Allow notifications to bgworkers without database connections.

Previously, if one background worker registered another background
worker and set bgw_notify_pid while for the second background worker,
it would not receive notifications from the postmaster unless, at the
time the "parent" was registered, BGWORKER_BACKEND_DATABASE_CONNECTION
was set.

To fix, instead instead of including only those background workers that
requested database connections in the postmater's BackendList, include
them all.  There doesn't seem to be any reason not do this, and indeed
it removes a significant amount of duplicated code.  The other option
is to make PostmasterMarkPIDForWorkerNotify look at BackgroundWorkerList
in addition to BackendList, but that adds more code duplication instead
of getting rid of it.

Patch by me.  Review and testing by Ashutosh Bapat.

Author: robertmhaas

src/backend/postmaster/postmaster.c

@@ -155,8 +155,7 @@
  * they will never become live backends.  dead_end children are not assigned a
  * PMChildSlot.
  *
- * Background workers that request shared memory access during registration are
- * in this list, too.
+ * Background workers are in this list, too.
  */
 typedef struct bkend
 {
@@ -404,13 +403,11 @@ static long PostmasterRandom(void);
 static void RandomSalt(char *md5Salt);
 static void signal_child(pid_t pid, int signal);
 static bool SignalSomeChildren(int signal, int targets);
-static bool SignalUnconnectedWorkers(int signal);
 static void TerminateChildren(int signal);
 
 #define SignalChildren(sig)			   SignalSomeChildren(sig, BACKEND_TYPE_ALL)
 
 static int	CountChildren(int target);
-static int	CountUnconnectedWorkers(void);
 static void maybe_start_bgworker(void);
 static bool CreateOptsFile(int argc, char *argv[], char *fullprogname);
 static pid_t StartChildProcess(AuxProcType type);
@@ -2414,7 +2411,6 @@ SIGHUP_handler(SIGNAL_ARGS)
 				(errmsg("received SIGHUP, reloading configuration files")));
 		ProcessConfigFile(PGC_SIGHUP);
 		SignalChildren(SIGHUP);
-		SignalUnconnectedWorkers(SIGHUP);
 		if (StartupPID != 0)
 			signal_child(StartupPID, SIGHUP);
 		if (BgWriterPID != 0)
@@ -2491,7 +2487,6 @@ pmdie(SIGNAL_ARGS)
 				/* and bgworkers too; does this need tweaking? */
 				SignalSomeChildren(SIGTERM,
 							   BACKEND_TYPE_AUTOVAC | BACKEND_TYPE_BGWORKER);
-				SignalUnconnectedWorkers(SIGTERM);
 				/* and the autovac launcher too */
 				if (AutoVacPID != 0)
 					signal_child(AutoVacPID, SIGTERM);
@@ -2543,11 +2538,11 @@ pmdie(SIGNAL_ARGS)
 				signal_child(BgWriterPID, SIGTERM);
 			if (WalReceiverPID != 0)
 				signal_child(WalReceiverPID, SIGTERM);
-			SignalUnconnectedWorkers(SIGTERM);
 			if (pmState == PM_RECOVERY)
 			{
+				SignalSomeChildren(SIGTERM, BACKEND_TYPE_BGWORKER);
 				/*
-				 * Only startup, bgwriter, walreceiver, unconnected bgworkers,
+				 * Only startup, bgwriter, walreceiver, possibly bgworkers,
 				 * and/or checkpointer should be active in this state; we just
 				 * signaled the first four, and we don't want to kill
 				 * checkpointer yet.
@@ -2999,25 +2994,21 @@ CleanupBackgroundWorker(int pid,
 		}
 
 		/* Get it out of the BackendList and clear out remaining data */
-		if (rw->rw_backend)
-		{
-			Assert(rw->rw_worker.bgw_flags & BGWORKER_BACKEND_DATABASE_CONNECTION);
-			dlist_delete(&rw->rw_backend->elem);
+		dlist_delete(&rw->rw_backend->elem);
 #ifdef EXEC_BACKEND
-			ShmemBackendArrayRemove(rw->rw_backend);
+		ShmemBackendArrayRemove(rw->rw_backend);
 #endif
 
-			/*
-			 * It's possible that this background worker started some OTHER
-			 * background worker and asked to be notified when that worker
-			 * started or stopped.  If so, cancel any notifications destined
-			 * for the now-dead backend.
-			 */
-			if (rw->rw_backend->bgworker_notify)
-				BackgroundWorkerStopNotifications(rw->rw_pid);
-			free(rw->rw_backend);
-			rw->rw_backend = NULL;
-		}
+		/*
+		 * It's possible that this background worker started some OTHER
+		 * background worker and asked to be notified when that worker
+		 * started or stopped.  If so, cancel any notifications destined
+		 * for the now-dead backend.
+		 */
+		if (rw->rw_backend->bgworker_notify)
+			BackgroundWorkerStopNotifications(rw->rw_pid);
+		free(rw->rw_backend);
+		rw->rw_backend = NULL;
 		rw->rw_pid = 0;
 		rw->rw_child_slot = 0;
 		ReportBackgroundWorkerPID(rw);	/* report child death */
@@ -3160,15 +3151,12 @@ HandleChildCrash(int pid, int exitstatus, const char *procname)
 			 * Found entry for freshly-dead worker, so remove it.
 			 */
 			(void) ReleasePostmasterChildSlot(rw->rw_child_slot);
-			if (rw->rw_backend)
-			{
-				dlist_delete(&rw->rw_backend->elem);
+			dlist_delete(&rw->rw_backend->elem);
 #ifdef EXEC_BACKEND
-				ShmemBackendArrayRemove(rw->rw_backend);
+			ShmemBackendArrayRemove(rw->rw_backend);
 #endif
-				free(rw->rw_backend);
-				rw->rw_backend = NULL;
-			}
+			free(rw->rw_backend);
+			rw->rw_backend = NULL;
 			rw->rw_pid = 0;
 			rw->rw_child_slot = 0;
 			/* don't reset crashed_at */
@@ -3505,7 +3493,6 @@ PostmasterStateMachine(void)
 		 * process.
 		 */
 		if (CountChildren(BACKEND_TYPE_NORMAL | BACKEND_TYPE_WORKER) == 0 &&
-			CountUnconnectedWorkers() == 0 &&
 			StartupPID == 0 &&
 			WalReceiverPID == 0 &&
 			BgWriterPID == 0 &&
@@ -3727,39 +3714,6 @@ signal_child(pid_t pid, int signal)
 #endif
 }
 
-/*
- * Send a signal to bgworkers that did not request backend connections
- *
- * The reason this is interesting is that workers that did request connections
- * are considered by SignalChildren; this function complements that one.
- */
-static bool
-SignalUnconnectedWorkers(int signal)
-{
-	slist_iter	iter;
-	bool		signaled = false;
-
-	slist_foreach(iter, &BackgroundWorkerList)
-	{
-		RegisteredBgWorker *rw;
-
-		rw = slist_container(RegisteredBgWorker, rw_lnode, iter.cur);
-
-		if (rw->rw_pid == 0)
-			continue;
-		/* ignore connected workers */
-		if (rw->rw_backend != NULL)
-			continue;
-
-		ereport(DEBUG4,
-				(errmsg_internal("sending signal %d to process %d",
-								 signal, (int) rw->rw_pid)));
-		signal_child(rw->rw_pid, signal);
-		signaled = true;
-	}
-	return signaled;
-}
-
 /*
  * Send a signal to the targeted children (but NOT special children;
  * dead_end children are never signaled, either).
@@ -3832,7 +3786,6 @@ TerminateChildren(int signal)
 		signal_child(PgArchPID, signal);
 	if (PgStatPID != 0)
 		signal_child(PgStatPID, signal);
-	SignalUnconnectedWorkers(signal);
 }
 
 /*
@@ -5093,33 +5046,6 @@ PostmasterRandom(void)
 	return random();
 }
 
-/*
- * Count up number of worker processes that did not request backend connections
- * See SignalUnconnectedWorkers for why this is interesting.
- */
-static int
-CountUnconnectedWorkers(void)
-{
-	slist_iter	iter;
-	int			cnt = 0;
-
-	slist_foreach(iter, &BackgroundWorkerList)
-	{
-		RegisteredBgWorker *rw;
-
-		rw = slist_container(RegisteredBgWorker, rw_lnode, iter.cur);
-
-		if (rw->rw_pid == 0)
-			continue;
-		/* ignore connected workers */
-		if (rw->rw_backend != NULL)
-			continue;
-
-		cnt++;
-	}
-	return cnt;
-}
-
 /*
  * Count up number of child processes of specified types (dead_end chidren
  * are always excluded).
@@ -5520,8 +5446,7 @@ do_start_bgworker(RegisteredBgWorker *rw)
 #endif
 		default:
 			rw->rw_pid = worker_pid;
-			if (rw->rw_backend)
-				rw->rw_backend->pid = rw->rw_pid;
+			rw->rw_backend->pid = rw->rw_pid;
 			ReportBackgroundWorkerPID(rw);
 	}
 }
@@ -5684,30 +5609,19 @@ maybe_start_bgworker(void)
 			rw->rw_crashed_at = 0;
 
 			/*
-			 * If necessary, allocate and assign the Backend element.  Note we
+			 * Allocate and assign the Backend element.  Note we
 			 * must do this before forking, so that we can handle out of
 			 * memory properly.
-			 *
-			 * If not connected, we don't need a Backend element, but we still
-			 * need a PMChildSlot.
 			 */
-			if (rw->rw_worker.bgw_flags & BGWORKER_BACKEND_DATABASE_CONNECTION)
-			{
-				if (!assign_backendlist_entry(rw))
-					return;
-			}
-			else
-				rw->rw_child_slot = MyPMChildSlot = AssignPostmasterChildSlot();
+			if (!assign_backendlist_entry(rw))
+				return;
 
 			do_start_bgworker(rw);		/* sets rw->rw_pid */
 
-			if (rw->rw_backend)
-			{
-				dlist_push_head(&BackendList, &rw->rw_backend->elem);
+			dlist_push_head(&BackendList, &rw->rw_backend->elem);
 #ifdef EXEC_BACKEND
-				ShmemBackendArrayAdd(rw->rw_backend);
+			ShmemBackendArrayAdd(rw->rw_backend);
 #endif
-			}
 
 			/*
 			 * Have ServerLoop call us again.  Note that there might not