Digging into this further I wondered why the mirror file was failing to be read. The release upgrader tarball includes Ubuntu.info but not Ubuntu.mirrors and the code tries to read from '/tmp/ubuntu-release-upgrader-XYZ/Ubuntu.mirrors'.
Digging into this further I wondered why the mirror file was failing to be read. The release upgrader tarball includes Ubuntu.info but not Ubuntu.mirrors and the code tries to read from '/tmp/ubuntu- release- upgrader- XYZ/Ubuntu. mirrors' .