Move synchronous_standbys_defined updates from WAL writer to BG writer.

This is advantageous because the BG writer is alive until much later in
the shutdown sequence than WAL writer; we want to make sure that it's
possible to shut off synchronous replication during a smart shutdown,
else it might not be possible to complete the shutdown at all.

Per very reasonable gripes from Fujii Masao and Simon Riggs.

src/backend/postmaster/bgwriter.c

@@ -55,6 +55,7 @@
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "postmaster/bgwriter.h"
+#include "replication/syncrep.h"
 #include "storage/bufmgr.h"
 #include "storage/fd.h"
 #include "storage/ipc.h"
@@ -363,6 +364,9 @@ BackgroundWriterMain(void)
 	if (RecoveryInProgress())
 		ThisTimeLineID = GetRecoveryTargetTLI();
 
+	/* Do this once before starting the loop, then just at SIGHUP time. */
+	SyncRepUpdateSyncStandbysDefined();
+
 	/*
 	 * Loop forever
 	 */
@@ -389,6 +393,8 @@ BackgroundWriterMain(void)
 		{
 			got_SIGHUP = false;
 			ProcessConfigFile(PGC_SIGHUP);
+			/* update global shmem state for sync rep */
+			SyncRepUpdateSyncStandbysDefined();
 		}
 		if (checkpoint_requested)
 		{
@@ -704,6 +710,8 @@ CheckpointWriteDelay(int flags, double progress)
 		{
 			got_SIGHUP = false;
 			ProcessConfigFile(PGC_SIGHUP);
+			/* update global shmem state for sync rep */
+			SyncRepUpdateSyncStandbysDefined();
 		}
 
 		AbsorbFsyncRequests();

src/backend/postmaster/walwriter.c

@@ -49,7 +49,6 @@
 #include "libpq/pqsignal.h"
 #include "miscadmin.h"
 #include "postmaster/walwriter.h"
-#include "replication/syncrep.h"
 #include "storage/bufmgr.h"
 #include "storage/fd.h"
 #include "storage/ipc.h"
@@ -217,9 +216,6 @@ WalWriterMain(void)
 	 */
 	PG_SETMASK(&UnBlockSig);
 
-	/* Do this once before starting the loop, then just at SIGHUP time. */
-	SyncRepUpdateSyncStandbysDefined();
-
 	/*
 	 * Loop forever
 	 */
@@ -241,8 +237,6 @@ WalWriterMain(void)
 		{
 			got_SIGHUP = false;
 			ProcessConfigFile(PGC_SIGHUP);
-			/* update global shmem state for sync rep */
-			SyncRepUpdateSyncStandbysDefined();
 		}
 		if (shutdown_requested)
 		{

src/backend/replication/syncrep.c

@@ -557,9 +557,9 @@ SyncRepWakeQueue(bool all)
 }
 
 /*
- * WAL writer calls this as needed to update the shared sync_standbys_defined
- * flag, so that backends don't remain permanently wedged if
- * synchronous_standby_names is unset.  It's safe to check the current value
+ * The background writer calls this as needed to update the shared
+ * sync_standbys_defined flag, so that backends don't remain permanently wedged
+ * if synchronous_standby_names is unset.  It's safe to check the current value
  * without the lock, because it's only ever updated by one process.  But we
  * must take the lock to change it.
  */