Enable HTTP headers to allow browser caching of assets (Meta Issue)

[sabine]

Problems with caching of assets (CSS, JavaScript, Fonts) are mentioned in these issues:

• Rendering discrepancies and CSS caching #704
• OCaml Playground's large bundle size #838
• Font loading is slow #797

Underlying problem: We cannot currently enable asset caching via HTTP headers because assets are being served under their exact filenames.

What needs to be done: serve assets under filenames that include a hash (file content or last modified time). This enables us to allow caching via HTTP headers indiscriminately while still maintaining the ability to make changes that will reach our users.

Idea: use a PPX that, at compile time hashes the file content (or inspects the last-modified time) and copies the file accordingly, then returns the path of the hash-named file. Then, we can use, e.g. [%asset "css/main.css" ] in our templates.

[sabine]

In aantron/dream#106 @thangngoc89 already looked into what the experience with a PPX for static assets rewriting/copying would look like.

Here's a radically simple idea that would work for our purposes:

What if we use a PPX that merely returns the content hash of an asset.
We can use this PPX to preprend the hash to the path: /{hash}/{asset_path}.
Then, all we need to do is serve the asset under all paths that match /*/{asset_path}.

[sabine]

Simpler idea:

We already use ocaml-crunch to generate assets.ml which exposes all the assets in-memory via mirage-kv as a read-only store.

mirage-kv provides file digests.

All we need to do:

1. make the digest part of the URL under which an asset file is being served.
2. every time we place an asset URL in a template, compute the correct URL by looking up the digest