Why call setsid after fork?
1. `Parent` = PID: 28084, PGID: 28084, SID: 28046The program has a controlling terminal at this point.
2. `Fork#1` = PID: 28085, PGID: 28084, SID: 28046The program still has a controlling terminal at this point. If the parent exited, this child would belong to an abandoned process group. It can setpgid() to another process group in the same session.
3. `Decouple#1`= PID: 28085, PGID: 28085, SID: 28085Now the program is in another session, and cannot setpgid() to switch to a process group in the original session.
4. `Fork#2` = PID: 28086, PGID: 28085, SID: 28085Reparent to init.