Bugfix: Payload encoding for PUBLISH on QoS 0

Re #19

src/packet.c

@@ -427,7 +427,9 @@ Buffer *encode_connack(ConnAckPayload *payload) {
 Buffer *encode_publish(PublishPayload *payload) {
     size_t sz = 0;
     sz += strlen(payload->topic) + 2; // topic
-    sz += 2; // packet id
+    if (payload->qos != MQTT_QOS_0) {
+        sz += 2; // packet id
+    }
     if (payload->message) {
         sz += strlen(payload->message);
     }
@@ -450,8 +452,10 @@ Buffer *encode_publish(PublishPayload *payload) {
 
     // Variable header
     utf8_string_encode(payload->topic, buffer);
-    buffer->data[buffer->position++] = (payload->packet_id & 0xff00) >> 8;
+    if (payload->qos != MQTT_QOS_0) {
-    buffer->data[buffer->position++] = (payload->packet_id & 0xff);
+        buffer->data[buffer->position++] = (payload->packet_id & 0xff00) >> 8;
+        buffer->data[buffer->position++] = (payload->packet_id & 0xff);
+    }
 
     // Payload
     if (payload->message) {

tests/encode_packet.c

@@ -314,6 +314,30 @@ TestResult test_encode_publish_with_msg(void) {
     );
 }
 
+TestResult test_encode_publish_with_msg_qos0(void) {
+    char data[] = {
+        0x31, 0x13, // header, qos1, retain
+        0x00, 0x0a, 't', 'e', 's', 't', '/', 't', 'o', 'p', 'i', 'c',
+        'p', 'a', 'y', 'l', 'o', 'a', 'd'
+    };
+    PublishPayload *payload = calloc(1, sizeof(PublishPayload));
+
+    payload->qos = MQTT_QOS_0;
+    payload->retain = true;
+    payload->duplicate = false;
+    payload->topic = "test/topic";
+    payload->packet_id = 10;
+    payload->message = "payload";
+
+    Buffer *encoded = mqtt_packet_encode(&(MQTTPacket){ PacketTypePublish, payload });
+    free(payload);
+
+    return TESTMEMCMP(
+        buffer_from_data_copy(data, sizeof(data)),
+        encoded
+    );
+}
+
 TestResult test_encode_puback(void) {
     char data[] = {
         0x40, 0x02, // header
@@ -528,6 +552,7 @@ TESTS(
     TEST("Encode Publish with no message", test_encode_publish_no_msg),
     TEST("Encode Publish with invalid flags", test_encode_publish_dup_qos0),
     TEST("Encode Publish with message", test_encode_publish_with_msg),
+    TEST("Encode Publish with message on QoS 0", test_encode_publish_with_msg_qos0),
     TEST("Encode PubAck", test_encode_puback),
     TEST("Encode PubRec", test_encode_pubrec),
     TEST("Encode PubRel", test_encode_pubrel),