nats-io/nats-server · error

error reading %s: %v

Error message

error reading %s: %v

What it means

mqttReader.readByte hits the end of the buffered packet (r.pos == len(r.buf)) while trying to read the named field, reporting io.EOF for that field. Usually means the packet is truncated relative to its declared remaining length, or a fixed-width field is missing.

Source

Thrown at server/mqtt.go:6350

	if l := len(r.pbuf); l > 0 {
		tmp := make([]byte, l+len(buf))
		copy(tmp, r.pbuf)
		copy(tmp[l:], buf)
		buf = tmp
		r.pbuf = nil
	}
	r.buf = buf
	r.pos = 0
	r.pstart = 0
}

func (r *mqttReader) hasMore() bool {
	return r.pos != len(r.buf)
}

func (r *mqttReader) readByte(field string) (byte, error) {
	if r.pos == len(r.buf) {
		return 0, fmt.Errorf("error reading %s: %v", field, io.EOF)
	}
	b := r.buf[r.pos]
	r.pos++
	return b, nil
}

func (r *mqttReader) readPacketLen(maxLen int32) (int, bool, error) {
	v, complete, err := r.readVarInt()
	if err != nil {
		return 0, false, err
	}
	if complete {
		packetEnd := r.pos + v
		packetLen := packetEnd - r.pstart
		if maxLen != jwt.NoLimit && int64(packetLen) > int64(maxLen) {
			return packetLen, false, ErrMaxPayload
		}
		if packetEnd <= len(r.buf) {

View on GitHub (pinned to 3a66a489d2)

Solutions

1. Check the client's remaining-length encoding matches the actual payload
2. Capture the raw packet to see which field is short
3. Verify no intermediary is truncating frames

When it happens

Trigger: Thrown at server/mqtt.go:6350 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of nats-io/nats-server@3a66a489d2 (2026-09-02). Data as JSON: /api/errors/d139cb737b1c2901. Report an issue: GitHub.