Parse files in parallel when possible

[ilevkivskyi]

The idea is simple: new parser doesn't need the GIL, so we can parse files in parallel. Not sure why, but the most I see is ~4-5x speed-up with 8 threads, if I add more threads, it doesn't get visibly faster (I have 16 physical cores).

Some notes on implementation:

• I use stdlib ThreadPoolExecutor, it seems to work OK.
• I refactored parse_file() a bit, so that we can parallelize (mostly) just the actual parsing. I see measurable degradation if I try to parallelize all of parse_file().
• I do not use psutil because it is an optional dependency. We may want to actually make it a required dependency at some point.
• It looks like there is a weird mypyc bug, that causes ast_serialize to be None sometimes in some threads. I simply add an ugly workaround for now.
• I only implement parallelization in the coordinator process. The workers counterpart can be done after Split type-checking into interface and implementation in parallel workers #21119 is merged (it will be trivial).

cc @JukkaL

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[mr-c]

Yes, len(psutil.Process().cpu_affinity()) is better everywhere except Darwin/macOS, where psutil doesn't support that; though I still suggest taking the minimum of that and os.cpu_count() as the later respects -X cpu_count and/or PYTHON_CPU_COUNT for Python 3.13+ users (especially containerized users).

If you don't want to add a psutil dependency yet, I recommend os.sched_getaffinity(0) which is how os.process_cpu_count() is implemented on Python 3.13+. (you should also still call os.cpu_count() and use it if it is smaller, for the same reasons as above).

[ilevkivskyi]

Yeah, I tried sched_getaffinity() but it is not available on Python 3.10 (which we still support). I guess we may need to write a separate helper with various fallbacks logic to make this ~reliable.

[mr-c]

Huh, I see os.sched_getaffinity() all the way back to Python 3.3: https://docs.python.org/3.3/library/os.html#os.sched_getaffinity

However,

They are only available on some Unix platforms

So maybe your platform didn't implement it until a later Python version.

Yeah, helper function + memoization is very helpful here