How to ensure that the child process also terminates after killing the parent process via signal in Python?

Python signal processing: gracefully terminate parent and child processes

In Python multi-process programming, after the parent process is terminated with a signal, the child process may continue to run, which usually requires a more granular process management strategy. This article discusses this issue and provides solutions.

Problem description

Suppose a.py creates a parent process and a child process, and the parent process ID is written to the file. b.py reads this ID and sends a termination signal (SIGTERM). However, after the parent process terminates, the child process may continue to run.

The following is the sample code (slightly different from the original text, it is more concise and easy to understand, and fixes the error in the original code):

a.py:

 import multiprocessing
import os
import signal
import time

def child_process():
    While True:
        print("Subprocess is running...")
        time.sleep(1)

if __name__ == "__main__":
    child = multiprocessing.Process(target=child_process)
    child.start()
    with open("pidfile.txt", "w") as f:
        f.write(str(os.getpid()))
    child.join() # Wait for the child process to end print("parent process end")

b.py:

 import os
import signal

try:
    with open("pidfile.txt", "r") as f:
        pid = int(f.read())
        os.kill(pid, signal.SIGTERM)
        print(f"Send SIGTERM signal to process {pid}")
except FileNotFoundError:
    print("pidfile.txt not found")
except Exception as e:
    print(f"Error occurred: {e}")

Solution: Utilize process groups

The key to solving this problem is to understand the concept of process groups. The parent process and its child process belong to the same process group. By sending signals to the process group, it is ensured that all processes receive signals.

Improved code:

a.py:

 import multiprocessing
import os
import signal
import time

def child_process():
    While True:
        print("Subprocess is running...")
        time.sleep(1)

if __name__ == "__main__":
    child = multiprocessing.Process(target=child_process)
    child.start()
    pgid = os.getpgid(0) # Get the current process group ID
    with open("pidfile.txt", "w") as f:
        f.write(str(pgid))
    child.join()
    print("Parent process ends")

b.py:

 import os
import signal

try:
    with open("pidfile.txt", "r") as f:
        pgid = int(f.read())
        os.killpg(pgid, signal.SIGTERM) # Send signal to process group print(f"Send SIGTERM signal to process group {pgid}")
except FileNotFoundError:
    print("pidfile.txt not found")
except Exception as e:
    print(f"Error occurred: {e}")

By using os.getpgid(0) to get the process group ID and write the process group ID to the file, b.py uses os.killpg() to send a SIGTERM signal to the entire process group to ensure that both the parent and child processes are terminated cleanly. In addition, child.join() in a.py ensures that the parent process waits for the child process to exit before exiting, avoiding race conditions. Finally, the code also performs more robust exception handling.

This improved solution is more reliable, avoids potential problems in the original code and provides a clearer code structure.

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