Lua非阻塞写日志

用nginx-lua搭的服务需要写日志，一般采用nginx自带的accesslog加logrotate脚本来实现，但是无法满足一次请求需要写多条日志的需求

之前用C和GO实现过类似的代码，思路是主进程写管道，子进程读管道并交由apache-rotatelogs写日志

这里用Lua也实现了一下--

-- Rlog is an non-blocking logging based on pipe.
-- Rotatelogs of Apache is recommanded to be the output of pipe.
-- Before using Rlog, you should install luaposix.
--

local posix = require("posix")
local os    = require("os")

-- input of pipe
local pipe_in

-- init rlog
-- @param pipe_cmd pipe operator and the reveiver command.
--  e.g. "| rotatelogs -l %Y%m%d%H.logfile 3600"
-- @return true when success, false when fail
local function init(pipe_cmd)
    -- if already inited, do nothing
    if pipe_in then return true end

    -- if pipe_cmd is not start with pipe operator, return false
    if string.sub(pipe_cmd, 1, 1) ~= "|" then return false end

    -- create pipe
    local pout, pin = posix.pipe()

    -- fork process to execute cmd
    if posix.fork() == 0 then
        -- close stdin
        io.close(io.stdin)

        -- set output of pipe as stdin
        posix.dup2(pout, posix.fileno(io.stdin))

        -- close input and output of pipe
        posix.close(pout)
        posix.close(pin)

        -- execute cmd
        os.execute(string.sub(pipe_cmd, 2, -1))
    end

    -- get input of pipe
    pipe_in = pin

    -- close output of pipe
    posix.close(pout)

    return true
end

-- write log
local function log(log_str)
    -- write log into pipe
    posix.write(pipe_in, log_str .. "\n")
end

return {
    init = init,
    log = log
}

下边是它的测试程序

local rlog = require("rlog")

-- init
ret = rlog.init("| rotatelogs -l %Y%m%d%H.log 3600")
if not ret then
    print("init fail")
end

-- log
rlog.log("Hello Rlog")

由于posix.write写管道是原子的，所以无须担心多进程或多线程写混乱的问题

不过这段代码依赖luaposix库，使用前需要安装一下