Rename getMetadata to prepareMetadata and ensure data access protected

Currently the setChanged flag is being cleared/set while the data mutex is not
locked.
As getMessage() already aquires this lock and is only ever used when sending a
message, move the flag clear inside this method and rename to prepareMetadata to
reflect the fact it is no longer simply retrieving data.
Add a lock within the error callback in case sending fails and the flag must be
reset
This looks scary as we have multiple locks being aquired within sendMetadata()
I think it's ok as on the send side we are already aquiring this lock, we're
just adding a data mutation. On the receive side we're adding a lock, but we
will only ever call the success or failure path, never both, so shouldn't be the
case where we might deadlock waiting for two different mutexes - in this way
it's no different to aquiring a lock in any of the setter methods.

ear-production-suite-plugins/lib/include/communication/object_metadata_sender.hpp

@@ -48,7 +48,7 @@ class ObjectMetadataSender {
  private:
   void sendMetadata();
   void startTimer();
-  MessageBuffer getMessage();
+  MessageBuffer prepareMessage();
   void handleTimeout(std::error_code ec);
   template<typename FunctionT>
   void setData(FunctionT&& set) {

ear-production-suite-plugins/lib/src/communication/object_metadata_sender.cpp

@@ -39,10 +39,11 @@ void ObjectMetadataSender::connect(const std::string& endpoint,
   startTimer();
 }
 
-MessageBuffer ObjectMetadataSender::getMessage() {
+MessageBuffer ObjectMetadataSender::prepareMessage() {
   std::lock_guard<std::mutex> lock(dataMutex_);
   MessageBuffer buffer = allocBuffer(data_.ByteSizeLong());
   data_.SerializeToArray(buffer.data(), buffer.size());
+  data_.set_changed(false);
   return buffer;
 }
 
@@ -65,16 +66,18 @@ void ObjectMetadataSender::sendMetadata() {
   if (!connectionId_.isValid()) {
     return;
   }
-  auto msg = getMessage();
-  data_.set_changed(false);
+  auto msg = prepareMessage();
   socket_.asyncSend(
       msg, [this](std::error_code ec, const nng::Message& ignored) {
         if (!ec) {
           std::lock_guard<std::mutex> lock(timeoutMutex_);
           lastSendTimestamp_ = std::chrono::system_clock::now();
         } else {
-          data_.set_changed(true);
-          EAR_LOGGER_WARN(logger_, "Metadata sending failed: {}", ec.message());
+          {
+            std::lock_guard<std::mutex> dataLock(dataMutex_);
+            data_.set_changed(true);
+          }
+          EAR_LOGGER_WARN(logger_, "Metadata sending failed: {}", ec.message())
         }
       });
 }