Replicates concurrency bug described in https://github.com/mitsuhiko/jinja2/issues/8 using two threads. `MyLRUCache` implements a fix using a lock. The first commandline argument is the amount of time to sleep between accesses to the cache. If set to 0, it triggers the bug almost right away. If set to 0.001 (the smallest nonzero value sleep will…

bug.py

from threading import Thread
from time import sleep, time
from random import choice, random

from jinja2.utils import LRUCache

class MyLRUCache(LRUCache):
	def __getitem__(self, key):
		self._wlock.acquire()
		try:
			return super(MyLRUCache, self).__getitem__(key)
		finally:
			self._wlock.release()

SIZE = 10
t0 = None
keys = 'abcdefghijklmnopqrstuvwxyz'
sleep_time = None

#d = LRUCache(SIZE)
d = MyLRUCache(SIZE)

def main(t):
	global sleep_time
	global t0
	
	sleep_time = t
	t0 = time()
	
	for _ in xrange(2):
		t = Thread(target = f)
		t.start()

def f():
	try:
		while True:
			k = choice(keys)
			if random() > 0.1:
				try:
					d[k]
				except KeyError:
					# This is ignorable, we might not have actually inserted this key
					pass
			else:
				d[k] = 1
			sleep(sleep_time)
	finally:
		print "Ran for {} seconds before failing".format(time() - t0)

if __name__ == '__main__':
	from sys import argv
	
	if len(argv) == 2:
		t = float(argv[1])
	else:
		t = 0
	
	main(t)