Execute code at exit

Question

In my code I have function like this:

def myfunc():
    # Don't do anything if there's an instance already
    if get_var('running') == 'true':
         return

    set_var('running', 'true')
    # In case things go wrong
    atexit.register(set_var, 'running', 'false')

    do_something()
    do_something_else()

    set_var('running', 'false')
    # Unregister handler because nothing bad happened
    atexit.unregister(set_var)

set_var sets variable that is contained in database.

The purpose of all those set_vars is to prevent multiple instances from running at the same time.

atexit handler works fine when program is being interrupted by Ctrl-C but doesn't when it's killed by system or something like that.

I know about signal but it doesn't allow to cancel handler.

How do I do that? Or how can change structure to achieve the same functionality?

Consider external monitoring. There is not much that a program can do if it receives e.g. `SIGKILL`. — Frédéric Hamidi, Jan 27 '15 at 17:25
@FrédéricHamidi Could you explain a bit more? How do I do that? — ivknv, Jan 27 '15 at 17:29
Basically, you write two programs. The first one performs your processing, the second one monitors the first and updates the database when it exits for whatever reason. — Frédéric Hamidi, Jan 27 '15 at 17:30

ivknv · Accepted Answer · 2015-01-28T14:21:29.313

I think I figured it out.

# Used to check if myfunc is running in current program
running_here = False

# Set 'running' variable inside database to 'false' if myfunc was running
# inside current program at the time of exiting or don't do anything otherwise
atexit.register(lambda: set_var('running', 'false') if running_here else None)
# Call atexit handler when SIGTERM is recieved by calling sys.exit
signal.signal(signal.SIGTERM, lambda x, frame: sys.exit(0))

def myfunc():
    global running_here

    # Don't do anything if there's an instance already
    if get_var('running') == 'true':
         return

    # Don't let multiple instances to run at the same time
    set_var('running', 'true')
    running_here = True

    do_something()
    do_something_else()

    # Allow other instances to run
    set_var('running', 'false')
    running_here = False

All I needed to do was just to make a handler that would not need to be canceled over and over again. I did that by adding global variable running_here.

When program is being terminated handler just checks if function is running in current program by checking running_here and if it's True then handler just sets variable running inside the database to 'false' so other instances won't start. If running_here is False it means that myfunc is not running and there's no need to reset running variable so it just quits.

Execute code at exit

1 Answers1