HTTP static pages served to HTTPS resulting in mixed content issues

[kraftbj]

Reported via w.org slack: https://wordpress.slack.com/archives/core-images/p1458158727000084 supercache creates files for both protocols, i.e. if you go to the home page of a site on http, SC creates index.html, and if you visit using ssl, then you get index-https.html

but for posts/pages not the home page, if you first visit the site on http it creates /sitename/pagename/index.html

and if you then visit on https it ​doesn’t​ create another file, it just serves the http version

so all of your assets get blocked

[jom]

I wasn't able to reproduce this on my local Docker image (w/ nginx proxying) or my main sandbox. All pages and posts generated the appropriate index.html and index-https.html files and served them correctly. I also tested this on mobile and it produced index-https-mobile.html and index-mobile.html.

I also don't see any obvious changes to supercache_filename() or its callers that would have prevented this from happening. Looking at the Slack messages, I'm not sure if we ever tried to reproduce out of their large instance. Have we had other reports?