attrs: add CheckSize function

api/stun1.txt

@@ -103,6 +103,7 @@ pkg github.com/gortc/stun, const MethodSend Method
 pkg github.com/gortc/stun, const TransactionIDSize = 12
 pkg github.com/gortc/stun, const TransactionIDSize ideal-int
 pkg github.com/gortc/stun, func Build(...Setter) (*Message, error)
+pkg github.com/gortc/stun, func CheckSize(AttrType, int, int) error
 pkg github.com/gortc/stun, func Decode([]uint8, *Message) error
 pkg github.com/gortc/stun, func Dial(string, string) (*Client, error)
 pkg github.com/gortc/stun, func FingerprintValue([]uint8) uint32
@@ -313,6 +314,7 @@ pkg github.com/gortc/stun, var BindingError MessageType
 pkg github.com/gortc/stun, var BindingRequest MessageType
 pkg github.com/gortc/stun, var BindingSuccess MessageType
 pkg github.com/gortc/stun, var ErrAgentClosed error
+pkg github.com/gortc/stun, var ErrAttrSizeInvalid error
 pkg github.com/gortc/stun, var ErrAttributeNotFound error
 pkg github.com/gortc/stun, var ErrBadIPLength error
 pkg github.com/gortc/stun, var ErrBadUnknownAttrsSize error

checksize.go

@@ -0,0 +1,11 @@
+// +build !debug
+
+package stun
+
+// CheckSize returns ErrAttrSizeInvalid if got is not equal to expected.
+func CheckSize(_ AttrType, got, expected int) error {
+	if got == expected {
+		return nil
+	}
+	return ErrAttrSizeInvalid
+}

checksize_debug.go

@@ -0,0 +1,15 @@
+// +build debug
+
+package stun
+
+// CheckSize returns *AttrLengthError if got is not equal to expected.
+func CheckSize(a AttrType, got, expected int) error {
+	if got == expected {
+		return nil
+	}
+	return &AttrLengthErr{
+		Got:      got,
+		Expected: expected,
+		Attr:     a,
+	}
+}

errors.go

@@ -1,5 +1,7 @@
 package stun
 
+import "errors"
+
 // DecodeErr records an error and place when it is occurred.
 type DecodeErr struct {
 	Place   DecodeErrPlace
@@ -52,3 +54,6 @@ func newDecodeErr(parent, children, message string) *DecodeErr {
 func newAttrDecodeErr(children, message string) *DecodeErr {
 	return newDecodeErr("attribute", children, message)
 }
+
+// ErrAttrSizeInvalid means that decoded attribute size is invalid.
+var ErrAttrSizeInvalid = errors.New("attribute size is invalid")

fingerprint.go

@@ -67,12 +67,8 @@ func (FingerprintAttr) Check(m *Message) error {
 	if err != nil {
 		return err
 	}
-	if len(b) != fingerprintSize {
-		return &AttrLengthErr{
-			Expected: fingerprintSize,
-			Got:      len(b),
-			Attr:     AttrFingerprint,
-		}
+	if err = CheckSize(AttrFingerprint, len(b), fingerprintSize); err != nil {
+		return err
 	}
 	val := bin.Uint32(b)
 	attrStart := len(m.Raw) - (fingerprintSize + attributeHeaderSize)