feature idea - cache callbacks

Migrated issue, originally created by jvanasco (jvanasco)

before trying to build this out, I wanted to propose these extensions to CacheRegion.get_or_create and CacheRegion.cache_on_arguments:

• callback_get_fn
• callback_create_fn

If these values are set, before returning the value from the cache it will be passed into the appropriate function as a filter (or a notification).

if GET and callback_get_fn:
       value = callback_get_fn(value)
elif CREATE and callback_create_fn:
       value = callback_create_fn(value)
return value

In some ways this is similar to a proxy, however this pattern would give a lot more flexibility when the cache generating functions have 'byproducts' of setting up miscellaneous data

Consider this example:

def expensive_function(self):
      self.foo = 'bar'
      return (1, 2)

def outer(self, args):
        @cache_on_arguments
        def inner():
              (a, b) = self.expensive_function()
              return (a, b)
        (a, b) = inner(args)

on the first run, self.foo is set; on the second run it is not.

when migrating functions to use dogpile, a lot of issues like this creep up. While the proper route is to avoid this pattern, that can take a lot of rewriting. If there was a mechanism such as a callback that could be used to derive that the value is cached -- then it would be relatively simple to write a few lines of code to ensure parity with whatever environment an uncached data generation would have created.

Start by reading CacheRegion.get_or_create and CacheRegion.cache_on_arguments, the two entry points named in the issue. Trace their cache-hit and value-creation paths, then determine how callback_get_fn and callback_create_fn should behave and verify that callbacks provide the requested state-restoration behavior without changing normal return values.