Add a bust_browser_cache flag to the static template tag

[David Kerkeslager]

The proposal is to add a bust_browser_cache flag to the static template tag using a similar syntax to passing context variables to the include template tag. It would look something like this:

{% static 'css/style.css' with bust_browser_cache=True %}

The flag would default to False to avoid breaking existing usages of the static tag.

What flag does is add a cache_hash get parameter to the URL of the static file, with a hash of the static file. So if the content of css/style.css looks like:

p {
    color: magenta;
}

The generated URL will look like /static/css/style.css?cache_hash=0756793258f5ea00cda3e445de6bb595.

If the content of css/style.css changes to:

p {
    color: magenta;
}
p::after {
    content: "Sorry about this awful color.";
}

Then the link would be /static/css/style.css?cache_hash=c3aebd6286841ce8f4d76aa8d3eba487.

This is not a security feature, so I think it would be best to use MD5 because it is fast.

Instead, the purpose of this is to allow us to serve static files with Cache-Control: immutable or Cache-Control: max-age=<some large number> without fear. As long as the content of the file does not change, the URL's cache_hash will not change, so the browser will use the cached version. But if the content of the static file changes, the cache_hash parameter will change, causing the browser to request the new version of the file. This gets us the best of both worlds: we maximize usage of browser cache for static files, while never having to worry that our users are seeing stale versions of static files.

It may make sense to store the MD5 hash in Django's cache so that it doesn't get calculated every time.

[David Kerkeslager]

I can implement this ticket, but I haven't contributed to Django before, and looking at the relevant source code it's going to take me a little while to figure out how to do it. If someone else wants to do it who is more familiar with the codebase, it would probably get done quicker.