Rename ReservedBackends variable to SuperuserReservedConnections.

This is in preparation for adding a new reserved_connections GUC,
but aligning the GUC name with the variable name is also a good
idea on general principle.

Patch by Nathan Bossart. Reviewed by Tushar Ahuja and by me.

Discussion: http://postgr.es/m/20230119194601.GA4105788@nathanxps13

src/backend/postmaster/postmaster.c

@@ -204,15 +204,15 @@ char	   *Unix_socket_directories;
 char	   *ListenAddresses;
 
 /*
- * ReservedBackends is the number of backends reserved for superuser use.
- * This number is taken out of the pool size given by MaxConnections so
- * number of backend slots available to non-superusers is
- * (MaxConnections - ReservedBackends).  Note what this really means is
- * "if there are <= ReservedBackends connections available, only superusers
- * can make new connections" --- pre-existing superuser connections don't
- * count against the limit.
+ * SuperuserReservedConnections is the number of backends reserved for
+ * superuser use.  This number is taken out of the pool size given by
+ * MaxConnections so number of backend slots available to non-superusers is
+ * (MaxConnections - SuperuserReservedConnections).  Note what this really
+ * means is "if there are <= SuperuserReservedConnections connections
+ * available, only superusers can make new connections" --- pre-existing
+ * superuser connections don't count against the limit.
  */
-int			ReservedBackends;
+int			SuperuserReservedConnections;
 
 /* The socket(s) we're listening to. */
 #define MAXLISTEN	64
@@ -908,11 +908,11 @@ PostmasterMain(int argc, char *argv[])
 	/*
 	 * Check for invalid combinations of GUC settings.
 	 */
-	if (ReservedBackends >= MaxConnections)
+	if (SuperuserReservedConnections >= MaxConnections)
 	{
 		write_stderr("%s: superuser_reserved_connections (%d) must be less than max_connections (%d)\n",
 					 progname,
-					 ReservedBackends, MaxConnections);
+					 SuperuserReservedConnections, MaxConnections);
 		ExitPostmaster(1);
 	}
 	if (XLogArchiveMode > ARCHIVE_MODE_OFF && wal_level == WAL_LEVEL_MINIMAL)

src/backend/utils/init/postinit.c

@@ -927,8 +927,8 @@ InitPostgres(const char *in_dbname, Oid dboid,
 	 * limited by max_connections or superuser_reserved_connections.
 	 */
 	if (!am_superuser && !am_walsender &&
-		ReservedBackends > 0 &&
-		!HaveNFreeProcs(ReservedBackends))
+		SuperuserReservedConnections > 0 &&
+		!HaveNFreeProcs(SuperuserReservedConnections))
 		ereport(FATAL,
 				(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
 				 errmsg("remaining connection slots are reserved for superusers")));

src/backend/utils/misc/guc_tables.c

@@ -2163,7 +2163,7 @@ struct config_int ConfigureNamesInt[] =
 			gettext_noop("Sets the number of connection slots reserved for superusers."),
 			NULL
 		},
-		&ReservedBackends,
+		&SuperuserReservedConnections,
 		3, 0, MAX_BACKENDS,
 		NULL, NULL, NULL
 	},

src/include/postmaster/postmaster.h

@@ -15,7 +15,7 @@
 
 /* GUC options */
 extern PGDLLIMPORT bool EnableSSL;
-extern PGDLLIMPORT int ReservedBackends;
+extern PGDLLIMPORT int SuperuserReservedConnections;
 extern PGDLLIMPORT int PostPortNumber;
 extern PGDLLIMPORT int Unix_socket_permissions;
 extern PGDLLIMPORT char *Unix_socket_group;