Create a global font cache

I'm using rustybuzz and wasm-bindgen to typeset text on an HTML5 canvas. I would like to create a global font cache, indexed by Strings, so that I can load and parse font files once and then reuse them multiple times (cue comments about how global variables are bad... if anyone has a better way to do this, please let me know). Specifically, I want some variation of a HashMap<String, rustybuzz::Face> that I can access anywhere. I then want to expose a register_font function to the JavaScript side so I can load in ArrayBuffers, something like:

#[wasm_bindgen]
pub fn register_font(name: &str, font_data: &[u8]) {
  MY_FONT_CACHE_SOMEHOW.insert(
    name.to_string(),
    rustybuzz::Face::from_slice(&font_data, 0).unwrap()
  );
}

And, for completion, I'd like an internal get_font function to retrieve:

fn get_font(name: &String) -> rustybuzz::Face {
  MY_FONT_CACHE_SOMEHOW.get(name).unwrap()
}

I know I've got issues with variable lifetimes, I just don't know how to solve them. The rustybuzz::Face struct just references its internal data, it doesn't own it. wasm-bindgen doesn't support marking the incoming ArrayBuffer/[u8] on the register_font function as 'static, which seems to be one of the main problems. But I'm new to this whole rust thing. Anyone know how to do this (or a better way to do this)?

Solution

If you only add to the hash table and never delete you could copy and leak font_data to make it static:

#[wasm_bindgen]
pub fn register_font(name: &str, font_data: &[u8]) {
    let font_data: &'static [u8] = font_data.to_owned().leak();
    //...
}

If you want to be able to delete fonts from the cache you have to keep the font_data and the Face together. That is the infamous issue of a self-referential type.

You can go the unsafe way and keep a pointer to a dynamically allocated *const [u8] type, or you can use one of the many crates that try to solve this issue.

Currently my favourite is ouroboros: it would be something like this (untested):

#[self_referencing]
struct MyFace {
    font_data: Vec<u8>,
    #[borrows(font_data)]
    face: Face<'this>,
}

#[wasm_bindgen]
pub fn register_font(name: &str, font_data: &[u8]) {
    let face = MyFaceBuilder {
        font_data: font_data.to_owned(),
        face_builder: |font_data: &[u8]| Face::from_slice(font_data).unwrap(),
    }.build();
    MY_FONT_CACHE_SOMEHOW.insert(
        name.to_string(),
        face,
    );
}