encoding/json: check if Number is valid

json.Number is a special case which didn't have any checks and could result in invalid JSON.

Fixes #10281

Change-Id: Ie3e726e4d6bf6a6aba535d36f6107013ceac913a
Reviewed-on: https://go-review.googlesource.com/12250
Reviewed-by: Russ Cox <rsc@golang.org>

src/encoding/json/decode.go

@@ -174,6 +174,124 @@ func (n Number) Int64() (int64, error) {
 	return strconv.ParseInt(string(n), 10, 64)
 }
 
+// IsValid returns if the number is a valid JSON number literal.
+func (n Number) IsValid() bool {
+	// This function implements the JSON numbers grammar.
+	// See https://tools.ietf.org/html/rfc7159#section-6
+	// and http://json.org/number.gif
+
+	l := len(n)
+	if l == 0 {
+		return false
+	}
+
+	i := 0
+	c := n[i]
+	i++
+
+	// Optional -
+	if c == '-' {
+		if i == l {
+			return false
+		}
+
+		c = n[i]
+		i++
+	}
+
+	// 1-9
+	if c >= '1' && c <= '9' {
+		// Eat digits.
+		for ; i < l; i++ {
+			c = n[i]
+			if c < '0' || c > '9' {
+				break
+			}
+		}
+		i++
+	} else if c != '0' {
+		// If it's not 0 or 1-9 it's invalid.
+		return false
+	} else {
+		if i == l {
+			// Just 0
+			return true
+		}
+
+		// Skip the 0
+		c = n[i]
+		i++
+	}
+
+	// . followed by 1 or more digits.
+	if c == '.' {
+		if i == l {
+			// Just 1. is invalid.
+			return false
+		}
+
+		// . needs to be followed by at least one digit.
+		c = n[i]
+		i++
+		if c < '0' || c > '9' {
+			return false
+		}
+
+		// Eat digits.
+		for ; i < l; i++ {
+			c = n[i]
+			if c < '0' || c > '9' {
+				break
+			}
+		}
+		i++
+	}
+
+	// e or E followed by an optional - or + and
+	// 1 or more digits.
+	if c == 'e' || c == 'E' {
+		if i == l {
+			// Just 1e is invalid.
+			return false
+		}
+
+		c = n[i]
+		i++
+
+		// Optional - or +
+		if c == '-' || c == '+' {
+			if i == l {
+				// Just 1e+ is invalid.
+				return false
+			}
+
+			c = n[i]
+			i++
+		}
+
+		// Need to have a digit.
+		if c < '0' || c > '9' {
+			return false
+		}
+
+		// Eat digits.
+		for ; i < l; i++ {
+			c = n[i]
+			if c < '0' || c > '9' {
+				break
+			}
+		}
+		i++
+	}
+
+	// Make sure we are at the end.
+	if i <= l {
+		return false
+	}
+
+	return true
+}
+
 // decodeState represents the state while decoding a JSON value.
 type decodeState struct {
 	data       []byte
@@ -781,6 +899,9 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool
 		default:
 			if v.Kind() == reflect.String && v.Type() == numberType {
 				v.SetString(s)
+				if !Number(s).IsValid() {
+					d.error(fmt.Errorf("json: invalid number literal, trying to unmarshal %q into Number", item))
+				}
 				break
 			}
 			if fromQuoted {