MessageProtocol: Add concurrency protection for callback functions

onBodyComplete() will be called on the transport thread, but
setXXHandler() will be called on other threads.  We must not
delete the function call object while it's being invoked or
bad things will happen.  Add a mutex to protect against this.

src/dbus_messageprotocol.cpp

@@ -73,24 +73,28 @@ void DBus::MessageProtocol::reset() { startMessage(); }
 void DBus::MessageProtocol::setMethodCallHandler(
     const DBus::Message::CallbackFunctionMethodCall& callback)
 {
+    std::lock_guard<std::recursive_mutex> lock(m_CallbackMutex);
     m_MethodCallCallback = callback;
 }
 
 void DBus::MessageProtocol::setMethodReturnHandler(
     const DBus::Message::CallbackFunctionMethodReturn& callback)
 {
+    std::lock_guard<std::recursive_mutex> lock(m_CallbackMutex);
     m_MethodReturnCallback = callback;
 }
 
 void DBus::MessageProtocol::setErrorHandler(
     const DBus::Message::CallbackFunctionError& callback)
 {
+    std::lock_guard<std::recursive_mutex> lock(m_CallbackMutex);
     m_ErrorCallback = callback;
 }
 
 void DBus::MessageProtocol::setSignalHandler(
     const DBus::Message::CallbackFunctionSignal& callback)
 {
+    std::lock_guard<std::recursive_mutex> lock(m_CallbackMutex);
     m_SignalCallback = callback;
 }
 
@@ -211,6 +215,7 @@ void DBus::MessageProtocol::onBodyComplete(const std::string& body)
     try
     {
         uint8_t type = Type::asByte(m_HeaderStruct[1]);
+        std::lock_guard<std::recursive_mutex> lock(m_CallbackMutex);
         switch (type) {
         case TYPE_METHOD:
             m_MethodCallCallback(Message::MethodCall(m_HeaderStruct, body));

src/dbus_messageprotocol.h

@@ -18,6 +18,7 @@
 #ifndef DBUS_MESSAGEPROTOCOL
 #define DBUS_MESSAGEPROTOCOL
 
+#include <mutex>
 #include "dbus_message.h"
 #include "dbus_octetbuffer.h"
 #include "dbus_type_struct.h"
@@ -61,6 +62,7 @@ class MessageProtocol {
     std::basic_string<uint8_t> m_octetCache;
 
     //
+    std::recursive_mutex m_CallbackMutex;
     Message::CallbackFunctionMethodCall m_MethodCallCallback;
     Message::CallbackFunctionMethodReturn m_MethodReturnCallback;
     Message::CallbackFunctionError m_ErrorCallback;