fix: Metrics: Exception when tracking Named Messages [MTT-5380] (#2426)

* MTT-5830 Fix
NetworkMetrics were tracking named message received events but it is possible for message handlers to unregister themselves.
We now cache the name of the message handler before invoking it so the Network Metrics can access the name even if its removed.
---------

Co-authored-by: Noel Stephens <[email protected]>

Author: samlucas-unity

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -30,6 +30,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Fixed an issue where Named Message Handlers could remove themselves causing an exception when the metrics tried to access the name of the message.(#2426)
 - Fixed registry of public `NetworkVariable`s in derived `NetworkBehaviour`s (#2423)
 - Fixed issue where runtime association of `Animator` properties to `AnimationCurve`s would cause `NetworkAnimator` to attempt to update those changes. (#2416)
 - Fixed issue where `NetworkAnimator` would not check if its associated `Animator` was valid during serialization and would spam exceptions in the editor console. (#2416)

com.unity.netcode.gameobjects/Runtime/Messaging/CustomMessageManager.cs

@@ -151,14 +151,18 @@ internal void InvokeNamedMessage(ulong hash, ulong sender, FastBufferReader read
                 // We dont know what size to use. Try every (more collision prone)
                 if (m_NamedMessageHandlers32.TryGetValue(hash, out HandleNamedMessageDelegate messageHandler32))
                 {
+                    // handler can remove itself, cache the name for metrics
+                    string messageName = m_MessageHandlerNameLookup32[hash];
                     messageHandler32(sender, reader);
-                    m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, m_MessageHandlerNameLookup32[hash], bytesCount);
+                    m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, messageName, bytesCount);
                 }
 
                 if (m_NamedMessageHandlers64.TryGetValue(hash, out HandleNamedMessageDelegate messageHandler64))
                 {
+                    // handler can remove itself, cache the name for metrics
+                    string messageName = m_MessageHandlerNameLookup64[hash];
                     messageHandler64(sender, reader);
-                    m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, m_MessageHandlerNameLookup64[hash], bytesCount);
+                    m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, messageName, bytesCount);
                 }
             }
             else
@@ -169,15 +173,19 @@ internal void InvokeNamedMessage(ulong hash, ulong sender, FastBufferReader read
                     case HashSize.VarIntFourBytes:
                         if (m_NamedMessageHandlers32.TryGetValue(hash, out HandleNamedMessageDelegate messageHandler32))
                         {
+                            // handler can remove itself, cache the name for metrics
+                            string messageName = m_MessageHandlerNameLookup32[hash];
                             messageHandler32(sender, reader);
-                            m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, m_MessageHandlerNameLookup32[hash], bytesCount);
+                            m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, messageName, bytesCount);
                         }
                         break;
                     case HashSize.VarIntEightBytes:
                         if (m_NamedMessageHandlers64.TryGetValue(hash, out HandleNamedMessageDelegate messageHandler64))
                         {
+                            // handler can remove itself, cache the name for metrics
+                            string messageName = m_MessageHandlerNameLookup64[hash];
                             messageHandler64(sender, reader);
-                            m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, m_MessageHandlerNameLookup64[hash], bytesCount);
+                            m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, messageName, bytesCount);
                         }
                         break;
                 }