Keep signature of Node::Shared to maintain ABI stability

`Node::Shared` is a private function but it's used in `details/Node.hh`
in templates which means there could be calls to `Node::Shared` embedded
in other libraries. Since we're mainly tracking the number of `Node`
instances, and since `Node::Shared` is private, it's okay to use raw pointers
internal use.

Signed-off-by: Addisu Z. Taddese <[email protected]>

include/gz/transport/Node.hh

@@ -765,7 +765,7 @@ namespace gz
       /// \brief Get a pointer to the shared node (singleton shared by all the
       /// nodes).
       /// \return The pointer to the shared node.
-      private: std::shared_ptr<NodeShared> Shared() const;
+      private: NodeShared *Shared() const;
 
       /// \brief Get the UUID of this node.
       /// \return The node UUID.

include/gz/transport/NodeShared.hh

@@ -285,10 +285,10 @@ namespace gz
                   const std::string &_topic) const;
 
       /// \brief Constructor.
-      public: NodeShared();
+      protected: NodeShared();
 
       /// \brief Destructor.
-      public: virtual ~NodeShared();
+      protected: virtual ~NodeShared();
 
       /// \brief Initialize all sockets.
       /// \return True when success or false otherwise. This function might

src/Node.cc

@@ -862,9 +862,9 @@ bool Node::EnableStats(const std::string &_topic, bool _enable,
 }
 
 //////////////////////////////////////////////////
-std::shared_ptr<NodeShared> Node::Shared() const
+NodeShared *Node::Shared() const
 {
-  return this->dataPtr->shared;
+  return this->dataPtr->shared.get();
 }
 
 //////////////////////////////////////////////////

src/NodeShared.cc

@@ -158,11 +158,12 @@ std::shared_ptr<NodeShared> NodeShared::Instance()
   // is not shared between different processes.
   static std::weak_ptr<NodeShared> nodeSharedWeak;
 
-  auto nodeShared = nodeSharedWeak.lock();
+  std::shared_ptr<NodeShared> nodeShared = nodeSharedWeak.lock();
   if (nodeShared)
     return nodeShared;
 
-  nodeShared = std::make_shared<NodeShared>();
+  class MakeSharedEnabler : public NodeShared {};
+  nodeShared = std::make_shared<MakeSharedEnabler>();
   nodeSharedWeak = nodeShared;
   return nodeShared;
 }