feat(mqtt): add MQTT packet parser, dispatch, and ReadLoop integration
Implement Task 2 of MQTT orchestration: - Create MqttParser.cs with loop-based packet parser and dispatch switch - Add MqttReader field to MqttHandler for per-connection parsing state - Wire ReadLoop to call MqttParser for MQTT connections - Implement PINGREQ handler (enqueues PINGRESP) - CONNECT-first enforcement (rejects non-CONNECT as first packet) - Partial packet handling via MqttReader pending buffer - 13 unit tests covering parser, dispatch, partial packets, and edge cases - Stub dispatch entries for CONNECT, PUBLISH, SUB, UNSUB, DISCONNECT (NotImplementedException)
This commit is contained in:
Joseph Doherty
162
dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttParser.cs
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162
dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttParser.cs
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// Copyright 2020-2026 The NATS Authors
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// Adapted from server/mqtt.go mqttParse() in the NATS server Go source.
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namespace ZB.MOM.NatsNet.Server.Mqtt;
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/// <summary>
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/// MQTT binary packet parser and dispatch.
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/// Reads packets from a byte buffer using <see cref="MqttReader"/> and dispatches
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/// to the appropriate handler based on packet type.
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/// Mirrors Go <c>mqttParse()</c> in server/mqtt.go.
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/// </summary>
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internal static class MqttParser
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{
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/// <summary>PINGRESP packet bytes: 0xD0 0x00.</summary>
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private static readonly byte[] PingRespPacket = [MqttPacket.PingResp, 0x00];
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/// <summary>
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/// Parses MQTT packets from <paramref name="buf"/> and dispatches to handlers.
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/// Returns null on success, or an exception describing the parse/dispatch error.
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/// Handles partial packets by saving state in the client's <see cref="MqttReader"/>.
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/// Mirrors Go <c>mqttParse(r *mqttReader, c *client, ...)</c>.
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/// </summary>
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public static Exception? Parse(ClientConnection c, byte[] buf, int len)
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{
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var mqtt = c.Mqtt!;
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var r = mqtt.Reader;
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// Slice buffer to actual length if needed.
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if (len < buf.Length)
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{
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var tmp = new byte[len];
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Buffer.BlockCopy(buf, 0, tmp, 0, len);
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buf = tmp;
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}
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r.Reset(buf);
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var connected = (c.Flags & ClientFlags.ConnectReceived) != 0;
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Exception? err = null;
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while (err == null && r.HasMore())
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{
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r.PacketStart = r.Position;
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// Read packet type + flags byte.
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byte b;
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try { b = r.ReadByte("packet type"); }
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catch (Exception ex) { err = ex; break; }
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var pt = (byte)(b & MqttPacket.Mask);
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// CONNECT must be the first packet.
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if (!connected && pt != MqttPacket.Connect)
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{
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err = new InvalidOperationException(
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$"the first packet should be a CONNECT (0x{MqttPacket.Connect:X2}), got 0x{pt:X2}");
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break;
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}
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// Read remaining length (variable-length encoding).
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int pl;
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bool complete;
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try
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{
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(pl, complete) = r.ReadPacketLen();
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}
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catch (Exception ex) { err = ex; break; }
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if (!complete)
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break; // Partial packet — state saved in reader.
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// Dispatch based on packet type.
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switch (pt)
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{
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case MqttPacket.Pub:
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// TODO: Task 4 — MqttParsePub + MqttProcessPub
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err = new NotImplementedException("PUBLISH not yet implemented");
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break;
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case MqttPacket.PubAck:
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// TODO: Task 5 — process PUBACK
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err = new NotImplementedException("PUBACK not yet implemented");
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break;
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case MqttPacket.PubRec:
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// TODO: Task 5 — process PUBREC
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err = new NotImplementedException("PUBREC not yet implemented");
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break;
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case MqttPacket.PubRel:
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// TODO: Task 5 — process PUBREL
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err = new NotImplementedException("PUBREL not yet implemented");
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break;
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case MqttPacket.PubComp:
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// TODO: Task 5 — process PUBCOMP
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err = new NotImplementedException("PUBCOMP not yet implemented");
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break;
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case MqttPacket.Sub:
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// TODO: Task 4 — MqttParseSubs + MqttProcessSubs
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err = new NotImplementedException("SUBSCRIBE not yet implemented");
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break;
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case MqttPacket.Unsub:
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// TODO: Task 4 — MqttParseUnsubs + MqttProcessUnsubs
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err = new NotImplementedException("UNSUBSCRIBE not yet implemented");
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break;
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case MqttPacket.Ping:
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HandlePingReq(c);
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break;
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case MqttPacket.Connect:
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if (connected)
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{
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// Second CONNECT on same connection is a protocol violation.
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err = new InvalidOperationException("second CONNECT packet not allowed");
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break;
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}
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// TODO: Task 3 — MqttParseConnect + MqttProcessConnect
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err = new NotImplementedException("CONNECT not yet implemented");
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break;
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case MqttPacket.Disconnect:
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// TODO: Task 3 — handle DISCONNECT
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err = new NotImplementedException("DISCONNECT not yet implemented");
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break;
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default:
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err = new InvalidOperationException($"unknown MQTT packet type: 0x{pt:X2}");
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break;
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}
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}
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return err;
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}
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/// <summary>
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/// Handles PINGREQ by enqueueing a PINGRESP packet.
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/// Mirrors Go <c>mqttEnqueuePingResp()</c>.
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/// </summary>
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private static void HandlePingReq(ClientConnection c)
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{
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lock (c)
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{
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c.EnqueueProto(PingRespPacket);
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}
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}
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}
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