mqtt0 - Known Issues

🟠 Major Issues

MQTT0-001: Go Client read path is not demultiplexed

File: go/pkg/mqtt0/client.go

Description: Subscribe, Unsubscribe, and other request/response operations read directly from the same stream as Recv(). If callers run Recv() concurrently with Subscribe() or Unsubscribe(), whichever acquires readMu first may consume packets that belong to the other operation, causing unexpected packet errors.

Impact: Hard-to-debug race between subscription changes and inbound message handling.

Suggestion: Introduce a single read loop with protocol demuxing, or document that Recv() must not run concurrently with subscribe/unsubscribe calls.

🟡 Minor Issues

MQTT0-002: Go Broker drops messages on backpressure

File: go/pkg/mqtt0/broker.go

Description: The broker uses a bounded channel for each client. When the channel is full, messages are dropped with a debug log.

Impact: Message loss under bursty load beyond QoS 0 expectations; may surprise users.

Suggestion: Document the drop behavior clearly or make buffer size configurable.

MQTT0-003: Rust WebSocket transport requires special handling

File: rust/mqtt0/src/transport.rs

Description: Transport::WebSocket implements AsyncRead/AsyncWrite by returning Unsupported errors. If code treats Transport uniformly, WebSocket connections will fail at runtime.

Impact: Surprising runtime errors for WebSocket clients if not handled explicitly.