Simon Willison’s Weblog

Reviving PyMiniRacer (via) PyMiniRacer is “a V8 bridge in Python”—it’s a library that lets Python code execute JavaScript code in a V8 isolate and pass values back and forth (provided they serialize to JSON) between the two environments.

It was originally released in 2016 by Sqreen, a web app security startup startup. They were acquired by Datadog in 2021 and the project lost its corporate sponsor, but in this post Ben Creech announces that he is revitalizing the project, with the approval of the original maintainers.

I’m always interested in new options for running untrusted code in a safe sandbox. PyMiniRacer has the three features I care most about: code can’t access the filesystem or network by default, you can limit the RAM available to it and you can have it raise an error if code execution exceeds a time limit.

The documentation includes a newly written architecture overview which is well worth a read. Rather than embed V8 directly in Python the authors chose to use ctypes—they build their own V8 with a thin additional C++ layer to expose a ctypes-friendly API, then the Python library code uses ctypes to call that.

I really like this. V8 is a notoriously fast moving and complex dependency, so reducing the interface to just a thin C++ wrapper via ctypes feels very sensible to me.

This blog post is fun too: it’s a good, detailed description of the process to update something like this to use modern Python and modern CI practices. The steps taken to build V8 (6.6 GB of miscellaneous source and assets!) across multiple architectures in order to create binary wheels are particularly impressive—the Linux aarch64 build takes several days to run on GitHub Actions runners (via emulation), so they use Mozilla’s Sccache to cache compilation steps so they can retry until it finally finishes.

On macOS (Apple Silicon) installing the package with “pip install mini-racer” got me a 37MB dylib and a 17KB ctypes wrapper module.