Support for debugging python multiprocessing

[pperanich]

Hi, I am trying to use dap-mode with debugpy to debug a simple python multiprocessing script. While the debugger seems responsive to breakpoints and debug navigation in the main process, the child process can never be reached. I will provide a simple test script below to demonstrate.

import multiprocessing
import traceback
import sys
import time

def sub_job_test():
    global trace_back_msg
    wait_time = 3 #random.randrange(0,13)
    time.sleep(wait_time)
    print("sub_job_test()")
    try:
        if wait_time < 4:
            raise FileExistsError
    except:
        trace_back_msg= "".join(traceback.format_exception(*sys.exc_info()))

if __name__ == "__main__":
    proc = multiprocessing.Process(target=sub_job_test)
    start_time = time.time()
    timeout = 7
    proc.start()
    proc.join(timeout=timeout)
    proc.terminate()

Lastly, I did try adding :subProcess t to my debugging configuration template, but this did not change the behaviour. See here for motivation.