expression: function format incompatible

expression/builtin_string.go

@@ -2969,10 +2969,69 @@ func evalNumDecArgsForFormat(f builtinFunc, row chunk.Row) (string, string, bool
 	} else if d > formatMaxDecimals {
 		d = formatMaxDecimals
 	}
+	xStr, err = roundFormatArgs(xStr, int(d))
+	if err != nil {
+		return "", "", true, err
+	}
 	dStr := strconv.FormatInt(d, 10)
 	return xStr, dStr, false, nil
 }
 
+func roundFormatArgs(xStr string, maxNumDecimals int) (string, error) {
+	if !strings.Contains(xStr, ".") {
+		return xStr, nil
+	}
+
+	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
+	}
+
+	xArr := strings.Split(xStr, ".")
+	integerPart := xArr[0]
+	decimalPart := xArr[1]
+
+	if len(decimalPart) > maxNumDecimals {
+		t := []byte(decimalPart)
+		carry := false
+		if t[maxNumDecimals] >= '5' {
+			carry = true
+		}
+		for i := maxNumDecimals - 1; i >= 0 && carry; i-- {
+			if t[i] == '9' {
+				t[i] = '0'
+			} else {
+				t[i] = t[i] + 1
+				carry = false
+			}
+		}
+		decimalPart = string(t)
+		t = []byte(integerPart)
+		for i := len(integerPart) - 1; i >= 0 && carry; i-- {
+			if t[i] == '9' {
+				t[i] = '0'
+			} else {
+				t[i] = t[i] + 1
+				carry = false
+			}
+		}
+		if carry {
+			integerPart = "1" + string(t)
+		} else {
+			integerPart = string(t)
+		}
+	}
+
+	xStr = integerPart + "." + decimalPart
+	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},