won't transform.context get clobbered here:

This is mostly safe with the GIL, so long as python doesn't switch to another thread that also owns a reference to transform, but in free-threading with the sort of parallel thread pool setup you have here it's much more likely to happen.

In general - how do you want to go about adding multithreaded tests? It looks like there's a decent amount of stateful objects that get passed around and updated. How much should we worry about threads simultaneously updating stateful objects?

oh yeah absolutely - we'll probably have to change how state is passed around between threads. In fact, it might be better to just make a new transformer instance for each file and only pass immutable data structures between threads/processes.

In general - how do you want to go about adding multithreaded tests?

We could run the test suite with something like unittest-ft, but the main thing I'd look for is the concurrent execution of the same set of visitors/transformers on multiple input files (basically what this PR sketches out), and then any bugs we find we can write targeted tests. How does that sound?

How does that sound?

Sounds great, that's more or less what I've been doing with pyca/cryptography this week: pyca/cryptography#12555

Another thing I learned there is moving code form pure python to Rust can also be a way to avoid thread safety issues. If we move state from Python to Rust then we have much more control over concurrent multithreaded access.

I'll look at this next week - thanks for merging my other PR!