script: add clock module

[myzhan]

Tring to add a clock module, so I can write per-thread request rate limiter in lua, with the global delay function.

[myzhan]

Aslo, I can use ffi.

local ffi = require("ffi")

ffi.cdef[[
    typedef long time_t;
    typedef int clockid_t;

    typedef struct timespec {
            time_t   tv_sec;        /* seconds */
            long     tv_nsec;       /* nanoseconds */
    } nanotime;
    int clock_gettime(clockid_t clk_id, struct timespec *tp);
]]

function get_current_time_in_ms()
    local pnano = ffi.new("nanotime[?]", 1)
    -- CLOCK_REALTIME  -> 0
    -- CLOCK_MONOTONIC -> 6
    ffi.C.clock_gettime(6, pnano)
    return pnano[0].tv_sec * 1000 + pnano[0].tv_nsec/1000000
end

function setup()
    local prev_time = get_current_time_in_ms()
end

local MAX_RPS = 2000
local current_rps = 0
local prev_time = 0

function delay()
    local now_ms = get_current_time_in_ms()
    local elapsed = now_ms - prev_time
    if elapsed > 1000 then
        prev_time = now_ms
        current_rps = 0
        return 0
    else
        current_rps = current_rps + 1
        if current_rps <= MAX_RPS then
            return 0
        end
        -- return sleep time in millis
        local sleep_time = 1000 - elapsed
        -- convert ffi's cdata into lua number
        return tonumber(sleep_time)
    end
end