lsp/frontend/utils: Thread safety

[flofriday]

@jaraonthe-dot-net recently ran into problems running multiple instances of OpenVADL in multiple threads causing thread safety issues.

This was mostly with the cache of Type.bits which is shared and multiple concurrent insertions causing issues.
Now @Jozott00 I think we often do caching like this across OpenVADL, right?

And @jaraonthe-dot-net can you quickly go into if could be possible to only ever invoke a single instance of the OpenVADL compiler?

[jaraonthe-dot-net]

And @jaraonthe-dot-net can you quickly go into if could be possible to only ever invoke a single instance of the OpenVADL compiler?

No, because different (independent) files may be compiled at the same time. Further, if a file is in the process of being edited, new versions of it may be compiled while an old version is not done compiling yet (with #995 the old compilation run should be stopped, but I'm not sure how clean/immediate that will be).

[Jozott00]

I see, so we have to make the caching thread-safe.
While I'm not 100% sure, I don't think that there are a lot of places where caching isn't bound to a instance/lifecylce of a specification. I think type construction is pretty unique in that sense.

[flofriday]

I think there is also the Deferred Diagnostic Store, which might be a bit harder to solve but otherwise I also don't think there are any other instances in the frontend.