"""Basically it's OK to collect
all the child exit codes if you record the results and return them when
requested. This would mean that waitpid and the like would have to check
a cached list of PIDs and exit statuses before actually waiting."""

note that this would still have problems.  PIDs are recycled by the OS
so as soon as you've waited on one and the process dies, the OS is free
to launch a new process using it.  If the new process happens to be
another one of ours launched by subprocess that would be a problem. 
Make the cached map of pids -> exit codes be a map of pids -> [list of
exit codes] instead?