Refactor MQTTMessageBuilder to accept external JsonDocument parameters for status and packet messages, reducing stack usage and improving memory management. Update MQTTBridge to allocate and reuse DynamicJsonDocument instances for JSON serialization, enhancing performance and efficiency during publish operations.

src/helpers/MQTTMessageBuilder.cpp

@@ -5,6 +5,7 @@
 #include "MeshCore.h"
 
 int MQTTMessageBuilder::buildStatusMessage(
+  JsonDocument& doc,
   const char* origin,
   const char* origin_id,
   const char* model,
@@ -25,8 +26,9 @@ int MQTTMessageBuilder::buildStatusMessage(
   int recv_errors,
   int internal_heap
 ) {
-  // Use StaticJsonDocument to avoid heap fragmentation (fixed-size stack allocation)
-  StaticJsonDocument<768> doc;  // Increased size to accommodate stats
+  // doc is provided by the caller (heap-allocated DynamicJsonDocument in MQTTBridge),
+  // keeping this 768-byte scratch space off the MQTT task stack.
+  doc.clear();
   JsonObject root = doc.to<JsonObject>();
   
   root["status"] = status;
@@ -78,6 +80,7 @@ int MQTTMessageBuilder::buildStatusMessage(
 }
 
 int MQTTMessageBuilder::buildPacketMessage(
+  JsonDocument& doc,
   const char* origin,
   const char* origin_id,
   const char* timestamp,
@@ -96,10 +99,9 @@ int MQTTMessageBuilder::buildPacketMessage(
   char* buffer,
   size_t buffer_size
 ) {
-  // Use StaticJsonDocument with fixed maximum size to avoid heap fragmentation
-  // Base JSON overhead ~200 bytes, raw hex can be up to 510 chars (255 bytes packet)
-  // Use maximum size (2048) to handle all packet sizes without heap allocation
-  StaticJsonDocument<2048> doc;
+  // doc is provided by the caller (heap-allocated DynamicJsonDocument in MQTTBridge),
+  // keeping this 2048-byte scratch space off the MQTT task stack.
+  doc.clear();
   JsonObject root = doc.to<JsonObject>();
   
   // Format numeric values as strings to avoid String object allocations
@@ -165,6 +167,7 @@ int MQTTMessageBuilder::buildRawMessage(
 }
 
 int MQTTMessageBuilder::buildPacketJSON(
+  JsonDocument& doc,
   mesh::Packet* packet,
   bool is_tx,
   const char* origin,
@@ -227,6 +230,7 @@ int MQTTMessageBuilder::buildPacketJSON(
   }
   
   return buildPacketMessage(
+    doc,
     origin, origin_id, timestamp,
     is_tx ? "tx" : "rx",
     time_str, date_str,
@@ -243,6 +247,7 @@ int MQTTMessageBuilder::buildPacketJSON(
 }
 
 int MQTTMessageBuilder::buildPacketJSONFromRaw(
+  JsonDocument& doc,
   const uint8_t* raw_data,
   int raw_len,
   mesh::Packet* packet,
@@ -309,6 +314,7 @@ int MQTTMessageBuilder::buildPacketJSONFromRaw(
   }
   
   return buildPacketMessage(
+    doc,
     origin, origin_id, timestamp,
     is_tx ? "tx" : "rx",
     time_str, date_str,