[core] Refax: added size cache to the group container (#2510).

srtcore/group.cpp

@@ -335,6 +335,7 @@ void CUDTGroup::GroupContainer::erase(CUDTGroup::gli_t it)
         }
     }
     m_List.erase(it);
+    --m_SizeCache;
 }
 
 void CUDTGroup::setOpt(SRT_SOCKOPT optName, const void* optval, int optlen)

srtcore/group.h

@@ -406,16 +406,21 @@ class CUDTGroup
     SRTSOCKET m_PeerGroupID;
     struct GroupContainer
     {
-        std::list<SocketData>        m_List;
+    private:
+        std::list<SocketData>  m_List;
+        sync::atomic<size_t>   m_SizeCache;
 
         /// This field is used only by some types of groups that need
         /// to keep track as to which link was lately used. Note that
         /// by removal of a node from the m_List container, this link
         /// must be appropriately reset.
         gli_t m_LastActiveLink;
 
+    public:
+
         GroupContainer()
-            : m_LastActiveLink(m_List.end())
+            : m_SizeCache(0)
+            , m_LastActiveLink(m_List.end())
         {
         }
 
@@ -425,13 +430,14 @@ class CUDTGroup
         gli_t        begin() { return m_List.begin(); }
         gli_t        end() { return m_List.end(); }
         bool         empty() { return m_List.empty(); }
-        void         push_back(const SocketData& data) { m_List.push_back(data); }
+        void         push_back(const SocketData& data) { m_List.push_back(data); ++m_SizeCache; }
         void         clear()
         {
             m_LastActiveLink = end();
             m_List.clear();
+            m_SizeCache = 0;
         }
-        size_t size() { return m_List.size(); }
+        size_t size() { return m_SizeCache; }
 
         void erase(gli_t it);
     };