parser: unexpected `unknown 1 in where clause` error

[qw4990]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Run the unit test below:

func TestUnknown1InWhereClause(t *testing.T) {
	store := testkit.CreateMockStore(t)
	tk := testkit.NewTestKit(t, store)
	tk.MustExec("use test")
	tk.MustExec("create table t(a varchar(32) NOT NULL primary key,b varchar(22) not null,key idx_a (a))")
	go func() {
		tkk := testkit.NewTestKit(t, store)
		tkk.MustExec(`use test`)
		for k := 0; k < 10000; k++ {
			tkk.ExecToErr("select * from xw.t1 where 1=1 a.  ")
		}
	}()
	for k := 0; k < 10000; k++ {
		tk.MustQuery(`select count(1) from t where a = 'Q13'`).Check(testkit.Rows("0"))
	}
}

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

No error.

3. What did you see instead (Required)

                Error:          Received unexpected error:
                                [planner:1054]Unknown column '1' in 'where clause'

4. What is your TiDB version? (Required)

[qw4990]

The value 1 is parsed as a column name:

[qw4990]

If we allocate a new Parser each time, then it can pass this test. So it seems like Parser's status is not reset correctly when we put it back to the pool:

[qw4990]

The minimum reproducible case:

func TestUnknown1InWhereClause2(t *testing.T) {
	p := parser.New()
	p.ParseSQL("a.")
	stmts, _, err := p.ParseSQL("select count(1) from t")
	require.NoError(t, err)
	var sb strings.Builder
	restoreCtx := format.NewRestoreCtx(format.DefaultRestoreFlags, &sb)
	sb.Reset()
	stmts[0].Restore(restoreCtx)
	fmt.Println("-->>> ", sb.String())
}

The result should be

SELECT COUNT(1) FROM `t`

instead of

SELECT COUNT(`1`) FROM `t`