Add private helper to avoid IO on each IndexFile lookup.

Alban Browaeysrequested to merge
prahal/python-apt:fix-io into master

Cache the IndexFile objects in the Cache Object. Otherwise unattended-upgrades reads the index files onces for each version of each package.

Aim is speedup unattended-upgrades: cprofile goes from 1900 to 60 cumulated time on this box.

Cache _find_index might is private to the implementation. apt_pkg.SourceList find_index might be overloaded so external users of this method benefit from the speedup. I wrapped PackageFile to make it hashable: maybe this should be handled in PackageFile itself.

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