query returns incorrect result when context done in the middle

[D3Hunter]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

suppose we have this patch added to help reproduce

diff --git a/pkg/executor/table_reader.go b/pkg/executor/table_reader.go
index 4d87861c95..fddbfe284a 100644
--- a/pkg/executor/table_reader.go
+++ b/pkg/executor/table_reader.go
@@ -250,6 +250,9 @@ func (e *TableReaderExecutor) Next(ctx context.Context, req *chunk.Chunk) error
                }
                return tableName
        }), e.ranges)
+       if e.columns[0].Name.L == "a" {
+               time.Sleep(5 * time.Second)
+       }
        if err := e.resultHandler.nextChunk(ctx, req); err != nil {
                return err
        }

then run this test

func TestIncorrectQuery(t *testing.T) {
	store := testkit.CreateMockStore(t)
	tk := testkit.NewTestKit(t, store)
	tk.MustExec("use test")
	tk.MustExec("create table t(a int)")
	tk.MustExec("insert into t values(1)")

	ctx, cancelFunc := context.WithTimeout(context.Background(), 2*time.Second)
	defer cancelFunc()
	ctx = util.WithInternalSourceType(ctx, "scheduler")
	rs, err := tk.Session().ExecuteInternal(ctx, "select * from test.t")
	rows, err2 := session.ResultSetToStringSlice(ctx, tk.Session(), rs)
	t.Log("result&err: ", len(rows), err, err2)
}

might need to run it for a few times

    scheduler_test.go:720: result&err:  0 <nil> <nil>

here we don't return context error, if context cancelled in the middle the query returns incorrect result.
Query from normal client don't have issue, as we don't cancel context except when kill where it's handled.

tidb/pkg/store/copr/coprocessor.go

Lines 945 to 951 in e8861cf

	case <-ctx.Done():
	// We select the ctx.Done() in the thread of `Next` instead of in the worker to avoid the cost of `WithCancel`.
	if atomic.CompareAndSwapUint32(&it.closed, 0, 1) {
	close(it.finishCh)
	}
	exit = true
	return

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

got err: context deadline exceed

3. What did you see instead (Required)

no error, result set is empty

4. What is your TiDB version? (Required)

[XuHuaiyu]

No reproducible step for now, I'll change the label to severity/major.