Okay, running dtrace on this

dtrace -qn '
proc:::signal-discard
{
        printf("Process %d discarded signal %d\n",
        args[1]->pr_pid,args[2]);
}'

I get Process 26312 discarded signal 20 so we're discarding the SIGCHLD.

@headius helped me find that if you add java.lang.UNIXProcess to the top of the script, it starts working. That class should probably be loaded early.

Root problem was that java.lang.UNIXProcess initializes after the user's trap is installed, and it then proceeds to reset SIGCHLD handling to default. According to sigaction man page, default behavior for SIGCHLD is to discard, so we silently lose the trap and the signal.

I have a fix that forces java.lang.Process and java.lang.UNIXProcess to load before setting up the Signal module. That should guarantee the JVM's SIGCHLD handler has been installed and that it will be overwritten by the user's trap.