Kill all child processes on program exit. Also catches (sets up a hande) the SIGTERM signal to process it gracefully. Python. Linux/Posix only.

KillAllChildProcessesOnExit.py

#!/usr/bin/python3
# It is for Python3 but may work for modern 2.x versions as well.

import os, signal, atexit

# Signletone which is made as a class with static methods.
# The "init()" method must be called before any child process are created.
# "killAllChildrenOnce" or "killAllChildren" methods could be called 
# any time to kill all child processes.
#   Creates a new process group during initialization and uses it to send
# the SIGTERM signal to all processes in that group on exit.
class KillAllChildrenProcesses:
	__initComplete=False # It can be initialized only once - this is the flag.
	__childrenKilled=False # Childrens  initialized only once - this is the flag.
	
	@classmethod
	def __atExit(cls):
		# Disable SIGTERM handler.
		signal.pthread_sigmask(signal.SIG_BLOCK, (signal.SIGTERM,))
		# Kill all children processes.
		cls.killAllChildrenOnce()
	#------------------------------------------------------------------------
	
	# Kills all child processes. Does it only on the first call. 
	# Does nothing on subsequent calls. 
	@classmethod
	def killAllChildrenOnce(cls):
		if cls.__childrenKilled: return
		cls.__childrenKilled=True
		cls.killAllChildren()
		os.killpg( os.getpid(), signal.SIGTERM )
	#------------------------------------------------------------------------

	# Kills all child processes. Does it every time is called. 
	@classmethod
	def killAllChildren(cls):
		# Kill all process in out process group. 
		os.killpg( os.getpid(), signal.SIGTERM )
	#------------------------------------------------------------------------
	
  # Initializes this singletone. Does it only once on the first call.
	@classmethod
	def init(cls):
		if cls.__initComplete: return
		cls.__initComplete = True
		os.setsid() # Create our own process group.
		atexit.register(cls.__atExit) # Register an exit handler.
	#------------------------------------------------------------------------
#----------------------------------------------------------------------------

# The TERM signal handler.  
def onSIGTERM( signum, frame ):
	# Must be executed only once - set the "executed" attribute as a flag.
	if hasattr(onSIGTERM, "executed"): return
	onSIGTERM.executed = True # Create a flag attribute - we've been allready executed.
	# Disable SIGTERM handling - just for sure.
	signal.pthread_sigmask(signal.SIG_BLOCK, (signal.SIGTERM,))
	# Replace the next line with custom code if you want other actions on SITERM. 
	quit()
#-----------------------------------------------------------------------------

if __name__ == '__main__' :
	# Setup the custom SIGTERM handler - might be not necessary.
	signal.signal(signal.SIGTERM, onSIGTERM)
	# Only for non-blocking code!
	# Uncomment the next line if you want SIGTERM not to interrupt system calls - 
	#signal.siginterrupt(signal.SIGTERM, False)
  
	# Initialize our singleton - all children will be killed on exit since now. 
	KillAllChildrenProcesses.init()
#-----------------------------------------------------------------------------

You cannot do

os.setsid() # Create our own process group.

instead, use

os.setpgrp()  # Create our own process group.