feature(core): implement core.sleep

[dabue]

fix #2170

Use a small time interval (such as 1s) to complete a relatively long sleep behavior, so that the worker process can be shut down gracefully.

local max_sleep_interval = 1

local function sleep(sec)
    if sec <= max_sleep_interval then
        ngx_sleep(sec)
        return
    end
    ngx_sleep(max_sleep_interval)
    if exiting() then
        return
    end
    sec = sec - max_sleep_interval
    sleep(sec)
end

[membphis]

LGTM, except for minor code style

[moonming]

Is there a long sleep in the project?

[moonming]

This change seems to be of little use to the current code

[dabue]

This change seems to be of little use to the current code

Among the current code, it really only work better when the worker process being shut down at the time of triggering request limiting and delaying processing. And we can keep it as a better way to sleep for the future usage scenarios.

[membphis]

I think this feature is useful.

When the APISIX instance restarts, we need to exit sleep as soon as possible.
Then the Nginx process can be gracefully shut down immediately, and free the system resources asap.

[membphis]

@dabue many thx