This intermittent error has been cropping up more lately than before. See: https://jenkins.freebsd.org/job/Build-UFS-image/599/ for an example failure (as well as the history for this Jenkins job and you'll see what I mean).
I looked at the error and the jenkins jobs a bit more and I think the real problem is that the FreeBSD_Head job is being run in parallel with the Build-UFS-image job, because the FreeBSD_Head job completes without issue and the Build-UFS-image job is not being run with -j <anything>. I know based on past experience if the tree is updated but not rebuilt with -DNO_CLEAN and installkernel/installworld is being run, sporadic build failures like this will occur, depending on whether or not something needs to be [re]built. One way to deal with this issue is to svn up a source tree, then rsync the checked out copy of the source tree to a unique memory disk, build from there, and install to a unique location as well. Using the jenkins job identifier should be enough… but I’m not sure what the best way with Jenkins is to do this, partly because I don’t know what plugins are installed on the Jenkins server. Thanks! I'm reassigning the bug to Craig for triage as this appears to be an artifact of how the Jenkins jobs are setup. If there's anything further I can do to help, please let me know!
I believe this has been fixed. Craig -- could you please confirm?
Closing bug due to lack of feedback and sightings.