nginx location block comprehension and using passenger with named location blocks

Question

I have a couple of questions regarding my nginx configuration as it pertains to serving webp files as well as using named locations with try_files.

Current config:

server {
    listen 80;
    server_name  assets.manager manager;
    passenger_app_env production;
    passenger_ruby /home/web-server/.rvm/gems/ruby-2.2.1@manager/wrappers/ruby;
    passenger_enabled on;

    error_log  /home/web-server/web-applications/manager/current/log/nginx-error.log;
    root       /home/web-server/web-applications/manager/current/public;

    satisfy any;
    allow 127.0.0.1;
    allow 192.168.0.0/24;
    deny all;

    location ~ ^/assets/ {
        gzip_static on;
        expires     max;
        add_header  Cache-Control public;
        add_header  Last-Modified "";
        add_header  ETag "";
    }

    location ~* .+\.(?:png|jpe?g|gif)$ {
        if ($webp_suffix != "") {
            add_header Vary Accept;
        }
        try_files $uri$webp_suffix $uri =404;
    }
}

As it stands, nginx is not serving the webp files. If I were to place add_header X-Webp-Uri "$uri$webp_suffix"; inside the first location block, the header is added. If I were to put that in the second png/jpeg matching location block the header doesn't get set. It was my understanding that regular expression location blocks are sequential (i.e., it doesn't stop matching at the 1st match).

1) I want to serve webp images if present (my attempt at this is the 2nd location block). Would having a nested location block help in this circumstance? I want to set gzip_static, expires, etc. for ALL files in /assets/, but I only want to serve webp version if they exist for certain file extensions within /assets/.

2) On another topic, I want to serve static .html files if present. To do this (after looking up tutorials on the web) I need a combination of try_files and a named location block that points to an upstream application (Rails). However I can't seem to find out how to declare the upstream block if I'm using Passenger (installed using passenger-install-nginx-module). The only configurations I can find for a Passenger/Nginx setup is to use passenger_enabled on;

---

EDIT: I found a sample configuration; here's an example (this ignores problem #1):

server {
    listen 80;
    server_name  assets.staging.pos staging.pos;
    passenger_app_env staging;
    passenger_ruby /home/vagrant/.rvm/gems/ruby-2.2.1@pos/wrappers/ruby;
    passenger_enabled on;

    error_log  /home/vagrant/rails/staging.pos/log/nginx-error.log;
    root       /home/vagrant/rails/staging.pos/public;

    try_files $uri /cache/$uri /cache/$uri.html @app;

    location @app {
        proxy_set_header X-Forwarded-Proto http;
    }

    location ~* ^/images/.+\.(png|jpe?g)$ {
      if ($webp_suffix != "") {
        add_header Vary Accept;
      }

      try_files $uri$webp_suffix $uri =404;
    }
}

EDIT 2: I've discovered that nginx completely wipes out any instruction outside of the if block!

This will result in only the Vary Accept header being set:

server {
    location ~* ^/images/.+\.(png|jpe?g)$ {
        add_header X-Whatever "Yo";
        if ($webp_suffix != "") {
          add_header Vary Accept;
        }
    }
}

This will result in both headers being set:

server {
    location ~* ^/images/.+\.(png|jpe?g)$ {
        if ($webp_suffix != "") {
          add_header Vary Accept;
          add_header X-Whatever "Yo";
        }
    }
}

EDIT 3: So now it's even more befuddling. It's like any prior add_header that isn't within the last code block (location or if statement) is completely ignored. i.e., with the following, only the X-Whatever header is set:

location ~ ^/assets/ {
    gzip_static on;
    expires     max;
    add_header  Cache-Control public; # Ignored?!
    add_header  Last-Modified ""; # Ignored?!
    add_header  ETag ""; # Ignored?!

    location ~* ^/assets/.+\.(?:png|gif|jpe?g)$ {
        add_header X-Something "$uri$webp_suffix"; # Not ignored!
    }
}

Answer 1

I had to remove the Vary Accept header condition and simply always apply it. I don't know if that's good or bad but I don't have any other choice considering it just removes every other header I've applied! I also had to move the webp location block above the assets block and duplicate code which sucks.

Working configuration:

server {
    listen 80;
    server_name  assets.staging.pos staging.pos;
    passenger_app_env staging;
    passenger_ruby /home/vagrant/.rvm/gems/ruby-2.2.1@pos/wrappers/ruby;
    passenger_enabled on;

    error_log  /home/vagrant/rails/staging.pos/log/nginx-error.log;
    root       /home/vagrant/rails/staging.pos/public;

    # Try to return cached responses without hitting the app
    try_files $uri /cache/$uri /cache/$uri.html @app;

    location @app {
        proxy_set_header X-Forwarded-Proto http;
    }

    # Return webp images when possible
    location ~* ^/assets/.+\.(?:png|gif|jpe?g)$ {
        expires     max;
        add_header  Cache-Control public;
        add_header  Vary Accept;
        add_header  Last-Modified "";
        add_header  ETag "";

        try_files $uri$webp_suffix $uri =404;
    }

    # Regular asset headers
    location ~ ^/assets/ {
        gzip_static on;
        expires     max;
        add_header  Cache-Control public;
        add_header  Last-Modified "";
        add_header  ETag "";
    }
}