domain: reduce the chance of "Information schema is out of date" error

pkg/domain/domain.go

@@ -810,15 +810,40 @@ func (do *Domain) Reload() error {
 		}
 	}
 
-	// lease renew, so it must be executed despite it is cache or not
-	do.SchemaValidator.Update(version, oldSchemaVersion, is.SchemaMetaVersion(), changes)
 	lease := do.GetSchemaLease()
 	sub := time.Since(startTime)
 	// Reload interval is lease / 2, if load schema time elapses more than this interval,
 	// some query maybe responded by ErrInfoSchemaExpired error.
 	if sub > (lease/2) && lease > 0 {
+		// If it is a full load and there are a lot of tables, this is likely to happen.
 		logutil.BgLogger().Warn("loading schema takes a long time", zap.Duration("take time", sub))
+
+		// We can optimize the case by updating the TS to a new value, as long as the schema version is the same.
+		// For example, lease is 45s, and the load process takes 2min, after the load process finish, this
+		// loaded infoschema because stale immediately.
+		// But if we re-check the schema version again and verify that it's still the newest, it is safe to use it.
+		var latestSchemaVer int64
+		var currentTS uint64
+		ctx := kv.WithInternalSourceType(context.Background(), kv.InternalTxnMeta)
+		err := kv.RunInNewTxn(ctx, do.store, false, func(_ context.Context, txn kv.Transaction) error {
+			var err error
+			m := meta.NewReader(txn)
+			latestSchemaVer, err = m.GetSchemaVersion()
+			if err != nil {
+				return errors.Trace(err)
+			}
+			currentTS = txn.StartTS()
+			return nil
+		})
+		if err == nil && latestSchemaVer == is.SchemaMetaVersion() {
+			version = currentTS
+			logutil.BgLogger().Info("use this schema and update ts",
+				zap.Int64("schema ver", latestSchemaVer),
+				zap.Uint64("reload ts", currentTS))
+		}
 	}
+	// lease renew, so it must be executed despite it is cache or not
+	do.SchemaValidator.Update(version, oldSchemaVersion, is.SchemaMetaVersion(), changes)
 
 	return nil
 }