nginx add_header not working

See this conf first:

location /i/ {

add_header X-Frame-Options "SAMEORIGIN" always;
if ($uri ~ "^/i/([0-9a-zA-Z]*)([/]{0,1})$") {
    set $id $1;
    add_header Set-Cookie "id=$id; Max-Age=60;" always;
#    return 404;
}
rewrite /i/(.*) /index.html last;

}

Then using curl http://test.com/i/abc -I
What happened about these code?
We CANNOT received any header by our definition.

What happened when uncommit return 404?
We CAN received Set-Cookie in the response header and 404-Not found.

为什么add_header不工作了？？
Interesting...

so...

See this article:
https://juejin.im/post/5c602a21f265da2dc00632be
And the official document: http://nginx.org/en/docs/http/ngx_http_headers_module.html

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.

There really do not have any sample on the official document unless a equivocal introduction.
I am a user and really do not have time to read the source code, I think by the author's and nginx's view, the if-block is an independent context, so the code outside if and inside if is not the same level.
Fortunately, the set_header inside if haven't been destroyed, and we can return it at least. The right code is here:

if ($uri ~ "^/i/([0-9a-zA-Z]*)([/]{0,1})$") {
    set $id $1;
    add_header X-Frame-Options "SAMEORIGIN" always;
    add_header Set-Cookie "id=$id; Max-Age=60;" always;
}

AND rewrite will jump our context into another context!
We should change the last to break to stem the jump.
rewrite /i/(.*) /index.html break; Will work fine.

我觉得这很坑，我是真真的仔细看了nginx的文档，它所谓的current level, previous level，就算if内外，都不算是一个level。
另外不小心用了rewrite last，跳走了，同样会销毁我们辛勤培养的add_header。。。