// writeMutation writes the mutation to the table descriptor. If the
// State or the Direction is undefined, these values are populated via
// picking random values before the mutation is written.
func (mt mutationTest) writeMutation(m csql.DescriptorMutation) {
	if m.Direction == csql.DescriptorMutation_NONE {
		// randomly pick ADD/DROP mutation.
		r := rand.Intn(2)
		if r == 0 {
			m.Direction = csql.DescriptorMutation_ADD
		} else {
			m.Direction = csql.DescriptorMutation_DROP
		}
	}
	if m.State == csql.DescriptorMutation_UNKNOWN {
		// randomly pick DELETE_ONLY/WRITE_ONLY state.
		r := rand.Intn(2)
		if r == 0 {
			m.State = csql.DescriptorMutation_DELETE_ONLY
		} else {
			m.State = csql.DescriptorMutation_WRITE_ONLY
		}
	}
	tableDesc := mt.desc.GetTable()
	tableDesc.Mutations = append(tableDesc.Mutations, m)
	if err := tableDesc.Validate(); err != nil {
		mt.Fatal(err)
	}
	if err := mt.kvDB.Put(mt.descKey, mt.desc); err != nil {
		mt.Fatal(err)
	}
}
// writeColumnMutation adds column as a mutation and writes the
// descriptor to the DB.
func (mt mutationTest) writeColumnMutation(t *testing.T, column string, state csql.DescriptorMutation_State) {
	tableDesc := mt.desc.GetTable()
	i, err := tableDesc.FindColumnByName(column)
	if err != nil {
		t.Fatal(err)
	}
	col := &tableDesc.Columns[i]
	m := csql.DescriptorMutation{Descriptor_: &csql.DescriptorMutation_Column{Column: col}, State: state}
	// randomly pick add/drop mutation.
	r := rand.Intn(2)
	if r == 0 {
		m.Direction = csql.DescriptorMutation_ADD
	} else {
		m.Direction = csql.DescriptorMutation_DROP
	}
	tableDesc.Mutations = append(tableDesc.Mutations, m)
	tableDesc.Columns = append(tableDesc.Columns[:i], tableDesc.Columns[i+1:]...)
	if err := tableDesc.Validate(); err != nil {
		t.Fatal(err)
	}
	if err := mt.kvDB.Put(mt.descKey, mt.desc); err != nil {
		t.Fatal(err)
	}
}