expression: function format incompatible

expression/builtin_string.go

@@ -2970,9 +2970,72 @@ func evalNumDecArgsForFormat(f builtinFunc, row chunk.Row) (string, string, bool
 		d = formatMaxDecimals
 	}
 	dStr := strconv.FormatInt(d, 10)
+	xStr, err = roundFormatArgs(xStr, dStr)
+	if err != nil {
+		return "", "", isNull, err
+	}
 	return xStr, dStr, false, nil
 }
 
+func roundFormatArgs(xStr, dStr string) (string, error) {
+	if strings.Contains(xStr, ".") {
+		sign := false
+		// xStr cannot have '+' prefix now.
+		// It is built in `evalNumDecArgsFormat` after evaluating `Evalxxx` method.
+		if strings.HasPrefix(xStr, "-") {
+			xStr = strings.Trim(xStr, "-")
+			sign = true
+		}
+		if strings.HasPrefix(xStr, "+") {
+			return "", errors.Errorf("xStr can not has `+` prefix")
+		}
+
+		xArr := strings.Split(xStr, ".")
+		x1 := xArr[0]
+		x2 := xArr[1]
+		digit, err := strconv.Atoi(dStr)
+		if err != nil {
+			return "", err
+		}
+		if len(x2) > digit {
+			t := []byte(x2)
+			carry := false
+			if t[digit] >= '5' {
+				carry = true
+			}
+			for i := digit - 1; i >= 0 && carry; i-- {
+				if t[i] == '9' {
+					t[i] = '0'
+				} else {
+					t[i] = t[i] + 1
+					carry = false
+				}
+			}
+			x2 = string(t)
+			t = []byte(x1)
+			for i := len(x1) - 1; i >= 0 && carry; i-- {
+				if t[i] == '9' {
+					t[i] = '0'
+				} else {
+					t[i] = t[i] + 1
+					carry = false
+				}
+			}
+			if carry {
+				x1 = "1" + string(t)
+			} else {
+				x1 = string(t)
+			}
+		}
+
+		xStr = x1 + "." + x2
+		if sign {
+			xStr = "-" + xStr
+		}
+	}
+	return xStr, nil
+}
+
 type builtinFormatWithLocaleSig struct {
 	baseBuiltinFunc
 }

expression/builtin_string_test.go

@@ -1608,6 +1608,13 @@ func (s *testEvaluatorSuite) TestFormat(c *C) {
 		ret       interface{}
 		warnings  int
 	}{
+		// issue #8796
+		{1.12345, 4, "1.1235", 0},
+		{9.99999, 4, "10.0000", 0},
+		{1.99999, 4, "2.0000", 0},
+		{1.09999, 4, "1.1000", 0},
+		{-2.5000, 0, "-3", 0},
+
 		{12332.123444, 4, "12,332.1234", 0},
 		{12332.123444, 0, "12,332", 0},
 		{12332.123444, -4, "12,332", 0},