#!/usr/bin/python3

import fcntl
import sys
import time

def library_function():
    # library function opens its own /var/tmp/testing file descriptor
    # We have no control over what library_function does
    # We have the lock so we expect this to happen
    with open('/var/tmp/testing', 'ab') as fd:
        fd.write(b'some data')
    # library_function closes its /var/tmp/testing file descriptor
    # Because we used lockf, we lose the file descriptor unexpectedly here

call_library_function = False
if len(sys.argv) >= 2:
    print('calling library function in this run')
    call_library_function = True

with open('/var/tmp/testing', 'ab') as fd:
    print('long-hold acquired lock')
    fcntl.lockf(fd, fcntl.LOCK_EX)
    if call_library_function:
        library_function()
    time.sleep(5)
print('long-hold exiting, expect to lose the lock when it exits')