http - 在 Nginx(和 lua)中提供大于最小文件大小的图像

Question

我正在尝试检查请求的文件是否超过特定大小(比如 250 字节)，如果是，则提供文件，否则提供默认图像。

但是，我得到了意想不到的结果。检查正文长度的行似乎没有得到尊重，并且 no_image_small.png 得到了服务。 res.status == 200 似乎有效，不过:

• 请求图片大小:17字节
• no_image_small.png 大小:3030 字节

nginx错误日志:

2014/12/01 11:52:57 [error] 6033#0: *1 subrequests cycle while processing "/images/blue_image_small.jpg", client: 127.0.0.1, server: images.dev, request: "GET /images/blue_image_small.jpg HTTP/1.1", subrequest: "/images/blue_image_small.jpg", host: "images.dev:8080"
2014/12/01 11:52:57 [error] 6033#0: *1 lua entry thread aborted: runtime error: rewrite_by_lua:2: failed to issue subrequest: -1
stack traceback:
coroutine 0:
    [C]: in function 'capture'
    rewrite_by_lua:2: in function <rewrite_by_lua:1>, client: 127.0.0.1, server: images.dev, request: "GET /images/blue_image_small.jpg HTTP/1.1", subrequest: "/images/blue_image_small.jpg", host: "images.dev:8080"

我的 nginx 配置:

server {
    listen 8080;
    server_name images.dev www.images.dev;
    root  /home/syz/dev/images/;


    location ~* ^/images/(.*_small.) {
        rewrite_by_lua '
            local res = ngx.location.capture(ngx.var.uri)
            if res.status == 200 then
                local blength = string.len(res.body)
                if blength > 250 then
                    ngx.say(res.body)
                    ngx.exit(ngx.OK)
                else
                    ngx.redirect("/images/no_image.small.png")
                end
            else
                ngx.redirect("/images/no_image_small.png")
            end
        ';
    }
}

Answer 1

您可以读取验证中的 Content-Length header (速度更快)，而不是捕获和读取响应正文长度。

我在生产中有类似的验证工作，有些像:

location ~*/img/.*\.(png|jpg|jpeg) {            
           set $max_image_size 15000;
           # default redirect if requested image too big
           error_page 412 = @default_image_loc; 

           #serving the content                   
           root  /local_server/root/;        

           #validating response
           header_filter_by_lua ' 
             local image_size = tonumber(ngx.header.content_length)                
             if image_size > tonumber(ngx.var.max_image_size) then
               ngx.log(ngx.NOTICE, string.format("invalid image size <%s>" , image_size))
               ngx.exit(412)
             end 
          '; 
}

location @default_image_loc {            
        try_files "/img/default_image.png" @none;            
}

访问日志将保持干净，因为 412 跳转是内部的。