Description of problem:
mock instances can share a ccache directory by using the --uniqueext option and a shared mock configuration file.

It is possible for one mock instance to be removing files from the ccache (for example if the cache fills up, or the various lock files used internally by ccache) while the other instance is trying to ensure consistent ownership of files in the cache via this code in ccache.py:

        self.buildroot.uid_manager.changeOwner(self.ccachePath, recursive=True)


If we look at the implementation of changeOwner we have:

        if recursive:
            for root, dirs, files in os.walk(path):
                for d in dirs:
                    os.chown(os.path.join(root, d), uid, gid)
                for f in files:
                    os.chown(os.path.join(root, f), uid, gid)


Presumably os.walk (path) enumerates all the files and directories, then the loop over them proceeds to change their ownership.  Obviously if a file is removed from the cache between the enumeration step and the actual chown call we'll get an error like this:

  File "/usr/lib/python2.7/site-packages/mockbuild/uid.py", line 84, in changeOwner
    os.chown(os.path.join(root, f), uid, gid)
OSError: [Errno 2] No such file or directory: '/var/cache/mock/epel-7-x86_64/ccache/u0/2/stats.lock'

Of course this aborts the mock build.

Version-Release number of selected component (if applicable):


How reproducible:
Keep many mock instances building SRPMS running which share a configuration file and use --uniqueext.  It's probably easier to trigger if the ccache size is kept small (which will encourage eviction of files from the cache).  Watch for error messages similar to the one above.

Steps to Reproduce:
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Actual results:


Expected results:


Additional info: