Function get_child_task_id_by_pid 

pub fn get_child_task_id_by_pid(parent: TaskId, pid: Pid) -> Option<TaskId>

Resolve a direct child of parent by POSIX pid.

Unlike the global pid index, this remains valid after the child has called exit and before it is reaped, because the task object stays in all_tasks until waitpid consumes the zombie.

Lock order: GLOBAL_SCHED_STATE before SCHED_IDENTITY (see module docs).