Lose some data when use lua-resty-upload to write to a file.

[itsre]

When I upload a file via lua-resty-upload ,I find the lastest uploaded file always lose some data,but when I upload another file, the previous file seems ok,it seems something has been buffered and not flush to the disk immediately。

    local upload = require "resty.upload"
    local chunk_size = 8192
    local form = upload:new(chunk_size)
    local uuid = require("resty.uuid")
    local file
    local currenttime = os.time()
    local filename = uuid().."-"..pid
    local file_name = uploadpath..filename
    if form then
    while true do
        local typ, res, err = form:read()
        if not typ then
              ngx.say(cjson.encode({code=1,message="nofile upload"}))
             return
        end

            if file_name then
                file = io.open(file_name, "a+")
                if not file then
                    ngx.say(cjson.encode({code=2,message="file save failed"}))
                    return
                end
            end


         if typ == "body" then
            if file then
                file:write(res)
            end

        elseif typ == "part_end" then
            file:close()
            file = nil
            file_name = nil
            ngx.say(cjson.encode({code=0,url=ngx.var.scheme.."://"..ngx.var.host.."/"..filename}))
            filename = nil
            return

        elseif typ == "eof" then
            break

        else
            -- do nothing
        end

    end
    else
        ngx.say(cjson.encode({code=1,message="nofile upload"}))
        return
    end

I call file:flush() before file:colse() ,there is no effect