nginx 配置add_header 'Access-Control-Allow-Origin' '*' 依然存在跨域问题

1.问题描述：
前端域名A 在POST请求后端域名为B 的一个接口时候请求成功时不存在跨域问题，请求失败时浏览器提示跨域。

解决：
当请求成功时，HTTP CODE 为200。而请求失败时HTTP CODE 为400， 此时add_header ‘Access-Control-Allow-Origin’ ‘*’ 配置无效！设置无论HTTP CODE 为何值时都生效需要加 always 。nginx版本>1.7.5时候无须加always。

add_header ‘Access-Control-Allow-Origin’ ‘*’ always;

nginx文档地址：
http://nginx.org/en/docs/http/ngx_http_headers_module.html

文档原文：

Syntax: add_header name value [always];
Default: —
Context: http, server, location, if in location

If the always parameter is specified (1.7.5), the header field will be added regardless of the response code.

2.附配置：后端为PHP时需要将 add_header ‘Access-Control-Allow-Origin’ ‘’ always; 增加在 location ~.php{ } 的模块内。以下配置中add_header ‘Access-Control-Allow-Origin’ '’ always; 写在location ~.php{ } 上面一行无效的原因（已测试，正确），文档有写：

There could be several add_header directives. These directives are inherited from the previous level if and only if there are no add_header directives defined on the current level.

当前层级无 add_header 指令时，则继承上一层级的add_header。相反的若当前层级有了add_header，就应该无法继承上一层的add_header。

server {
        listen 80;
        server_name localhost 127.0.0.1 demo.com;
        root /www;
        location ~ \.php {
        client_body_timeout 6s;
        	if ($request_method = 'OPTIONS') {
            	add_header 'Access-Control-Allow-Origin' '*' always;
            	add_header 'Access-Control-Allow-Credentials' 'true';
            	add_header 'Access-Control-Allow-Methods' 'GET, POST, PATCH, DELETE, PUT, OPTIONS';
             	add_header 'Access-Control-Allow-Headers' 'DNT,X-Mx-ReqToken,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,  Access-Control-Expose-Headers, Token, Authorization';
            	add_header 'Access-Control-Max-Age' 1728000;
            	add_header 'Content-Type' 'text/plain charset=UTF-8';
            	add_header 'Content-Length' 0;
            	return 204;
        	}
        	add_header 'Access-Control-Allow-Origin' '*' always;
        	fastcgi_pass    unix:/run/php7.0-fpm.sock;
        	include         snippets/fastcgi-php.conf;
        	include         fastcgi_params;
        	fastcgi_param   SCRIPT_FILENAME $document_root$fastcgi_script_name;
    	}
}