serveJson: Ensure semaphore scope lasts until reply is done

Extend the JSON response class to hold the global JSON buffer lock
until the transaction is completed.

Fixes Aircoookie#3641

wled00/json.cpp

@@ -1009,6 +1009,17 @@ void serializeModeNames(JsonArray arr)
   }
 }
 
+
+// Global buffer locking response helper class
+class GlobalBufferAsyncJsonResponse: public JSONBufferGuard, public AsyncJsonResponse {
+  public:
+  inline GlobalBufferAsyncJsonResponse(bool isArray) : JSONBufferGuard(17), AsyncJsonResponse(pDoc, isArray) {};
+  virtual ~GlobalBufferAsyncJsonResponse() {};
+
+  // Other members are inherited
+};
+
+
 static volatile bool servingClient = false;
 void serveJson(AsyncWebServerRequest* request)
 {
@@ -1050,12 +1061,12 @@ void serveJson(AsyncWebServerRequest* request)
     return;
   }
 
-  if (!requestJSONBufferLock(17)) {
+  GlobalBufferAsyncJsonResponse *response = new GlobalBufferAsyncJsonResponse(subJson==JSON_PATH_FXDATA || subJson==JSON_PATH_EFFECTS); // will clear and convert JsonDocument into JsonArray if necessary
+  if (!response->owns_lock()) {
     serveJsonError(request, 503, ERR_NOBUF);
     servingClient = false;
     return;
   }
-  AsyncJsonResponse *response = new AsyncJsonResponse(pDoc, subJson==JSON_PATH_FXDATA || subJson==JSON_PATH_EFFECTS); // will clear and convert JsonDocument into JsonArray if necessary
 
   JsonVariant lDoc = response->getRoot();
 
@@ -1098,7 +1109,6 @@ void serveJson(AsyncWebServerRequest* request)
   DEBUG_PRINT(F("JSON content length: ")); DEBUG_PRINTLN(len);
 
   request->send(response);
-  releaseJSONBufferLock();
   servingClient = false;
 }