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rust-fontconfig

Pure-Rust rewrite of the Linux fontconfig library (no system dependencies) - using allsorts as a font parser in order to parse .woff, .woff2, .ttc, .otf and .ttf

NOTE: Also works on Windows and macOS - without external dependencies!

Motivation

There are a number of reasons why I want to have a pure-Rust version of fontconfig:

  • fontconfig with all dependencies (expat and freetype) is ~190.000 lines of C (extremely bloated for what it does)
  • fontconfig, freetype, expat and basically any kind of parsing in C is a common attack vector (via maliciously crafted fonts). The Rust version (allsorts) checks the boundaries before accessing memory, so attacks via font files should be less common.
  • it gets rid of the cmake / cc dependencies necessary to build azul on Linux
  • fontconfig isn't really a "hard" library to rewrite, it just parses fonts and selects fonts by name
  • Rust has existing xml parsers and font parsers, just use those
  • It allows fontconfig libraries to be purely statically linked
  • Font parsing / loading can be easily multithreaded (parsing font files in parallel)
  • It reduces the number of necessary non-Rust dependencies on Linux for azul to 0
  • fontconfig (or at least the Rust bindings) do not allow you to store an in-memory cache, only an on-disk cache, requiring disk access on every query (= slow)
  • Potential no_std support for minimal binaries?

Now for the more practical reasons:

  • libfontconfig 0.12.x sometimes hangs and crashes (see issue)
  • libfontconfig introduces build issues with cmake / cc (see issue)
  • To support font fallback in CSS selectors and text runs based on Unicode ranges, you have to do several calls into C, since fontconfig doesn't handle that
  • The rust rewrite uses multithreading and memory mapping, since that is faster than reading each file individually
  • The rust rewrite only parses the font tables necessary to select the name, not the entire font
  • The rust rewrite uses very few allocations (some are necessary because of UTF-16 / UTF-8 conversions and multithreading lifetime issues)

Usage

use rust_fontconfig::{FcFontCache, FcPattern};

fn main() {
    let cache = FcFontCache::build(); let result =

    cache.query(&FcPattern {
      name: Some(String::from("Arial")),
      .. Default::default()
    });

    println!("font path: {:?}", result);
}

Performance

  • cache building: ~90ms for ~530 fonts
  • cache query: ~4µs

License

MIT