I am using python 2.7
I want to create a wrapper function around fcntl.flock() that will timeout after a set interval:
wrapper_function(timeout):
For Python 3.5+, Glenn Maynard's solution no longer works because of PEP-475. This is a modified version:
import signal, errno
from contextlib import contextmanager
import fcntl
@contextmanager
def timeout(seconds):
def timeout_handler(signum, frame):
# Now that flock retries automatically when interrupted, we need
# an exception to stop it
# This exception will propagate on the main thread, make sure you're calling flock there
raise InterruptedError
original_handler = signal.signal(signal.SIGALRM, timeout_handler)
try:
signal.alarm(seconds)
yield
finally:
signal.alarm(0)
signal.signal(signal.SIGALRM, original_handler)
with timeout(1):
f = open("test.lck", "w")
try:
fcntl.flock(f.fileno(), fcntl.LOCK_EX)
except InterruptedError:
# Catch the exception raised by the handler
# If we weren't raising an exception, flock would automatically retry on signals
print("Lock timed out")
Timeouts for system calls are done with signals. Most blocking system calls return with EINTR when a signal happens, so you can use alarm
to implement timeouts.
Here's a context manager that works with most system calls, causing IOError to be raised from a blocking system call if it takes too long.
import signal, errno
from contextlib import contextmanager
import fcntl
@contextmanager
def timeout(seconds):
def timeout_handler(signum, frame):
pass
original_handler = signal.signal(signal.SIGALRM, timeout_handler)
try:
signal.alarm(seconds)
yield
finally:
signal.alarm(0)
signal.signal(signal.SIGALRM, original_handler)
with timeout(1):
f = open("test.lck", "w")
try:
fcntl.flock(f.fileno(), fcntl.LOCK_EX)
except IOError, e:
if e.errno != errno.EINTR:
raise e
print "Lock timed out"
I'm sure there are several ways, but how about using a non-blocking lock? After some n attempts, give up and exit?
To use non-blocking lock, include the fcntl.LOCK_NB
flag, as in:
fcntl.flock(self.__lock_file.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
I'm a fan of shelling out to flock here, since attempting to do a blocking lock with a timeout requires changes to global state, which makes it harder to reason about your program, especially if threading is involved.
You could fork off a subprocess and implement the alarm as above, or you could just exec http://man7.org/linux/man-pages/man1/flock.1.html
import subprocess
def flock_with_timeout(fd, timeout, shared=True):
rc = subprocess.call(['flock', '--shared' if shared else '--exclusive', '--timeout', str(timeout), str(fd)])
if rc != 0:
raise Exception('Failed to take lock')
If you have a new enough version of flock you can use -E
to specify a different exit code for the command otherwise succeeding, but failed to take the lock after a timeout, so you can know whether the command failed for some other reason instead.