try_files doesn't work sometimes

[fuweichin@…]

My site uses HTML5 History state and now I need to configure nginx to support it. Nginx supports HTML5 History state with directive 'try_files' but there is a downside: 'try_files' doesn't try files according to acceptable types of a request, which may cause developers hard to find broken non-html links, e.g. a browser wants an image/css while the server may return a html. To overcome the downside, I'm trying to do some tricks.

site configuration

server {
        listen 80;
        listen 443 ssl;
        server_name qin-workstation.bldgos.net;

        ssl_certificate /etc/ssl/certs/bldgos.net.cer;
        ssl_certificate_key /etc/ssl/private/bldgos.net.key;

        location / {
                root /var/www/html;
                index index.html;
                set $fallback_file $uri/;
                if ($http_accept ~ text/html) {
                        set $fallback_file /index.html;
                }
                add_header Try-Files "$uri $fallback_file" always;   # just to debug
                try_files $uri $fallback_file;
        }
}

server root (ls /var/www/html)

index.html

request headers

GET /new-state HTTP/1.1
Host: qin-workstation.bldgos.net
Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9,en-US;q=0.8,en;q=0.7

response headers

HTTP/1.1 404 Not Found
Server: nginx/1.15.7
Date: Thu, 10 Jan 2019 09:37:38 GMT
Content-Type: text/html
Content-Length: 555
Connection: keep-alive
Try-Files: /new-state /index.html

the problem
Now that try_files: $uri $fallback_file is equivalent try_files: /new-state /index.html for the request, why nginx returns 404 and not to try file '/index.html'?

my workaround to the problem

                set $fallback_file /index.html;
                if ($uri ~ "\.(js|css|png|jpg|gif|ico)$") {
                       set $fallback_file $uri/;
                }
                try_files $uri $fallback_file;

conclusion
Since 'try_files' can only be put into scope 'server' and 'location', but not 'if', I guess that the processing of 'try_files' may be prior to 'if'.

[Maxim Dounin]

The try_files directive is not inherited into the if block, and hence not used when the if matches, see ticket #86, in particular this comment.

Also note that using

set $fallback_file $uri/;
try_files $uri $fallback_file;

won't work, as trailing "/" as a flag to test directory existence is only recognized directly in the try_files arguments, not when it comes via a variable.