Notify bgworker registrant after freeing worker slot.

Tom Lane observed buildfarm failures caused by the select_parallel
regression test trying to launch new parallel queries before the
worker slots used by the previous ones were freed.  Try to fix this by
having the postmaster free the worker slots before it sends the
SIGUSR1 notifications to the registering process.  This doesn't
completely eliminate the possibility that the user backend might
(correctly) observe the worker as dead before the slot is free, but I
believe it should make the window significantly narrower.

Patch by me, per complaint from Tom Lane.  Reviewed by Amit Kapila.

Discussion: http://postgr.es/m/30673.1487310734@sss.pgh.pa.us

src/backend/postmaster/bgworker.c

@@ -429,6 +429,39 @@ ReportBackgroundWorkerPID(RegisteredBgWorker *rw)
 		kill(rw->rw_worker.bgw_notify_pid, SIGUSR1);
 }
 
+/*
+ * Report that the PID of a background worker is now zero because a
+ * previously-running background worker has exited.
+ *
+ * This function should only be called from the postmaster.
+ */
+void
+ReportBackgroundWorkerExit(slist_mutable_iter *cur)
+{
+	RegisteredBgWorker *rw;
+	BackgroundWorkerSlot *slot;
+
+	rw = slist_container(RegisteredBgWorker, rw_lnode, cur->cur);
+
+	Assert(rw->rw_shmem_slot < max_worker_processes);
+	slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
+	slot->pid = rw->rw_pid;
+
+	/*
+	 * If this worker is slated for deregistration, do that before notifying
+	 * the process which started it.  Otherwise, if that process tries to
+	 * reuse the slot immediately, it might not be available yet.  In theory
+	 * that could happen anyway if the process checks slot->pid at just the
+	 * wrong moment, but this makes the window narrower.
+	 */
+	if (rw->rw_terminate ||
+		rw->rw_worker.bgw_restart_time == BGW_NEVER_RESTART)
+		ForgetBackgroundWorker(cur);
+
+	if (rw->rw_worker.bgw_notify_pid != 0)
+		kill(rw->rw_worker.bgw_notify_pid, SIGUSR1);
+}
+
 /*
  * Cancel SIGUSR1 notifications for a PID belonging to an exiting backend.
  *

src/backend/postmaster/postmaster.c

@@ -3050,9 +3050,9 @@ CleanupBackgroundWorker(int pid,
 						int exitstatus) /* child's exit status */
 {
 	char		namebuf[MAXPGPATH];
-	slist_iter	iter;
+	slist_mutable_iter	iter;
 
-	slist_foreach(iter, &BackgroundWorkerList)
+	slist_foreach_modify(iter, &BackgroundWorkerList)
 	{
 		RegisteredBgWorker *rw;
 
@@ -3126,7 +3126,7 @@ CleanupBackgroundWorker(int pid,
 		rw->rw_backend = NULL;
 		rw->rw_pid = 0;
 		rw->rw_child_slot = 0;
-		ReportBackgroundWorkerPID(rw);	/* report child death */
+		ReportBackgroundWorkerExit(&iter);	/* report child death */
 
 		LogChildExit(EXIT_STATUS_0(exitstatus) ? DEBUG1 : LOG,
 					 namebuf, pid, exitstatus);

src/include/postmaster/bgworker_internals.h

@@ -42,6 +42,7 @@ extern void BackgroundWorkerShmemInit(void);
 extern void BackgroundWorkerStateChange(void);
 extern void ForgetBackgroundWorker(slist_mutable_iter *cur);
 extern void ReportBackgroundWorkerPID(RegisteredBgWorker *);
+extern void ReportBackgroundWorkerExit(slist_mutable_iter *cur);
 extern void BackgroundWorkerStopNotifications(pid_t pid);
 extern void ResetBackgroundWorkerCrashTimes(void);