I did not add a version as it should happen in all versions. Before leaving the version unset I did test with 4.9.0 and the problem still happens. I assume it happens in 4.10.0.

Checking latest source code in https://git.savannah.gnu.org/cgit/findutils.git/tree/xargs/xargs.c I do not see how it could not also happen in the latest changes as it still has:

  /* Make sure to listen for the kids.  */
  signal (SIGCHLD, SIG_DFL);

The fix probably should be to install a signal handler for SIGCHLD and call waitpid in the signal handler.

This is a copy of the report at https://issues.redhat.com/browse/RHEL-27230

---

User has a shell script in the pattern:

tail -n0 -F /var/log/messages | stdbuf -i0 -o0 grep -o ... | stdbuf -i0 -o0 awk ... | stdbuf -i0 -o0 awk ... | stdbuf -i0 -o0 xargs -n1 /path/to/handler

A sample reproducer is:

term1# xargs -n1 echo
test
test
foo
foo

term2# ps ax | grep Z
PID pts/0 Z+ 0:00 [echo] <defunct>

I understand the root cause is because xargs has the code:

   /* Make sure to listen for the kids.  */
  signal (SIGCHLD, SIG_DFL);

and this means it does not synchronously handle SIGCHLD.

There is also the code:

      /* Before forking, reap any already-exited child. We do this so
         that we don't leave unreaped children around while we build a
         new command line.  For example this command will spend most
         of its time waiting for sufficient arguments to launch
         another command line:

         seq 1 1000 | fmt | while read x ; do echo $x; sleep 1 ; done |
         ./xargs -P 200 -n 20  sh -c 'echo "$@"; sleep $((1 + $RANDOM % 5))' sleeper
      */
      wait_for_proc (false, 0u);

It is understood that changing this might require a significant rework of how xargs work.

The reproducer should be a very uncommon usage, as input usually should be very fast, or xargs not used in the way in the user script, and instead use some shell loop construct.

Still, this might be considered a valid usage of xargs, and the process in zombie state looks as if something wrong is happening.