Accordng to Windows build hangs after Done exit 0, this happens if some background processes are still running.
To compare process lists before and after, e.g. run ps -W | sort at appropriate moments, then diff the outputs. To get the command line of the culprit, read /proc/<pid>/cmdline as per https://superuser.com/questions/1247844/full-command-text-with-ps-on-cygwin.