util/timeutil: fix data race caused by forgetting set stats lease to 0 (

#7901) (#8485)

util/kvencoder/kv_encoder_test.go

@@ -49,6 +49,7 @@ func newStoreWithBootstrap() (kv.Storage, *domain.Domain, error) {
 		return nil, nil, errors.Trace(err)
 	}
 	session.SetSchemaLease(0)
+	session.SetStatsLease(0)
 	dom, err := session.BootstrapSession(store)
 	return store, dom, errors.Trace(err)
 }

util/timeutil/time.go

@@ -117,9 +117,13 @@ func SystemLocation() *time.Location {
 	return loc
 }
 
+var setSysTZOnce sync.Once
+
 // SetSystemTZ sets systemTZ by the value loaded from mysql.tidb.
 func SetSystemTZ(name string) {
-	systemTZ = name
+	setSysTZOnce.Do(func() {
+		systemTZ = name
+	})
 }
 
 // getLoc first trying to load location from a cache map. If nothing found in such map, then call