Offer cartridge role

[Totktonada]

It'll easy setup periodical expiration in a cartridge application.

[oleg-jukovec]

We had a pull request before. It's a simple role for tests:
#67

The role is not an independent unit. It just saves the states of expirationd's tasks. Tasks must be started from the client code manually.

So I propose an another approach:

1. Launching tasks from a configuration file.
2. Arguments for launching are similar as for expirationd.start + the role parameters.
3. The role parameter can be: restart or try to restore from previous state? Something else?

expirationd:
  first_role_name:
    space_id: "space_id"
    is_expired: "is_expired_func"
    options:
       args: "arg"
       force: true
       full_scan_delay: 1
       etc
    resume: true
  second_role_name:
    space_id: "space_id_second"
    is_expired: "is_expired_func_second"
    options:
       args: "arg"
       force: true
       full_scan_delay: 1
       etc
    resume: false

We can load functions by name from _G table:

local function load_function(func_name)
    if func_name == nil or type(func_name) ~= 'string' then
        return nil
    end

    local func = rawget(_G, func_name)
    if func == nil or type(func) ~= 'function' then
        return nil
    end
    return func
end

The role function stop will be very simple:

local function stop()
    for _, task in tasks_list do
        log.info('stop expirationd task: %s', task)
        expirationd.kill(task)
    end
    tasks_list = {}
    return true
end

validate_config and apply_config will be much more complicated. We must to validate very many parameters in validate_config. apply_config is a good place to start/realod tasks. Something like that:

local function apply_config(conf, opts) -- luacheck: no unused args
    local conf = conf[role_name] or {}

    stop()
    if opts.is_master then
        start(conf)
    end

    return true
end

I have two questions:

1. What additional parameters are needed for the role? Do we need to try to resume tasks, or is it enough just to start it? Do we need a parameter for this?
2. Do we need additional methods for configuration from a client code, as it is done in metrics.lua?

But I propose to start with a simple implementation. And add additional parameters/functions later.

[no1seman]

• register expirationd as a cartridge service for easy access to API: cartridge.service_get('expiratind')
• stop all tasks on stop
• optionally start/stop tasks on apply config according to is_master state