Python Daemon: checking to have one daemon run at all times

myalert.py

from daemon import Daemon
import os, time, sys

class alertDaemon(Daemon):
    def run(self):
        while True:
            time.sleep(1)

if __name__ == "__main__":
    alert_pid = '/tmp/ex.pid'

    # if pid doesnt exists run
    if os.path.isfile(alert_pid):            # is this check enough?
        sys.exit(0)

    daemon = alertDaemon(alert_pid)
    daemon.start()

Given that no other programs or users will create the pid file:

1) Is there a case where pid does not exists yet the daemon process still running?
2) Is there a case where pid does exists yet the daemon isnt running?

Because if answer is yes to at least one of the questions above, then simply checking for the existence of pid file isnt enough if my goal is have one daemon running at all times.

Q: If i have to check for the process then, I am hoping of avoid something like system call ps -ef and grep for the name of the script. Is there a standard way of doing this?

Note: the script, myalert.py, will be a cronjob

Solution

The python-daemon library, which is the reference implementation for PEP 3143: "Standard daemon process library", handles this by using a file lock (via the lockfile library) on the pid file you pass to the DaemonContext object. The underlying OS guarantees that the file lock will be released when the daemon process exits, even if its uncleanly exited. Here's a simple usage example:

import daemon
from daemon.pidfile import PIDLockFile

context = daemon.DaemonContext(
    pidfile= PIDLockFile('/var/run/spam.pid'),
    )

with context:
    main()

So, if a new instance starts up, it doesn't have to determine if the process that created the existing pid file is still running via the pid itself; if it can acquire the file lock, then no other instances are running (since they'd have acquired the lock). If it can't acquire the lock, then another daemon instance must be running.

The only way you'd run into trouble is if someone came along and manually deleted the pid file while the daemon was running. But I don't think you need to worry about someone deliberately breaking things in that way.

Ideally, python-daemon would be part of the standard library, as was the original goal of PEP 3143. Unfortunately, the PEP got deferred, essentially because there was no one willing to actually do the remaining work needed to get in added to the standard library:

Further exploration of the concepts covered in this PEP has been deferred for lack of a current champion interested in promoting the goals of the PEP and collecting and incorporating feedback, and with sufficient available time to do so effectively.