I’ll open another reports for the possible bug in _generate_cache and for review of the other tests calling get_distribution.

> Test test_install and test_command_install_data interference cache
I’ve added checks in regrtest to make sure that each tests clears the caches some time ago.

> Most important is why packaging use global variables.
[citation needed].  First, mutable objects with a global name (let’s not use “variable” for Python) are not unconditionally evil; see sys.path and sys.modules for example.  Second, how concretely would you change that implementation?  We need shared caches for functions in packaging.database.  They should be built on demand, and should be cleared by higher-level libraries or applications when the filesystem has changed (after an installation or removal for example).  With these constraints, do you have implementation ideas?