nginx: redirect everything from http to https, except one url-pattern

Question

I have a website which should only be reachable over HTTPS except one URL-pattern (because I have on some pages http-iframe's and I would like to avoid security warnings)

E.g. this pages should be redirected to https:
http://example.com
http://example.com/a/this-is-an-article
http://example.com/v/this-is-a-video

This pages should not be redirected to https (or should be redirected form https to http)
http://example.com/l/page-with-unsafe-iframe
http://example.com/l/other-page-with-unsafe-iframe

Answer 1

If the iframe pages are always in the same directory, simple prefix locations could be used.

server {
    listen 443;

    location /l/ {  # redirect https iframe requests to http server
        return 301 http://$server_name$request_uri;
    }
    # ...
}

server {
    listen 80;

    location / {  # the default location redirects to https
        return 301 https://$server_name$request_uri;
    }

    location /l/ {}  # do not redirect requests for iframe location
    # ...
}