How to Gracefully Handle SIGTERM Signals in Python Daemons?

Graceful Handling of SIGTERM Signal

In daemonic operations, terminating a process using SIGTERM (TERM) signal can abruptly interrupt critical tasks. To address this, let's explore how to gracefully handle this signal.

Problem Statement

Given a simple daemon written in Python:

def mainloop():
    while True:
        # Perform important jobs
        # Sleep

daemonizing it using start-stop-daemon sends SIGTERM (TERM) upon termination. If the signal is received during a critical operation (e.g., step #2), it terminates immediately.

Using signal.signal()

Attempts to handle the signal event using signal.signal(signal.SIGTERM, handler) can also interrupt current execution and redirect control to the handler.

Solution: Separated Thread for Signal Handling

To avoid interrupting current execution, we can create a separated thread to handle the TERM signal. This thread can set shutdown_flag = True, allowing the main loop to gracefully exit.

Implementation

Here's a class-based solution for graceful signal handling:

import signal
import time

class GracefulKiller:
  kill_now = False
  def __init__(self):
    signal.signal(signal.SIGINT, self.exit_gracefully)
    signal.signal(signal.SIGTERM, self.exit_gracefully)

  def exit_gracefully(self, signum, frame):
    self.kill_now = True

if __name__ == '__main__':
  killer = GracefulKiller()
  while not killer.kill_now:
    time.sleep(1)
    print("doing something in a loop ...")
   
  print("End of the program. I was killed gracefully :)")

Benefits

This solution allows the daemon to handle SIGTERM gracefully, ensuring that critical operations can complete before termination. It also provides a clean and maintainable implementation for handling signals in multi-threaded environments.

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