ticket fields may be generated more than once

[Stephen Compall <stephen.compall@…>]

In this definition of get_ticket_fields:

def get_ticket_fields(self):
    """Returns the list of fields available for tickets."""
    # This is now cached - as it makes quite a number of things faster,
    # e.g. #6436
    if self._fields is None:
        self._fields_lock.acquire()
        try:
            self._fields = self._get_ticket_fields()
        finally:
            self._fields_lock.release()
    return [f.copy() for f in self._fields]

there is a race condition. Obviously it is difficult to test, but may be easily imagined by simply imagining two threads trying to execute the body of get_ticket_fields at exactly the same time.

1. Both threads test self._fields, discovering it is None, and entering the body of the if.
2. Both threads attempt to acquire the lock. Thread Jabba gets it first.
3. Jabba enters the try, calls _get_ticket_fields, the slow computation, and saves it.
4. Jabba releases the lock, so Thread Solo gets it.
5. Solo enters the try, calls _get_ticket_fields, the slow computation, and saves it.
6. And so on…

self._fields must be rechecked after acquiring the lock to avoid this race condition.

[Christian Boos]

Agreed, except for the point 6, as this race happens at most once (until the self._fields gets eventually cleared again).

It's nevertheless better to fix this, of course.

[Christian Boos]

Patch fixing the race condition, applies on r7853 0.11-stable branch

[Christian Boos]

Above patch applied in r7865. Thanks for noticing this issue and keep up the good work, it's much appreciated!