cache.py stalls causing my 17.10->18.04 LTS upgrade to hang

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It looks like when you instantiate the apt/cache.py class with rootdir="/" on my plain vanilla 17.10 system, it hangs. CPU spikes to 100%+ and just sits there. It smells like a deadlock. This is right after the first "Reading cache..." message after calling do-release-upgrade (which spawns the "bionic" process which uses apt-pkg).

Looking at the python test code, it seems to hang when "rootdir" = "/" in the Ctor.

It does enter the C module apt_pkg, during this call:

apt_pkg.read_config_file(...)

I'm not sure if it's the locking causing the issue (that is what it feels like again). This is a 32 CPU Threadripper system (AMD64).

I have libapt-pkg5.0, pythona-pt 1.4.0-beta3build2 installed. I'm not sure what to do other than to reinstall my system from scratch which I really don't want to do.