Dependency between bgw_notify_pid and bgw_flags

In BackgroundWorkerStateChange
 318         /*
 319          * Copy the PID to be notified about state changes, but only if the
 320          * postmaster knows about a backend with that PID.  It isn't an error
 321          * if the postmaster doesn't know about the PID, because the backend
 322          * that requested the worker could have died (or been killed) just
 323          * after doing so.  Nonetheless, at least until we get some experience
 324          * with how this plays out in the wild, log a message at a relative
 325          * high debug level.
 326          */
 327         rw->rw_worker.bgw_notify_pid = slot->worker.bgw_notify_pid;
 328         if (!PostmasterMarkPIDForWorkerNotify(rw->rw_worker.bgw_notify_pid))
 329         {
 330             elog(DEBUG1, "worker notification PID %lu is not valid",
 331                  (long) rw->rw_worker.bgw_notify_pid);
 332             rw->rw_worker.bgw_notify_pid = 0;
 333         }