This issue seems to be caused by concurrent run of createrepo_c with build.
I still need to have a look at it.

...
DEBUG util.py:563:  child environment: None
DEBUG util.py:495:  Executing command: ['/bin/mount', '-n', '-t', 'tmpfs', 'tmpfs', '/var/lib/mock/fedora-23-x86_64-mockbuilder-5704/root/dev/shm'] with env {'HOSTNAME': 'mock', 'HOME': '/builddir', 'TERM': 'vt100', 'SHELL': '/bin/sh', 'PATH': '/usr/bin:/bin:/usr/sbin:/sbin', 'LANG': 'en_US.UTF-8'} and shell False
DEBUG util.py:542:  Child return code was: 0
DEBUG util.py:563:  child environment: None
DEBUG util.py:495:  Executing command: ['/bin/mount', '-n', '-t', 'devpts', '-o', 'gid=5,mode=0620,ptmxmode=0666,newinstance', 'devpts', '/var/lib/mock/fedora-23-x86_64-mockbuilder-5704/root/dev/pts'] with env {'HOSTNAME': 'mock', 'HOME': '/builddir', 'TERM': 'vt100', 'SHELL': '/bin/sh', 'PATH': '/usr/bin:/bin:/usr/sbin:/sbin', 'LANG': 'en_US.UTF-8'} and shell False
DEBUG util.py:542:  Child return code was: 0
DEBUG util.py:563:  child environment: None
DEBUG util.py:495:  Executing command: ['/bin/mount', '-n', '--bind', '/var/cache/mock/fedora-23-x86_64/yum_cache/', '/var/lib/mock/fedora-23-x86_64-mockbuilder-5704/root/var/cache/yum/'] with env {'HOSTNAME': 'mock', 'HOME': '/builddir', 'TERM': 'vt100', 'SHELL': '/bin/sh', 'PATH': '/usr/bin:/bin:/usr/sbin:/sbin', 'LANG': 'en_US.UTF-8'} and shell False
DEBUG util.py:542:  Child return code was: 0
DEBUG util.py:563:  child environment: None
DEBUG util.py:495:  Executing command: ['/bin/mount', '-n', '--bind', '/tmp/mock-selinux-plugin.gvbfzpxj', '/var/lib/mock/fedora-23-x86_64-mockbuilder-5704/root/proc/filesystems'] with env {'HOSTNAME': 'mock', 'HOME': '/builddir', 'TERM': 'vt100', 'SHELL': '/bin/sh', 'PATH': '/usr/bin:/bin:/usr/sbin:/sbin', 'LANG': 'en_US.UTF-8'} and shell False
DEBUG util.py:542:  Child return code was: 0
DEBUG buildroot.py:138:  rootdir = /var/lib/mock/fedora-23-x86_64-mockbuilder-5704/root
DEBUG buildroot.py:139:  resultdir = /var/tmp/mockremote-xcsUn/build/results/fedora-23-x86_64/cov-im-utils-1.0.2.20160429.144027.gef57d34-1.fc23
DEBUG util.py:97:  ensuring that dir exists: /var/lib/mock/fedora-23-x86_64-mockbuilder-5704/root/etc/pki/mock
DEBUG util.py:100:  creating dir: /var/lib/mock/fedora-23-x86_64-mockbuilder-5704/root/etc/pki/mock
DEBUG util.py:97:  ensuring that dir exists: /var/lib/mock/fedora-23-x86_64-mockbuilder-5704/root/etc/yum/pluginconf.d
DEBUG util.py:100:  creating dir: /var/lib/mock/fedora-23-x86_64-mockbuilder-5704/root/etc/yum/pluginconf.d
DEBUG package_manager.py:199:  configure yum
DEBUG package_manager.py:205:  configure yum priorities
DEBUG package_manager.py:207:  configure yum rhnplugin
DEBUG util.py:97:  ensuring that dir exists: /var/lib/mock/fedora-23-x86_64-mockbuilder-5704/root/etc
DEBUG package_manager.py:96:  ['/usr/bin/yum-deprecated', '--installroot', '/var/lib/mock/fedora-23-x86_64-mockbuilder-5704/root/', '--releasever', '23', 'install', '@buildsys-build']
DEBUG util.py:563:  child environment: None
DEBUG util.py:495:  Executing command: ['/usr/bin/yum-deprecated', '--installroot', '/var/lib/mock/fedora-23-x86_64-mockbuilder-5704/root/', '--releasever', '23', 'install', '@buildsys-build', '--setopt=tsflags=nocontexts'] with env {'PROMPT_COMMAND': 'printf "\x1b]0;<mock-chroot>\x07<mock-chroot>"', 'HOSTNAME': 'mock', 'LANG': 'en_US.UTF-8', 'LC_MESSAGES': 'C', 'TERM': 'vt100', 'HOME': '/builddir', 'SHELL': '/bin/bash', 'PATH': '/usr/bin:/bin:/usr/sbin:/sbin'} and shell False
DEBUG util.py:417:  Yum command has been deprecated, use dnf instead.
DEBUG util.py:417:  See 'man dnf' and 'man yum2dnf' for more information.
DEBUG util.py:417:  http://coprbe.devel.redhat.com/repos/kdudka/covscan-testing/fedora-23-x86_64/devel/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found
DEBUG util.py:417:  Trying other mirror.
DEBUG util.py:417:   One of the configured repositories failed (updates),
DEBUG util.py:417:   and yum doesn't have enough cached data to continue. At this point the only
DEBUG util.py:417:   safe thing yum can do is fail. There are a few ways to work "fix" this:
DEBUG util.py:417:       1. Contact the upstream for the repository and get them to fix the problem.
DEBUG util.py:417:       2. Reconfigure the baseurl/etc. for the repository, to point to a working
DEBUG util.py:417:          upstream. This is most often useful if you are using a newer
DEBUG util.py:417:          distribution release than is supported by the repository (and the
DEBUG util.py:417:          packages for the previous distribution release still work).
DEBUG util.py:417:       3. Disable the repository, so yum won't use it by default. Yum will then
DEBUG util.py:417:          just ignore the repository until you permanently enable it again or use
DEBUG util.py:417:          --enablerepo for temporary usage:
DEBUG util.py:417:              yum-config-manager --disable updates
DEBUG util.py:417:       4. Configure the failing repository to be skipped, if it is unavailable.
DEBUG util.py:417:          Note that yum will try to contact the repo. when it runs most commands,
DEBUG util.py:417:          so will have to try and fail each time (and thus. yum will be be much
DEBUG util.py:417:          slower). If it is a very temporary problem though, this is often a nice
DEBUG util.py:417:          compromise:
DEBUG util.py:417:              yum-config-manager --save --setopt=updates.skip_if_unavailable=true
DEBUG util.py:417:  Cannot retrieve metalink for repository: updates/23/x86_64. Please verify its path and try again
DEBUG util.py:542:  Child return code was: 1
DEBUG util.py:176:  kill orphans