I'm fairly sure that this is because the new livecd-rootfs-built xenial buildd images include makedev, which was Priority: required in xenial (but not in later releases) but not in the historical buildd chroots. The best approach is likely to be to just take makedev out again.
xenial mir builds recently started failing as follows:
https:/ /launchpadlibra rian.net/ 442878565/ buildlog_ ubuntu- xenial- amd64.mir_ 1.4.0+dev83- g939ca20348- 0ubuntu16. 04_BUILDING. txt.gz
ERROR: src/umockdev. c:4991: umockdev_ testbed_ create_ node_for_ device: assertion failed: (Linux.openpty (out ptym, out ptys, ptyname_array, null, null) == 0)
I'm fairly sure that this is because the new livecd-rootfs-built xenial buildd images include makedev, which was Priority: required in xenial (but not in later releases) but not in the historical buildd chroots. The best approach is likely to be to just take makedev out again.