emerge -uD world
Calculating world dependencies... done!

>>> Emerging (1 of 12) sys-libs/libcap-1.10-r8 to /
Traceback (most recent call last):
  File "/usr/bin/emerge", line 3897, in ?
    emerge_main()
  File "/usr/bin/emerge", line 3892, in emerge_main
    myopts, myaction, myfiles, spinner)
  File "/usr/bin/emerge", line 3435, in action_build
    mydepgraph.merge(pkglist, favorites, mtimedb)
  File "/usr/bin/emerge", line 1911, in merge
    mydbapi=portdb, tree="porttree")
  File "/usr/lib/portage/pym/portage.py", line 2774, in doebuild
    return spawn(EBUILD_SH_BINARY+" clean",mysettings,debug=debug,free=1,logfile=None)
  File "/usr/lib/portage/pym/portage.py", line 1831, in spawn
    retval = spawn_func(mystring, env=env, **keywords)
  File "/usr/lib/portage/pym/portage_exec.py", line 39, in spawn_bash
    return spawn(args, opt_name=opt_name, **keywords)
  File "/usr/lib/portage/pym/portage_exec.py", line 184, in spawn
    retval = os.waitpid(pid, 0)[1]
OSError: [Errno 10] No child processes