broken_gevent_timeout.py

import random
import gevent
from collections import defaultdict

class ReadError(Exception):
    pass

class Connection(object):
    def __init__(self):
        pass

    def cleanup(self):
        self.broken_state = False
    
    def read(self):
        # changes somes internal state
        self.broken_state = True
        # a request takes a while to com back
        time_to_wait = random.randrange(990, 1100) / 100000.0
        gevent.sleep(time_to_wait)
        # clean everything up!
        self.cleanup()
        # imagine that this sometimes raises ReadError
        # what could go wrong?


def safe_read(cxn):
    try:
        cxn.read()
    except ReadError:
        # We are careful! We know that `read()` can raise ReadError
        # so we catch that and clean up if it happens
        # you'll notice that this is what the Redis client library does -- it only catches exceptions
        # that the code it's calling can raise: https://github.com/redis/redis-rb/blob/master/lib/redis/client.rb#L238
        cxn.cleanup()

broken_results = defaultdict(int)
for _ in xrange(100):
    cxn = Connection()
    with gevent.Timeout(0.01):
        try:
            safe_read(cxn)
        except:
            # if there's a timeout, keep going -- everything should be fine right?
            pass
        broken_results[cxn.broken_state] += 1
print broken_results

# defaultdict(<type 'int'>, {False: 28, True: 72})
# oh no everything is broken