This is a bit of advanced rpm trickery that probably falls in the "don't do that" category. :-/
Short version:
How do I query in %post, when executed during an upgrade, for the version we are upgrading from?
Long version:
A custom rpm (that OLPC's XS uses to manage configuration) needs to nuke a dynamically generated config file - /etc/udev/rules.d/70-persistent-net.rules - _only once_. Before version 0.5 of the pkg, the script that drove the device naming was subtly wrong.
The problem is: bad data is almost indistinguishable from good data. So I can't make the condition just on the bad data, I want to also check that we're upgrading from a known-bad version.
Users will face a small disruption - any manual tweaks of device assignation will be lost, but we don't have many users. This will change soon, so better break it now than later... :-)
cheers,
m