*: fix cast decimal allocates large memory. (#2146)

If precision is too large, encode decimal allocates large amount of memory.
This commit set cast decimal unspecified length to default decimal length (10),
also add check on precision and frac on `MyDecimal.ToBin`.

executor/executor_test.go

@@ -982,6 +982,8 @@ func (s *testSuite) TestBuiltin(c *C) {
 	result.Check(testkit.Rows("1991-09-05 11:11:11"))
 	result = tk.MustQuery("select cast('11:11:11' as time)")
 	result.Check(testkit.Rows("11:11:11"))
+	result = tk.MustQuery("select * from t where a > cast(2 as decimal)")
+	result.Check(testkit.Rows("3 2"))
 
 	// test unhex and hex
 	result = tk.MustQuery("select unhex('4D7953514C')")

parser/parser.y

@@ -3115,6 +3115,10 @@ CastType:
 		x := types.NewFieldType(mysql.TypeNewDecimal)
 		x.Flen = fopt.Flen
 		x.Decimal = fopt.Decimal
+		if fopt.Flen == types.UnspecifiedLength {
+			x.Flen = mysql.GetDefaultFieldLength(mysql.TypeNewDecimal)
+			x.Decimal = mysql.GetDefaultDecimal(mysql.TypeNewDecimal)
+		}
 		$$ = x
 	}
 |	"TIME" OptFieldLen

util/types/mydecimal.go

@@ -1040,6 +1040,9 @@ with the correct -1/0/+1 result
                 7E F2 04 C7 2D FB 2D
 */
 func (d *MyDecimal) ToBin(precision, frac int) ([]byte, error) {
+	if precision > digitsPerWord*maxWordBufLen || precision < 0 || frac > MaxFraction || frac < 0 {
+		return nil, ErrBadNumber
+	}
 	var err error
 	var mask int32
 	if d.negative {

util/types/mydecimal_test.go

@@ -393,6 +393,21 @@ func (s *testMyDecimalSuite) TestToBinFromBin(c *C) {
 		str := dec2.ToString()
 		c.Assert(string(str), Equals, ca.output)
 	}
+	var dec MyDecimal
+	dec.FromInt(1)
+	errCases := []struct {
+		prec int
+		frac int
+	}{
+		{82, 1},
+		{-1, 1},
+		{10, 31},
+		{10, -1},
+	}
+	for _, ca := range errCases {
+		_, err := dec.ToBin(ca.prec, ca.frac)
+		c.Assert(ErrBadNumber.Equal(err), IsTrue)
+	}
 }
 
 func (s *testMyDecimalSuite) TestCompare(c *C) {