Op_Lock could not trigger pessimistic lock write conflict

[cfzjywxk]

Consider such case

func (s *testMvccSuite) TestPessimisticLockConflict(c *C) {
	store, err := NewTestStore("TestPessimisticLockConflict", "TestPessimisticLockConflict", c)
	c.Assert(err, IsNil)
	defer CleanTestStore(store)

	// txn1
	k := []byte("tk")
	v := []byte("v")
	MustPrewritePut(k, k, v, 5, store)
	MustCommit(k, 5, 10, store)

        // txn2
	MustPrewriteLock(k, k, 12, store)
	MustCommit(k, 12, 13, store)

        // txn3
	_, err = PessimisticLock(k, k, 11, lockTTL, 11, false, false, store)
	c.Assert(err, NotNil)
}

The last PessimsiticLock request should get an error since its startTS is smaller than the commitTS of the txn2, but it succeeded.
Seems Op_Lock write record could not be found from the storage engine or is collapsed without keeping a newset one?

[coocood]

It is not collapsed, it's stored in a different key space, so they don't conflict each other, the commit status of Op_Lock can still be checked by CheckTxnStatus.

[cfzjywxk]

It is not collapsed, it's stored in a different key space, so they don't conflict each other, the commit status of Op_Lock can still be checked by CheckTxnStatus.

The getDBItems does't return the Op_Lock record, it's needed to judge if a pessmistic lock request should get write conflict error. Some unit test cases in TiDB will fail using mockstore but not using TiKV because of this incompatibility.

[coocood]

OK, you can change it to be compatible.

[cfzjywxk]

For optimistic transactions, the write conflict for Op_Lock could not be triggered, examples:
Start a tidb instance with unistore

mysql> create table t1(c1 int primary key, c2 int);
Query OK, 0 rows affected (0.09 sec)

mysql> insert into t1 values(1, 10);
Query OK, 1 row affected (0.01 sec)

session1:
mysql> begin optimistic;
Query OK, 0 rows affected (0.00 sec)

session1:
mysql> update t1 set c2 = c2 + 1;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

session2:
mysql> begin optimistic;
Query OK, 0 rows affected (0.01 sec)

session2:
mysql> select * from t1 for update;
+----+------+
| c1 | c2   |
+----+------+
|  1 |   10 |
+----+------+
1 row in set (0.00 sec)

session2:
mysql> commit;

session1:
mysql> commit; // expected to report write conflict error