types: fix a bug in decimal multiplication (#7208)

expression/integration_test.go

@@ -3366,3 +3366,12 @@ func (s *testIntegrationSuite) TestPrefixIndex(c *C) {
 	res := tk.MustQuery("select * from t1 where name = '借款策略集_网页';")
 	res.Check(testkit.Rows("借款策略集_网页"))
 }
+
+func (s *testIntegrationSuite) TestDecimalMul(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("USE test")
+	tk.MustExec("create table t(a decimal(38, 17));")
+	tk.MustExec("insert into t select 0.5999991229316*0.918755041726043;")
+	res := tk.MustQuery("select * from t;")
+	res.Check(testkit.Rows("0.55125221922461136"))
+}

tablecodec/tablecodec_test.go

@@ -19,6 +19,7 @@ import (
 	"testing"
 	"time"
 
+	gofail "github.com/coreos/gofail/runtime"
 	. "github.com/pingcap/check"
 	"github.com/pingcap/tidb/mysql"
 	"github.com/pingcap/tidb/sessionctx/stmtctx"
@@ -278,6 +279,16 @@ func (s *testTableCodecSuite) TestCutKey(c *C) {
 	c.Assert(handleVal, DeepEquals, types.NewIntDatum(100))
 }
 
+func (s *testTableCodecSuite) TestDecodeBadDecical(c *C) {
+	gofail.Enable("github.com/pingcap/tidb/util/codec/errorInDecodeDecimal", `return(true)`)
+	defer gofail.Disable("github.com/pingcap/tidb/util/codec/errorInDecodeDecimal")
+	dec := types.NewDecFromStringForTest("0.111")
+	b := codec.EncodeDecimal(nil, dec, 0, 0)
+	// Expect no panic.
+	_, _, err := codec.DecodeOne(b)
+	c.Assert(err, NotNil)
+}
+
 func (s *testTableCodecSuite) TestIndexKey(c *C) {
 	tableID := int64(4)
 	indexID := int64(5)

types/mydecimal.go

@@ -85,6 +85,24 @@ func add(a, b, carry int32) (int32, int32) {
 	return sum, carry
 }
 
+// add2 adds a and b and carry, returns the sum and new carry.
+// It is only used in DecimalMul.
+func add2(a, b, carry int32) (int32, int32) {
+	sum := int64(a) + int64(b) + int64(carry)
+	if sum >= wordBase {
+		carry = 1
+		sum -= wordBase
+	} else {
+		carry = 0
+	}
+
+	if sum >= wordBase {
+		sum -= wordBase
+		carry++
+	}
+	return int32(sum), carry
+}
+
 // sub subtracts b and carry from a, returns the diff and new carry.
 func sub(a, b, carry int32) (int32, int32) {
 	diff := a - b - carry
@@ -1777,7 +1795,7 @@ func DecimalMul(from1, from2, to *MyDecimal) error {
 			p := int64(from1.wordBuf[idx1]) * int64(from2.wordBuf[idx2])
 			hi = int32(p / wordBase)
 			lo = int32(p - int64(hi)*wordBase)
-			to.wordBuf[idxTo], carry = add(to.wordBuf[idxTo], lo, carry)
+			to.wordBuf[idxTo], carry = add2(to.wordBuf[idxTo], lo, carry)
 			carry += hi
 			idx2--
 			idxTo--
@@ -1786,7 +1804,7 @@ func DecimalMul(from1, from2, to *MyDecimal) error {
 			if idxTo < 0 {
 				return ErrOverflow
 			}
-			to.wordBuf[idxTo], carry = add(to.wordBuf[idxTo], 0, carry)
+			to.wordBuf[idxTo], carry = add2(to.wordBuf[idxTo], 0, carry)
 		}
 		for idxTo--; carry > 0; idxTo-- {
 			if idxTo < 0 {

types/mydecimal_test.go

@@ -619,6 +619,8 @@ func (s *testMyDecimalSuite) TestMul(c *C) {
 		{"123", "0.01", "1.23", nil},
 		{"123", "0", "0", nil},
 		{"1" + strings.Repeat("0", 60), "1" + strings.Repeat("0", 60), "0", ErrOverflow},
+		{"0.5999991229316", "0.918755041726043", "0.5512522192246113614062276588", nil},
+		{"0.5999991229317", "0.918755041726042", "0.5512522192247026369112773314", nil},
 	}
 	for _, tt := range tests {
 		var a, b, product MyDecimal

util/codec/codec.go

@@ -336,10 +336,12 @@ func DecodeOne(b []byte) (remain []byte, d types.Datum, err error) {
 	case decimalFlag:
 		var dec *types.MyDecimal
 		b, dec, err = DecodeDecimal(b)
-		precision, frac := dec.PrecisionAndFrac()
-		d.SetMysqlDecimal(dec)
-		d.SetLength(precision)
-		d.SetFrac(frac)
+		if err == nil {
+			precision, frac := dec.PrecisionAndFrac()
+			d.SetMysqlDecimal(dec)
+			d.SetLength(precision)
+			d.SetFrac(frac)
+		}
 	case durationFlag:
 		var r int64
 		b, r, err = DecodeInt(b)

util/codec/decimal.go

@@ -36,6 +36,10 @@ func EncodeDecimal(b []byte, dec *types.MyDecimal, precision, frac int) []byte {
 
 // DecodeDecimal decodes bytes to decimal.
 func DecodeDecimal(b []byte) ([]byte, *types.MyDecimal, error) {
+	// gofail: var errorInDecodeDecimal bool
+	// if errorInDecodeDecimal {
+	//	 return b, nil, errors.New("gofail error")
+	// }
 	if len(b) < 3 {
 		return b, nil, errors.New("insufficient bytes to decode value")
 	}