added methods to support QoS1/2 retry logic

README.md

@@ -211,6 +211,17 @@ bool publish(const char topic[], const char payload[], int length, bool retained
 - Beginning with version 2.6, payloads of arbitrary length may be published, see [Notes](#notes).
 - The functions return a boolean that indicates if the publishing has been successful (true).
 
+Obtain the last used packet ID and prepare the publication of a duplicate message using the specified packet ID:
+
+```c++
+uint16_t lastPacketID();
+void prepareDuplicate(uint16_t packetID);
+```
+
+- These functions may be used to implement a retry logic for failed publications of QoS1 and QoS2 messages.
+- The `lastPacketID()` function can be used after calling `publish()` to obtain the used packet ID.
+- The `prepareDuplicate()` function may be called before `publish()` to temporarily change the next used packet ID and flag the message as a duplicate.
+
 Subscribe to a topic:
 
 ```c++

src/MQTTClient.cpp

@@ -392,8 +392,16 @@ bool MQTTClient::publish(const char topic[], const char payload[], int length, b
   message.retained = retained;
   message.qos = lwmqtt_qos_t(qos);
 
+  // prepare options
+  lwmqtt_publish_options_t options = lwmqtt_default_publish_options;
+
+  // set duplicate packet id if available
+  if (this->nextDupPacketID > 0) {
+    options.dup_id = &this->nextDupPacketID;
+  }
+
   // publish message
-  this->_lastError = lwmqtt_publish(&this->client, nullptr, lwmqtt_string(topic), message, this->timeout);
+  this->_lastError = lwmqtt_publish(&this->client, &options, lwmqtt_string(topic), message, this->timeout);
   if (this->_lastError != LWMQTT_SUCCESS) {
     // close connection
     this->close();
@@ -404,6 +412,16 @@ bool MQTTClient::publish(const char topic[], const char payload[], int length, b
   return true;
 }
 
+uint16_t MQTTClient::lastPacketID() {
+  // get last packet id from client
+  return this->client.last_packet_id;
+}
+
+void MQTTClient::prepareDuplicate(uint16_t packetID) {
+  // set next duplicate packet id
+  this->nextDupPacketID = packetID;
+}
+
 bool MQTTClient::subscribe(const char topic[], int qos) {
   // return immediately if not connected
   if (!this->connected()) {

src/MQTTClient.h

@@ -88,6 +88,7 @@ class MQTTClient {
   lwmqtt_client_t client = lwmqtt_client_t();
 
   bool _connected = false;
+  uint16_t nextDupPacketID = 0;
   lwmqtt_return_code_t _returnCode = (lwmqtt_return_code_t)0;
   lwmqtt_err_t _lastError = (lwmqtt_err_t)0;
   uint32_t _droppedMessages = 0;
@@ -170,6 +171,9 @@ class MQTTClient {
   }
   bool publish(const char topic[], const char payload[], int length, bool retained, int qos);
 
+  uint16_t lastPacketID();
+  void prepareDuplicate(uint16_t packetID);
+
   bool subscribe(const String &topic) { return this->subscribe(topic.c_str()); }
   bool subscribe(const String &topic, int qos) { return this->subscribe(topic.c_str(), qos); }
   bool subscribe(const char topic[]) { return this->subscribe(topic, 0); }