Merge pull request #431 from capnproto/improve-maxseg-error

Add context to 'too many segments' error in decoder.

message.go

@@ -736,7 +736,7 @@ func (d *Decoder) Decode() (*Message, error) {
 	}
 	maxSeg := SegmentID(binary.LittleEndian.Uint32(d.wordbuf[:]))
 	if maxSeg > maxStreamSegments {
-		return nil, errors.New("decode: too many segments to decode")
+		return nil, errSegIDTooLarge(maxSeg)
 	}
 
 	// Read the rest of the header if more than one segment.
@@ -804,6 +804,14 @@ func (d *Decoder) Decode() (*Message, error) {
 	return &d.msg, nil
 }
 
+type errSegIDTooLarge SegmentID
+
+func (err errSegIDTooLarge) Error() string {
+	id := str.Utod(err)
+	max := str.Itod(maxStreamSegments)
+	return "decode: segment id" + id + "exceeds max segment count (max=" + max + ")"
+}
+
 func resizeSlice(b []byte, size int) []byte {
 	if cap(b) < size {
 		return make([]byte, size)