Multiprocessing: Parent/Child w/ GUI (tkinter)

Question

I am creating a parent program with UI that spawns multiple subprograms, also with GUI. All child processes need to talk to a parent (trying PIPE atm). Parent uses a QUEUE to terminate all processes.

EDIT: The programs run on Raspberry Pi 4 w/ Raspbian OS and python 3.7.3.

Main program: bridge.py

import tkinter as tk
from time import sleep
import multiprocessing as mp
import os
import sys

import simple

class Main_Comm():
    def __init__(self):
        self.kill_queue = mp.Queue()
        self.p_conn, self.ch_conn = mp.Pipe()
        print("MAIN: PIPE: child: {}".format(self.ch_conn))
        self.proc = mp.Process(target=simple.my_dev, \
                    args=(self.kill_queue, self.ch_conn, ))
        self.proc.start()

    def Stop_Devices(self):
        #self.kill_queue.put(True)
        self.proc.join()
        print("Wait for processes to finish ...")
        sleep(2)
        print("Device OFF!")

    def Info(self):
        print("Info: QUEUE {}, PIPE {}".format(self.kill_queue, self.ch_conn))

class MainApp_bridge(tk.Tk):
    def __init__(self, master=None, title="Default"):
        #super().__init__()
        tk.Tk.__init__(self)
        self.title(title)
        self.btn = tk.Button(self, text="QUIT", command=self.on_quit)
        self.btn.pack(padx=20, pady=20)

        self.communicator = Main_Comm()
        self.communicator.Info()

        self.mainloop()

    def on_quit(self):
        print("Sending termination message via QUEUE ...")
        self.communicator.Stop_Devices()
        sleep(1)
        print("Shutting down main tread, HAL ...")
        sleep(1)
        self.destroy()

def main():
    root_bridge = MainApp_bridge(title="BRIDGE")

if __name__ == "__main__":
    main()

# EOF

and one child (simple.py)

import tkinter as tk
import os
import random
from time import sleep
import sys

class MainApp_simple(tk.Tk):
    def __init__(self, parent=None, title="Device",
            FLAG=False, kq=None, chc=None):
        #super().__init__()
        tk.Tk.__init__(self)
        self.title(title)
        self.b_QUIT = tk.Button(self, text="QUIT", command=self.on_quit)
        self.b_QUIT.pack(side="top", padx=30, pady=30)
        self.window=self
        self.kq = kq
        self.chc = chc
        self.comm_agent = communicator( self.window, self.kq, self.chc )
        self.mainloop()

    def on_quit(self):
        print("CHILD: Quitting ...")
        self.destroy()

class Dev_comm():
    def __init__(self, win, kq, chc):
        self.kq = kq
        self.chc = chc
        self.win = win
        self.mypid = os.getpid()
        print("CHILD: PID is {} and PIPE is {}".format(self.mypid, chc))

def my_dev( kill_queue, child_comm ):
    root_simple = MainApp_simple(
            parent=None,
            title="CHILD", 
            FLAG=False, 
            kq=kill_queue, 
            chc=child_comm
            )

# EOF sim.py

Each programs work fine on their own. If I take out GUI from the bridge, it works. All together, however, I get this:

CHILD: MainApp - pre __init__ .... flushing
MAIN: PIPE: child: <multiprocessing.connection.Connection object at 0xb5989750>
Info: QUEUE <multiprocessing.queues.Queue object at 0xb5dc39d0>, PIPE <multiprocessing.connection.Connection object at 0xb5989750>
CHILD: Entered my_dev function ... flushing ...
XIO:  fatal IO error 25 (Inappropriate ioctl for device) on X server ":0.0"
      after 47 requests (47 known processed) with 2 events remaining.
X Error of failed request:  BadIDChoice (invalid resource ID chosen for this connection)
  Major opcode of failed request:  45 (X_OpenFont)
  Resource id in failed request:  0x2c00004
  Serial number of failed request:  114
  Current serial number in output stream:  120

I just cannot figure it out, Btw; flushing did not provide any new information. the error message begins with XIO...

First I thought it was related to polling the pipes and queues interfering with mainloop()... but apparently not.

Help is greatly appreciated.

Cheers, Radek

EDIT: I thought that perhaps there is some interference between two tk.Tk calls, but I can run multiple child processes with GUI as long as the parent runs in the terminal. Even pipes and queue worked... it's the parent GUI...

Answer 1

Jason Harper actually provided a solution: SPAWN instead of FORK.

The start up seems a bit slower, but everything runs as expected. Even polling the queue and pipes between process works fairly well.

Thank you. Radek