Cannot Call Pure Function From Multiple Threads

[Pebaz]

Hello, I'm the author of Nimporter and I'm trying to debug a multithreading issue (as described here for completeness but also happens without using Nimporter).

Put simply, calling any Nim function in another thread causes Python to crash.

Reproduction Steps

calc.nim:

import nimpy

proc add(a: int, b: int): int {.exportpy.} =
    return a + b

Compiled with: nim c --threads:on --app:lib --out:calc.pyd calc

Import and use in Python file:

import threading, calc

def foo():
    print(calc.add(1, 2))

threading.Thread(target=foo).start()  # Crashes Python

Starting the second thread is what causes the crash.

[yglukhov]

Just ran it on linux, and it doesn't crash. There seems to be a typo in your code target=calc which causes python exception

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python3.8/threading.py", line 932, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.8/threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
TypeError: 'module' object is not callable

But if corrected, the program runs just fine. So it might be something windows specific.

[Pebaz]

Ah, yes sorry for the copy paste error.
That sounds familiar, now that you say that it works on Linux, I believe that there may be a Nim flag I can set for this.
Thanks for looking into it!