【高级技巧】信号量：并发访问的计数闸门封装

local function newSemaphore(total)
    local s = { left = total, wait = {} }
    function s.acquire(key)
        if s.left > 0 then
            s.left = s.left - 1
            return true
        end
        s.wait[#s.wait + 1] = key
        return false
    end
    function s.release()
        s.left = s.left + 1
        local nxt = table.remove(s.wait, 1)
        if nxt then
            s.left = s.left - 1
            return nxt
        end
        return nil
    end
    return s
end

local gate = newSemaphore(3)
local ok = gate.acquire("队伍甲")
print(ok, gate.left)
local nxt = gate.release()
print(nxt)