Undesired error report when schema is changed (not null column is dropped) in rc or for-update-read.

[SpadeA-Tang]

Bug Report

This is another variant of #21498.

1. Minimal reproduce step (Required)

create table t (id int primary key, c int not null)
insert into t values (1, 1), (2, 2)

session 1

begin

session 2

alter table t drop column c
insert into t values

session 1

select * from t for update  // error

In READ-COMMITTED, the error is the same.

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

3. What did you see instead (Required)

ERROR 1105 (HY000): [components/tidb_query_executors/src/table_scan_executor.rs:411]: Data is corrupted, missing data for NOT NULL column (offset = 1)

4. What is your TiDB version? (Required)

[cfzjywxk]

@SpadeA-Tang
Better to link this issue to the existing schema issues.

[SpadeA-Tang]

@SpadeA-Tang Better to link this issue to the existing schema issues.

Okay，done.

[cfzjywxk]

Fixed on master branch a6d194b after the MDL is introduced.