How I built a scriptable Lua extension engine inside my C++ activity tracker (and almost lost my mind to deadlocks)

Hey!

I've been building HPR, an open source activity tracker and window tracker written in C++23. It watches your active window, builds a history of where your time went, and runs locally with zero accounts and zero telemetry. Think of it as a lightweight ActivityWatch alternative with native Wayland support.

At some point I decided it wasn't enough to just track windows. I wanted people to be able to extend it. So I built a Lua extension engine into it. Here's how that went.

The basic idea

Every .lua file you drop into the extensions folder gets picked up automatically. HPR scans the directory recursively, loads each file into its own sol::state, registers the API, and spins up a dedicated thread for it. Each extension runs completely isolated from everything else. A slow or broken extension can't freeze the UI or corrupt the tracking loop.