cache_lookup

Lookup attributes in the cache, or retrieve them from the object if necessary.

If this is a ObjectCache then this will lookup the attributes there first if available, otherwise will read them from the object and update the cache.

If this is not a ObjectCache, then the attributes will simply be read from the object.

This may block, use the asynchronous version when this is not desirable

Parameters:

this	the object
attr_types	the types of attributes to update
cancellable	optional cancellation object
n_attr_types	the number of attribute types

Returns:

the attributes retrieved or null on failure