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

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.

Start with the reported hang in apt/cache.py when constructing the class with rootdir="/", then trace the call into apt_pkg.read_config_file during the do-release-upgrade flow. Reproduce it on the described Ubuntu 17.10 setup and determine whether the issue is resolved by preventing the stall without breaking normal cache initialization.

Written by the indexing model from the issue text.