mock error in koji: No such file or directory: '%{buildroot}/etc/mtab'

[T.C. Hollingsworth]

A koji rawhide build [1] just failed with:

INFO: Mock Version: 1.1.39
Start: chroot ['fedpkg', 'sources']
Start: device setup
Finish: device setup
Finish: chroot ['fedpkg', 'sources']
ERROR: [Errno 2] No such file or directory:
'/var/lib/mock/f21-build-2179813-398918/root/etc/mtab'
Traceback (most recent call last):
  File "/usr/sbin/mock", line 306, in <module>
    def do_buildsrpm(config_opts, chroot, options, args):
  File "/usr/sbin/mock", line 579, in main
    chroot.chroot(args, options)
  File "/usr/lib/python2.7/site-packages/mockbuild/backend.py", line
832, in chroot
    self._setupFiles()
  File "<peak.util.decorators.rewrap wrapping
mockbuild.backend._setupFiles at 0x01FED050>", line 3, in _setupFiles
    def _setupFiles(self): return __decorated(self)
  File "/usr/lib/python2.7/site-packages/mockbuild/trace_decorator.py",
line 70, in trace
    result = func(*args, **kw)
  File "/usr/lib/python2.7/site-packages/mockbuild/backend.py", line
552, in _setupFiles
    mockbuild.util.touch(item)
  File "<peak.util.decorators.rewrap wrapping mockbuild.util.touch at
0x01FA7B18>", line 3, in touch
    def touch(fileName): return __decorated(fileName)
  File "/usr/lib/python2.7/site-packages/mockbuild/trace_decorator.py",
line 70, in trace
    result = func(*args, **kw)
  File "/usr/lib/python2.7/site-packages/mockbuild/util.py", line 77, in touch
    fo = open(fileName, 'w')
IOError: [Errno 2] No such file or directory:
'/var/lib/mock/f21-build-2179813-398918/root/etc/mtab'

It might have something to do with yesterday's util-linux update in
rawhide? [2]  But /etc/mtab still shows up on that build's filelist in
koji, so I'm not sure why it's missing in the buildroot.

Thanks!
-T.C.

[1] http://koji.fedoraproject.org/koji/taskinfo?taskID=7105815
[2] http://koji.fedoraproject.org/koji/buildinfo?buildID=541536