Created attachment 937243 [details] build.log Description of problem: This is a placeholder for the extremely mysterious heisenbug where the /dev/sda1 device node does not get created, or magically disappears. Version-Release number of selected component (if applicable): It started happening in Fedora 21 and above. How reproducible: Rare, but annoying. Happens a lot more often under Koji than anywhere else, but can be reproduced in a VM occasionally. Only appears to happen after we write to the first sector of /dev/sda. In the attached build log, it happens twice. Search for the error message "/dev/sda1: No such file or directory"
Ugh. Turns out to be a stupid udev rule. Whenever you close a device, the kernel sends a "change" event to udev which fires off a whole set of rules. It runs at least: scsi_id, path_id, blkid. And these operations overlap the subsequent mkfs or other operation. More udev workarounds needed. Thanks agk for pointing me in the right direction.
Added a workaround: https://github.com/libguestfs/libguestfs/commit/4515413217c7d4a6fe026254bffa6f82746047c4