_key 
 if options.start_key ~= nil or options.start_key == box.NULL then 
     if type(options.start_key) == "function" then 
         task.start_key = function(...) return options.start_key(...) end 
     else 
         task.start_key = function() return options.start_key end 
     end 
 end 

-- default iterate_with function
local function default_iterate_with(task)
    return task.index:pairs(task.start_key(task.args), { iterator = task.iterator_type })
       :take_while(
            function()
                return task:process_while()
            end
        )
end

Or should we pass task as it says in README?

expirationd/README.md

Lines 94 to 129 in c3b86c8

	```lua
	expirationd.start(job_name, space.id, is_expired, {
	-- name or id of the index in the specified space to iterate over
	index = "exp",
	-- one transaction per batch
	-- default is false
	atomic_iteration = true,
	-- delete data that was added a year ago
	-- default is nil
	start_key = function( task )
	return clock.time() - (365*24*60*60)
	end,
	-- delete it from the oldest to the newest
	-- default is ALL
	iterator_type = "GE",
	-- stop full_scan if delete a lot
	-- returns true by default
	process_while = function( task )
	if task.args.max_expired_tuples >= task.expired_tuples_count then
	task.expired_tuples_count = 0
	return false
	end
	return true
	end,
	-- this function must return an iterator over the tuples
	iterate_with = function( task )
	return task.index:pairs({ task.start_key() }, { iterator = task.iterator_type })
	:take_while( function( tuple )
	return task:process_while()
	end )
	end,
	args = {
	max_expired_tuples = 1000
	}
	})
	```

[oleg-jukovec]

Thank you for the issue!

It seems to me that we should to fix the README. I don't see a real case of usage the task argument in the start_key function considering that we only provide to an user the interface:

task.start (self)
task.stop (self)
task.restart (self)
task.kill (self)
task.statistics (self)

and the user should not use other fields or methods:

expirationd/expirationd.lua

Lines 210 to 213 in c3b86c8

	-- NOTE: task object contains a number of properties that available for users.
	-- However these properties are not a part of expirationd API. Property name
	-- can be changed or property itself can be removed in future version. Be
	-- careful!

[ArtDu]

Thank you for the issue!

It seems to me that we should to fix the README. I don't see a real case of usage the task argument in the start_key function considering that we only provide to an user the interface:

task.start (self)
task.stop (self)
task.restart (self)
task.kill (self)
task.statistics (self)

and the user should not use other fields or methods:

expirationd/expirationd.lua

Lines 210 to 213 in c3b86c8

	-- NOTE: task object contains a number of properties that available for users.
	-- However these properties are not a part of expirationd API. Property name
	-- can be changed or property itself can be removed in future version. Be
	-- careful!

We can pass args instead task. It's useful when you want to start iteration from point and you depends on user data(args).
For example, if I wanted to remove by time with some threshold I would use start_key

function get_lower_bound(args)
    return fiber.time() - args.threshold
end

expirationd.start(name, space, is_expired, {
  index = 'created_at_index',
  start_key = get_lower_bound,
  iterator_type = "LT"
  args = {
    threshold = 30 -- seconds
  }
})

And we've already had the flexibility of user callbacks which have task as argument

expirationd/expirationd/init.lua

Lines 347 to 358 in 977c8cc

	task.on_full_scan_start(task)
	local state, err = pcall(task.do_worker_iteration, task)
	-- Following functions are on_full_scan*,
	-- but we probably did not complete the full scan,
	-- so we should check for cancellation here.
	if task.worker_cancelled then
	fiber.self():cancel()
	end
	if state then
	task.on_full_scan_success(task)
	else
	task.on_full_scan_error(task, err)

That's why I prefer to pass task for having consistent with others callbacks