Ensure ThreadIdent's lock is always released; improve comment PYTHON-509

Author: A. Jesse Jiryu Davis

pymongo/thread_util.py

@@ -73,13 +73,17 @@ class ThreadVigil(object):
  pass
 
  def _make_vigil(self):
- # Assigning to a threadlocal isn't thread-safe in Python <= 2.7.0
+ # Threadlocals in Python <= 2.7.0 have race conditions when setting
+ # attributes and possibly when getting them, too, leading to weakref
+ # callbacks not getting called later.
  self._lock.acquire()
- vigil = getattr(self._local, 'vigil', None)
- if not vigil:
- self._local.vigil = vigil = ThreadIdent.ThreadVigil()
+ try:
+ vigil = getattr(self._local, 'vigil', None)
+ if not vigil:
+ self._local.vigil = vigil = ThreadIdent.ThreadVigil()
+ finally:
+ self._lock.release()
 
- self._lock.release()
  return vigil
 
  def get(self):