[Django] #26463: Allowing Callbacks/Handlers to be called on Cache entry Expiration

[Django]

#26463: Allowing Callbacks/Handlers to be called on Cache entry Expiration
------------------------------+------------------------
     Reporter:  oneTimePad    |      Owner:  oneTimePad
         Type:  New feature   |     Status:  new
    Component:  Core (Other)  |    Version:  master
     Severity:  Normal        |   Keywords:
 Triage Stage:  Unreviewed    |  Has patch:  1
Easy pickings:  0             |      UI/UX:  0
------------------------------+------------------------
 I read a suggestion about this here:
 [http://stackoverflow.com/questions/20866460/django-cache-backend-how-to-
 implement-callback-when-cache-timeout] .


 I have written up an implementation of it with some tests for
 FileBasedCache (filebased.py) and want to add it to locmem.py too (not
 done  yet).

 Here are code snippets from filebased.py that have been tested:
    The edits  are in set and _is_expired
 {{{
  def set(self, key, value, timeout=DEFAULT_TIMEOUT,
 version=None,handler=None):
         self._createdir()  # Cache dir can be deleted at any time.
         fname = self._key_to_file(key, version)
         self._cull()  # make some room if necessary
         fd, tmp_path = tempfile.mkstemp(dir=self._dir)
         renamed = False
         try:
             with io.open(fd, 'wb') as f:
                 expiry = self.get_backend_timeout(timeout)
                 f.write(pickle.dumps(expiry, pickle.HIGHEST_PROTOCOL))
                 #check if handler exists
                 if handler:
                   #write handler to pickle
                   f.write(pickle.dumps(handler,pickle.HIGHEST_PROTOCOL))
                 f.write(zlib.compress(pickle.dumps(value,
 pickle.HIGHEST_PROTOCOL)))
             file_move_safe(tmp_path, fname, allow_overwrite=True)
             renamed = True
         finally:
             if not renamed:
                 os.remove(tmp_path)


  def _is_expired(self, f):
         """
         Takes an open cache file and determines if it has expired,
         deletes the file if it is has passed its expiry time.
         """
         exp = pickle.load(f)
         if exp is not None and exp < time.time():
             #if there is a handler, call it
             try:
               handler = pickle.load(f)
               handler(exp)
             except pickle.UnpicklingError:
              #handler not added(no handler specified), so pickling error
 occurs
               pass
             f.close()  # On Windows a file has to be closed before
 deleting
             self._delete(f.name)
             return True
         return False
 }}}

 The handler has the following format

 {{{
         class Handler(object):
               def __call__(self,exp):
                     ...
 }}}

--
Ticket URL: <https://code.djangoproject.com/ticket/26463>
Django <https://code.djangoproject.com/>
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