Adafruit_MQTT (API): the payload of publish method is read-only

Adafruit_MQTT.cpp

@@ -311,13 +311,14 @@ bool Adafruit_MQTT::disconnect() {
 }
 
 bool Adafruit_MQTT::publish(const char *topic, const char *data, uint8_t qos) {
-  return publish(topic, (uint8_t *)(data), strlen(data), qos);
+  return publish(topic, (const uint8_t *)(data), strlen(data), qos);
 }
 
-bool Adafruit_MQTT::publish(const char *topic, uint8_t *data, uint16_t bLen,
-                            uint8_t qos) {
+bool Adafruit_MQTT::publish(const char *topic, const uint8_t *data,
+                            uint16_t bLen, uint8_t qos) {
   // Construct and send publish packet.
-  uint16_t len = publishPacket(buffer, topic, data, bLen, qos);
+  uint16_t len =
+      publishPacket(buffer, topic, data, bLen, qos, (uint16_t)sizeof(buffer));
   if (!sendPacket(buffer, len))
     return false;
 
@@ -665,11 +666,22 @@ uint8_t Adafruit_MQTT::connectPacket(uint8_t *packet) {
   return len;
 }
 
+uint16_t Adafruit_MQTT::packetAdditionalLen(uint16_t currLen) {
+  /* Increase length field based on current length */
+  if (currLen < 128)
+    return 0;
+  if (currLen < 16384)
+    return 1;
+  if (currLen < 2097151)
+    return 2;
+  return 3;
+}
+
 // as per
 // http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718040
 uint16_t Adafruit_MQTT::publishPacket(uint8_t *packet, const char *topic,
-                                      uint8_t *data, uint16_t bLen,
-                                      uint8_t qos) {
+                                      const uint8_t *data, uint16_t bLen,
+                                      uint8_t qos, uint16_t maxPacketLen) {
   uint8_t *p = packet;
   uint16_t len = 0;
 
@@ -679,7 +691,21 @@ uint16_t Adafruit_MQTT::publishPacket(uint8_t *packet, const char *topic,
   if (qos > 0) {
     len += 2; // qos packet id
   }
-  len += bLen; // payload length
+  // calculate additional bytes for length field (if any)
+  uint16_t additionalLen = packetAdditionalLen(len + bLen);
+
+  // payload length
+  if (len + bLen + 2 + additionalLen <= maxPacketLen) {
+    len += bLen + additionalLen;
+  } else {
+    // If we make it here, we got a pickle: the payload is not going
+    // to fit in the packet buffer. Instead of corrupting memory, let's
+    // do something less damaging by reducing the bLen to what we are
+    // able to accomodate. Alternatively, consider using a bigger
+    // maxPacketLen.
+    bLen = maxPacketLen - (len + 2 + packetAdditionalLen(maxPacketLen));
+    len = maxPacketLen - 4;
+  }
 
   // Now you can start generating the packet!
   p[0] = MQTT_CTRL_PUBLISH << 4 | qos << 1;
@@ -831,7 +857,7 @@ bool Adafruit_MQTT_Publish::publish(const char *payload) {
 }
 
 // publish buffer of arbitrary length
-bool Adafruit_MQTT_Publish::publish(uint8_t *payload, uint16_t bLen) {
+bool Adafruit_MQTT_Publish::publish(const uint8_t *payload, uint16_t bLen) {
 
   return mqtt->publish(topic, payload, bLen, qos);
 }

Adafruit_MQTT.h

@@ -187,7 +187,7 @@ class Adafruit_MQTT {
   // Publish a message to a topic using the specified QoS level.  Returns true
   // if the message was published, false otherwise.
   bool publish(const char *topic, const char *payload, uint8_t qos = 0);
-  bool publish(const char *topic, uint8_t *payload, uint16_t bLen,
+  bool publish(const char *topic, const uint8_t *payload, uint16_t bLen,
                uint8_t qos = 0);
 
   // Add a subscription to receive messages for a topic.  Returns true if the
@@ -257,8 +257,10 @@ class Adafruit_MQTT {
   // Functions to generate MQTT packets.
   uint8_t connectPacket(uint8_t *packet);
   uint8_t disconnectPacket(uint8_t *packet);
-  uint16_t publishPacket(uint8_t *packet, const char *topic, uint8_t *payload,
-                         uint16_t bLen, uint8_t qos);
+  static uint16_t packetAdditionalLen(uint16_t currLen);
+  uint16_t publishPacket(uint8_t *packet, const char *topic,
+                         const uint8_t *payload, uint16_t bLen, uint8_t qos,
+                         uint16_t maxPacketLen);
   uint8_t subscribePacket(uint8_t *packet, const char *topic, uint8_t qos);
   uint8_t unsubscribePacket(uint8_t *packet, const char *topic);
   uint8_t pingPacket(uint8_t *packet);
@@ -278,7 +280,7 @@ class Adafruit_MQTT_Publish {
               // This might be ignored and a higher precision value sent.
   bool publish(int32_t i);
   bool publish(uint32_t i);
-  bool publish(uint8_t *b, uint16_t bLen);
+  bool publish(const uint8_t *b, uint16_t bLen);
 
 private:
   Adafruit_MQTT *mqtt;