Completes this job. The result is
true if this job was completed as a result of this invocation and
false otherwise (if it was already completed).
Subsequent invocations of this function have no effect and always produce
This function transitions this job into _completed- state if it was not completed or cancelled yet. However, that if this job has children, then it transitions into completing state and becomes complete once all its children are complete. See Job for details.