This won't solve your problem especially for library code, but prctl(2) with PR_SET_CHILD_SUBREAPER is worth mentioning: by setting the flag your process become

This won't solve your problem especially for library code, but prctl(2) with PR_SET_CHILD_SUBREAPER is worth mentioning: by setting the flag your process becomes a subreaper, and will become the parent of any orphaned child processes. Unlike cgroups, PR_SET_CHILD_SUBREAPER can be used without special privileges in commonly deployed Linux versions.