MINIFICPP-2438 Do not parse json response for acknowledge on C2

Signed-off-by: Ferenc Gerlits <[email protected]>
This closes #1851

docker/test/integration/features/minifi_c2_server.feature

@@ -27,6 +27,7 @@ Feature: MiNiFi can communicate with Apache NiFi MiNiFi C2 server
     When all instances start up
     Then the MiNiFi C2 server logs contain the following message: "acknowledged with a state of FULLY_APPLIED(DONE)" in less than 30 seconds
     And a flowfile with the content "test" is placed in the monitored directory in less than 10 seconds
+    And the Minifi logs do not contain the following message: "Failed to parse json response: The document is empty. at 0" after 0 seconds
 
   Scenario: MiNiFi flow config is updated from MiNiFi C2 server through SSL with SSL controller service
     Given a file with the content "test" is present in "/tmp/input"

libminifi/src/c2/protocols/RESTProtocol.cpp

@@ -38,9 +38,12 @@
 namespace org::apache::nifi::minifi::c2 {
 
 C2Payload RESTProtocol::parseJsonResponse(const C2Payload &payload, std::span<const std::byte> response) const {
-  rapidjson::Document root;
+  if (payload.getOperation() == Operation::acknowledge) {
+    return {payload.getOperation(), state::UpdateState::READ_COMPLETE};
+  }
 
   try {
+    rapidjson::Document root;
     rapidjson::ParseResult ok = root.Parse(reinterpret_cast<const char*>(response.data()), response.size());
     if (ok) {
       std::string identifier;