fix(python): release GIL on most operations

[adriangb]

Fixes #2269

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[adriangb]

@ion-elgreco could you kick off CI please? It might be worth allowing repeat contributor's to run CI automatically.

Never mind, it was just stuck.

[davidhewitt]

Although I am not familiar with the delta-rs codebase, @adriangb suggested I might want to take a quick look at this given the pain with blocking on IO and holding the GIL at the same time.

For what it's worth, I agree that it's a problem and this PR is the current solution. It does sadden me a bit that it's so verbose to have to do this, though.

Upstream in PyO3 we're looking at a #[pyo3(allow_threads)] attribute on functions which will do this for you behind the scenes - PyO3/pyo3#3610

There's also the question of more radical changes like automatically releasing the GIL when none of the input arguments need the GIL but there's a performance cost to releasing the GIL so I think it needs to be user choice.

Finally, I see that you use futures here so maybe there's a good argument that more investment in PyO3's async story would solve this in a different way by allowing you to make these functions async and allow a Python reader to await them. There are definitely complexities in mixing Rust and Python async runtimes so that's not an easy solution either.

[ion-elgreco]

@davidhewitt @adriangb was quite busy this week, so hadn't gotten time to look at until now : )

Yeah it's fine to release the GIL where possible, there was never really a need for it before, so I guess that's why it wasn't brought up outside of that other PR which wasn't finished.

@davidhewitt I think a derive allow_threads is definitely useful if the whole function can release the GIL.

I am not sure, what pyo3 async would provide us though

[ion-elgreco]

Thanks! @adriangb