Threadgroup: bug concerning the wait/stop methods

The wait method does not always wait for all the (green)threads to be processed and the stop method does not always stop all threads.

Reason: in the method we iterate over the self.threads list. The problem is that this list can potentially be concurrently edited by the _thread_done callbacks, that remove some items in the list and thus can fool the iterator in the wait method.

1. Below a basic "test" to reproduce the problem (the reason why I have imbricated threadgroups is that it is the only way I managed to reproduce the issue. Statistically, I had to wait for around one minute to see the issue happen: END STUFF printed after the WAIT STUFF things).

2. Attached a correction proposal: one line in the ThreadGroup.wait/stop methods, shallow copy of the list before we iterate over it.
In a thread environment, I'm not even sure this would really fix the bug: indeed, if the shallow copy itself was not thread-safe, we would have to synchronize the _thread_done callbacks with the copy.
In a green thread environment, I may be wrong, but I think the shallow copy fixes the issue: we are saved by the fact that there won't be any greenthread switching during the shallow copy (no IO, nor explicit greenthread switching method called). The copy is thus greenthread safe

#######################################################################################
#TEST
#######################################################################################
#!/usr/bin/env python

import eventlet
eventlet.monkey_patch()
import random
from openstack.common import threadgroup

class A(object):
    def __init__(self):
        #self.thread_pool = eventlet.GreenPool()
        self.tg = threadgroup.ThreadGroup(100)

    def test1(self, i):
        eventlet.sleep(random.randint(1,3))
        print "END TEST %s" % i

    def run_test(self):

        while True:
            print "#"*100
# threads = []
            for i in range(0,100):
# threads.append(self.thread_pool.spawn(self.test1, i))
                self.tg.add_thread(self.test1, i)
            self.tg.wait()
# for thd in threads:
# thd.wait()
            print "WAIT SELF TG"
            eventlet.sleep(3)

if __name__ == '__main__':
    a = A()
    tg = threadgroup.ThreadGroup()
    tg.add_thread(a.run_test)
    tg.wait()