net: mark CNode unique_ptr members as const

Dereferencing a unique_ptr is not necessarily thread safe. The reason
these are safe is because their values are set at construction and do
not change later; so mark them as const and set them via the initializer
list to guarantee that.

Author: ajtowns

src/net.cpp

@@ -2723,7 +2723,9 @@ CNode::CNode(NodeId idIn,
              ConnectionType conn_type_in,
              bool inbound_onion,
              std::unique_ptr<i2p::sam::Session>&& i2p_sam_session)
-    : m_sock{sock},
+    : m_deserializer{std::make_unique<V1TransportDeserializer>(V1TransportDeserializer(Params(), idIn, SER_NETWORK, INIT_PROTO_VERSION))},
+      m_serializer{std::make_unique<V1TransportSerializer>(V1TransportSerializer())},
+      m_sock{sock},
       m_connected{GetTime<std::chrono::seconds>()},
       addr{addrIn},
       addrBind{addrBindIn},
@@ -2746,9 +2748,6 @@ CNode::CNode(NodeId idIn,
     } else {
         LogPrint(BCLog::NET, "Added connection peer=%d\n", id);
     }
-
-    m_deserializer = std::make_unique<V1TransportDeserializer>(V1TransportDeserializer(Params(), id, SER_NETWORK, INIT_PROTO_VERSION));
-    m_serializer = std::make_unique<V1TransportSerializer>(V1TransportSerializer());
 }
 
 bool CConnman::NodeFullyConnected(const CNode* pnode)

src/net.h

@@ -341,8 +341,8 @@ class CNode
     friend struct ConnmanTestMsg;
 
 public:
-    std::unique_ptr<TransportDeserializer> m_deserializer;
-    std::unique_ptr<const TransportSerializer> m_serializer;
+    const std::unique_ptr<TransportDeserializer> m_deserializer; // Used only by SocketHandler thread
+    const std::unique_ptr<const TransportSerializer> m_serializer;
 
     NetPermissionFlags m_permissionFlags{NetPermissionFlags::None};