Staleread does not work correctly

[xhebox]

Bug Report

Please answer these questions before submitting your issue. Thanks!

A bug introduced by #40768, check #40740 (comment) for more infomation.

1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

MySQL [consistent_replicate_storage_file]> set @@tidb_snapshot=439512290927575059;
Query OK, 0 rows affected (0.007 sec)
MySQL [consistent_replicate_storage_file]> show tables;
+---------------------------------------------+
| Tables_in_consistent_replicate_storage_file |
+---------------------------------------------+
| check1                                      |
+---------------------------------------------+
1 row in set (0.001 sec)

3. What did you see instead (Required)

MySQL [consistent_replicate_storage_file]> set @@tidb_snapshot=439512492570574858;
Query OK, 0 rows affected (0.005 sec)

MySQL [consistent_replicate_storage_file]> show tables;
Empty set (0.001 sec)

MySQL [consistent_replicate_storage_file]> set @@tidb_snapshot=439512492570574859;
Query OK, 0 rows affected (0.004 sec)

MySQL [consistent_replicate_storage_file]> show tables;
Empty set (0.001 sec)

MySQL [consistent_replicate_storage_file]> set @@tidb_snapshot=439512492570574860;
Query OK, 0 rows affected (0.006 sec)

MySQL [consistent_replicate_storage_file]> show tables;
+---------------------------------------------+
| Tables_in_consistent_replicate_storage_file |
+---------------------------------------------+
| check1                                      |
+---------------------------------------------+
1 row in set (0.001 sec)

4. What is your TiDB version? (Required)

v6.7- nightly - 5df1913