Random hangs, likely on waitpid

When running certain shells or complex scripts, the script will hang, and Ctrl-C will not exit. kill 1 {pid} does work.

People have reported that for both fish (Josh Megnauth) and zsh (Wildan) it appears to be hanging on waitpid. I have seen it when running os-test, where it is using command evaluation:

mkdir -p -- "$(dirname "$OUT_PATH/$FILE")"

seems to be hanging after running dirname.

If you do ps, it shows the shell script running but no child processes.

The problem does not seem to occur when running with a single core.