No way to install Julia signal handler when other dependency uses juliacall

[MilesCranmer]

Affects: JuliaCall

Describe the bug

Related to #219. There's currently a scenario with no possible workaround other than forcing a downstream user to set up environment variables to avoid a segfault.

Say that I import a package which uses serial Julia code. That package runs

from juliacall import Main as jl

to start Julia.

Then, I import a different package. That package calls multithreaded Julia code, and thus to avoid a segfault (https://juliapy.github.io/PythonCall.jl/stable/faq/#Is-PythonCall/JuliaCall-thread-safe?) instead performs the import instead as:

import os
os.environ["PYTHON_JULIACALL_HANDLE_SIGNALS"] = "yes"

from juliacall import Main as jl

However, at this point, the Julia runtime has already started. So now when I run a multithreaded portion of code, I will get a segfault, even though I implemented special workarounds in the multithreaded package.

How could I avoid this scenario? Indeed the user could always set up the environment variables themselves. But managing to figure this out from a random bus error will only be ~5% of users.

[MilesCranmer]

My temporary workaround is to put this code:

if "juliacall" in sys.modules:
    warnings.warn(
        "juliacall module already imported. Make sure that you have set `PYTHON_JULIACALL_HANDLE_SIGNALS=yes` to avoid segfaults."
    )

before the import statement.

[cjdoris]

Well firstly, packages shouldn't generally be setting global config such as that env var, though I understand why you're doing it.

Over in #219 I suggested adding a warning to set PYTHON_JULIACALL_HANDLE_SIGNALS=yes if Julia is started with multiple threads. Do you think this would be a sufficient warning?

Also, given Julia starts with 1 thread by default, presumably you're already telling your users to set PYTHON_JULIACALL_NUM_THREADS? In which case you can also tell them to set PYHON_JULIACALL_HANDLE_SIGNALS?