wait unblocks before signals processed

[Elliott Forney]

While trying to modify some code I found on an earlier post for
running N jobs in parallel I came across the interesting behavior
illustrated below.  It appears that the wait command proceeds before
my SIGUSR's are all processed.  Is this a bug or just a fact of life?
I understand that it isn't possible to know if a process will receive
a signal in the future but I am surprised that the signals aren't
received and processed in time in this case.

On a related note, I think it would be very nice if there were a way
to wait for ANY background job to finish.  Currently it seems like one
can only wait for either ALL jobs or else a single job with a given
PID.  Would it be possible to have something like 'wait -' that would
block until any of the current background jobs completes?  This would
make writing simple parallel loops much easier.  The busy-wait/SIGUSR
solution is kindof a hack and for such a simple problem I would prefer
not to depend on gnu parallel.

#!/bin/bash

nrunning=0
nmax=3

function job_wrap
{
  echo "sleeping: $2 nrunning: $nrunning"
  eval "$@"
  kill -s USR2 $$
}

trap ': $(( --nrunning ))' USR2
for x in {1..20}
do
  while [[ nrunning -ge nmax ]]
  do
    : # busy wait
  done

  : $(( ++nrunning ))
  job_wrap sleep $(( RANDOM % 3 )) &
done

echo 'start wait'
wait
trap - USR2
echo 'end wait'

$ ./par_sigusr
sleeping: 0 nrunning: 1
sleeping: 2 nrunning: 2
sleeping: 0 nrunning: 3
sleeping: 1 nrunning: 3
sleeping: 0 nrunning: 3
sleeping: 2 nrunning: 3
sleeping: 0 nrunning: 3
sleeping: 2 nrunning: 3
sleeping: 2 nrunning: 3
sleeping: 0 nrunning: 3
sleeping: 2 nrunning: 3
sleeping: 2 nrunning: 3
sleeping: 0 nrunning: 3
sleeping: 1 nrunning: 3
sleeping: 2 nrunning: 3
sleeping: 2 nrunning: 3
sleeping: 2 nrunning: 3
sleeping: 1 nrunning: 3
sleeping: 2 nrunning: 3
start wait
sleeping: 2 nrunning: 3
end wait
$ ./par_sigusr: line 10: kill: (16287) - No such process
./par_sigusr: line 10: kill: (16287) - No such process

Thanks!
-------
Elliott Forney