Downstream bug: https://bugs.kde.org/show_bug.cgi?id=394360

The symptom is that QProcess:waitForFinished never returns although the child exited and it stays around as a zombie.

It's not always reproducible, but the cause was found with strace to be the removal of the sigchld handler (quoting the comment on the downstream bug):

16233 exit_group(0) = ?
16233 +++ exited with 0 +++
13987 <... ppoll resumed> ) = ? ERESTARTNOHAND (To be restarted if no handler)
13987 — SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=16233, si_uid=500, si_status=0, si_utime=0, si_stime=0} ---
13987 ppoll([\{fd=-1}, \{fd=-1}, \{fd=-1}, \{fd=46, events=POLLIN}], 4, NULL, NULL, 8 <unfinished ...>
13990 <... poll resumed> ) = 1 ([\{fd=3, revents=POLLIN}])

If I interpret this correctly, kdeinit was currently doing a ppoll (no sigmask, so == poll) and it had no handler, as otherwise it would've donene of the calls in the handler and not continue with ppoll.

This seems to explain it:

13987 rt_sigaction(SIGCHLD, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7f0242406110}, NULL, 8) = 0

It's the last call to rt_sigaction in that process, so it removed its signal handler, so it's no surprise it stays a zombie...

Now the question is where the rt_sigaction call comes from. It might be somewhere in forkfd.

With QT_NO_GLIB=1 it seems to work.