Improve juliacall async documentation

[charleskawczynski]

Hello Julia folks,

I recently to use Julia in a python project involving async function definitions, and came across some behavior / requirements that I think could be better documented. For example, it seems that precompiling methods may be needed (or perhaps this is related to GC during compiling) in async functions calls. Here is an example that hangs unless functions are precompiled:

import asyncio
from concurrent.futures import ThreadPoolExecutor
from juliacall import Main as jl
async def main(i: int):
    jl.seval("module MyMod; foo(x::Int, y::Int) = x+y+1; end")
    MM = jl.seval("MyMod")
    # MM.foo(0, 0)                            # fixes hang
    # jl.precompile(MM.foo, (jl.Int, jl.Int)) # fixes hang
    with ThreadPoolExecutor(1) as pool:
        future = pool.submit(MM.foo, i, 1)
        result = future.result()
    return result

result = asyncio.run(main(1))
print(f"Final result: {result}")

[cjdoris]

Oh I need to add big notices to the docs saying that any support for multithreading is experimental and subject to change.

One thing that I do know doesn't work very well is calling Julia from a Python thread that isn't the main thread. I think that as soon as Julia enters the task scheduler, it will never return to work on the Python thread, so that thread hangs. I think this is what's happening for you - so this is basically unsupported usage.

I guess it would be handy to include known cases where it doesn't work, such as this, in the FAQ.

[charleskawczynski]

I'm mostly concerned about concurrency, not necessarily multithreading.

So, are you saying that Julia can't be used with any backend that leverages python's asyncio / FastAPI? (that's many applications).

Strangely, I was able to get something to partly work, but I'm now a bit concerned with the potential fragility.

[cjdoris]

I see - you combined both in your example and I think the issue was with multithreading. AFAIK async concurrency works just fine with JuliaCall.

[charleskawczynski]

Ah, ok. Now that I'm thinking about it, in my real world example, I've removed the use of ThreadPoolExecutor and it works in some settings but not other. Maybe a parent process is launching some of this on multiple threads and I just can't use it for that reason. Bummer.