Fix task timezone

When set the GUC task_timezone, it will assign and check the new value
by hook function assign_timezone and show_timezone. But this two
functions are associated with system GUC timezone.

We should add our own hook function for this.

src/backend/task/pg_cron.c

@@ -145,6 +145,8 @@ char *task_timezone = "GMT";
 int max_running_tasks = 5;
 char *task_host_addr = "127.0.0.1";
 
+static pg_tz *task_timezone_tz = NULL;
+
 /* flags set by signal handlers */
 static volatile sig_atomic_t got_sigterm = false;
 
@@ -298,6 +300,25 @@ bgw_generate_returned_message(StringInfoData *display_msg, ErrorData edata)
 		appendStringInfo(display_msg, "\nCONTEXT: %s", edata.context);
 }
 
+void
+assign_task_timezone(const char *newval, void *extra)
+{
+	task_timezone_tz = *((pg_tz **) extra);
+}
+
+const char *show_task_timezone(void)
+{
+	const char *tzn;
+
+	/* Always show the zone's canonical name */
+	tzn = pg_get_timezone_name(task_timezone_tz);
+
+	if (tzn != NULL)
+		return tzn;
+
+	return "unknown";
+}
+
 bool PgCronStartRule(Datum main_arg)
 {
 	return (Gp_role == GP_ROLE_DISPATCH);

src/backend/utils/misc/guc_gp.c

@@ -4733,7 +4733,7 @@ struct config_string ConfigureNamesString_gp[] =
 		},
 		&task_timezone,
 		"GMT",
-		check_timezone, assign_timezone, show_timezone
+		check_timezone, assign_task_timezone, show_task_timezone
 	},
 
 	{

src/include/task/pg_cron.h

@@ -33,5 +33,7 @@ extern void CronBackgroundWorker(Datum arg);
 extern pid_t PgCronLauncherPID(void);
 extern Size PgCronLauncherShmemSize(void);
 extern void PgCronLauncherShmemInit(void);
+extern void assign_task_timezone(const char *newval, void *extra);
+extern const char *show_task_timezone(void);
 
 #endif      /* PG_CRON_H */