Hi,
Documentation here
http://www.postgresql.org/docs/devel/static/bgworker.html does not indicate any relation between the fields bgw_notify_pid and bgw_flags of BackgroundWorker structure. But in one has to set BGWORKER_BACKEND_DATABASE_CONNECTION in order to use bgw_notify_pid feature.
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 }
bgw_notify_pid gets wiped out (and that too silently) if PostmasterMarkPIDForWorkerNotify() returns false. PostmasterMarkPIDForWorkerNotify() only looks at the BackendList, which does not contain all the background worker created by Register*BackgroundWorker() calls. Only a baground worker which has set BGWORKER_BACKEND_DATABASE_CONNECTION, gets added into BackendList in maybe_start_bgworker()
5629 if (rw->rw_worker.bgw_flags & BGWORKER_BACKEND_DATABASE_CONNECTION)
5630 {
5631 if (!assign_backendlist_entry(rw))
5632 return;
5633 }
5634 else
5635 rw->rw_child_slot = MyPMChildSlot = AssignPostmasterChildSlot();
5636
5637 do_start_bgworker(rw); /* sets rw->rw_pid */
5638
5639 if (rw->rw_backend)
5640 {
5641 dlist_push_head(&BackendList, &rw->rw_backend->elem);
Should we change the documentation to say "one needs to set BGWORKER_BACKEND_DATABASE_CONNECTION" in order to use bgw_notify_pid feature? OR we should fix the code not to wipe out bgw_notify_pid in the code above (may be change PostmasterMarkPIDForWorkerNotify() to scan the list of other background workers as well)?
--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company
