Motivation:

Since 3.1.1 mqtt protocol version SUBACK message can now indicate the failure in payload.

Modification:

Do not erase failure payload in for SUBACK message.

Result:

Fixes #7665

codec-mqtt/src/main/java/io/netty/handler/codec/mqtt/MqttDecoder.java

@@ -392,7 +392,10 @@ public final class MqttDecoder extends ReplayingDecoder<DecoderState> {
         final List<Integer> grantedQos = new ArrayList<Integer>();
         int numberOfBytesConsumed = 0;
         while (numberOfBytesConsumed < bytesRemainingInVariablePart) {
-            int qos = buffer.readUnsignedByte() & 0x03;
+            int qos = buffer.readUnsignedByte();
+            if (qos != MqttQoS.FAILURE.value()) {
+                qos &= 0x03;
+            }
             numberOfBytesConsumed++;
             grantedQos.add(qos);
         }

codec-mqtt/src/test/java/io/netty/handler/codec/mqtt/MqttCodecTest.java

@@ -214,6 +214,30 @@ public class MqttCodecTest {
         validateSubAckPayload(message.payload(), decodedMessage.payload());
     }
 
+    @Test
+    public void testSubAckMessageWithFailureInPayload() throws Exception {
+        MqttFixedHeader mqttFixedHeader =
+                new MqttFixedHeader(MqttMessageType.SUBACK, false, MqttQoS.AT_MOST_ONCE, false, 0);
+        MqttMessageIdVariableHeader mqttMessageIdVariableHeader = MqttMessageIdVariableHeader.from(12345);
+        MqttSubAckPayload mqttSubAckPayload = new MqttSubAckPayload(MqttQoS.FAILURE.value());
+        MqttSubAckMessage message =
+                new MqttSubAckMessage(mqttFixedHeader, mqttMessageIdVariableHeader, mqttSubAckPayload);
+
+        ByteBuf byteBuf = MqttEncoder.doEncode(ALLOCATOR, message);
+
+        List<Object> out = new LinkedList<Object>();
+        mqttDecoder.decode(ctx, byteBuf, out);
+
+        assertEquals("Expected one object but got " + out.size(), 1, out.size());
+
+        MqttSubAckMessage decodedMessage = (MqttSubAckMessage) out.get(0);
+        validateFixedHeaders(message.fixedHeader(), decodedMessage.fixedHeader());
+        validateMessageIdVariableHeader(message.variableHeader(), decodedMessage.variableHeader());
+        validateSubAckPayload(message.payload(), decodedMessage.payload());
+        assertEquals(1, decodedMessage.payload().grantedQoSLevels().size());
+        assertEquals(MqttQoS.FAILURE, MqttQoS.valueOf(decodedMessage.payload().grantedQoSLevels().get(0)));
+    }
+
     @Test
     public void testUnSubscribeMessage() throws Exception {
         final MqttUnsubscribeMessage message = createUnsubscribeMessage();