fix: NetworkServer/NetworkClient batch processing now detects ever growing batches (as caused by #2882)

Assets/Mirror/Runtime/Batching/Unbatcher.cs

@@ -16,6 +16,8 @@ public class Unbatcher
         // just in case.
         Queue<PooledNetworkWriter> batches = new Queue<PooledNetworkWriter>();
 
+        public int BatchesCount => batches.Count;
+
         // NetworkReader is only created once,
         // then pointed to the first batch.
         NetworkReader reader = new NetworkReader(new byte[0]);

Assets/Mirror/Runtime/NetworkClient.cs

@@ -364,6 +364,24 @@ internal static void OnTransportData(ArraySegment<byte> data, int channelId)
                         return;
                     }
                 }
+
+                // if we weren't interrupted by a scene change,
+                // then all batched messages should have been processed now.
+                // if not, we need to log an error to avoid debugging hell.
+                // otherwise batches would silently grow.
+                // we need to log an error to avoid debugging hell.
+                //
+                // EXAMPLE: https://github.com/vis2k/Mirror/issues/2882
+                // -> UnpackAndInvoke silently returned because no handler for id
+                // -> Reader would never be read past the end
+                // -> Batch would never be retired because end is never reached
+                //
+                // IMPORTANT: always keep this check to detect memory leaks.
+                //            this took half a day to debug last time.
+                if (!isLoadingScene && unbatcher.BatchesCount > 0)
+                {
+                    Debug.LogError($"Still had {unbatcher.BatchesCount} batches remaining after processing, even though processing was not interrupted by a scene change. This should never happen, as it would cause ever growing batches.");
+                }
             }
             else Debug.LogError("Skipped Data message handling because connection is null.");
         }

Assets/Mirror/Runtime/NetworkServer.cs

@@ -497,6 +497,23 @@ internal static void OnTransportData(int connectionId, ArraySegment<byte> data,
                         return;
                     }
                 }
+
+                // if we weren't interrupted by a scene change,
+                // then all batched messages should have been processed now.
+                // otherwise batches would silently grow.
+                // we need to log an error to avoid debugging hell.
+                //
+                // EXAMPLE: https://github.com/vis2k/Mirror/issues/2882
+                // -> UnpackAndInvoke silently returned because no handler for id
+                // -> Reader would never be read past the end
+                // -> Batch would never be retired because end is never reached
+                //
+                // IMPORTANT: always keep this check to detect memory leaks.
+                //            this took half a day to debug last time.
+                if (!isLoadingScene && connection.unbatcher.BatchesCount > 0)
+                {
+                    Debug.LogError($"Still had {connection.unbatcher.BatchesCount} batches remaining after processing, even though processing was not interrupted by a scene change. This should never happen, as it would cause ever growing batches.");
+                }
             }
             else Debug.LogError("HandleData Unknown connectionId:" + connectionId);
         }

Assets/Mirror/Tests/Editor/NetworkServerTest.cs

@@ -864,7 +864,9 @@ public void UnregisterHandler()
             Assert.That(variant1Called, Is.EqualTo(1));
 
             // unregister, send again, should not be called again
+            // error about remaining batches is expected
             NetworkServer.UnregisterHandler<TestMessage1>();
+            LogAssert.Expect(LogType.Error, new Regex("Still had 1 batches remaining after processing.*"));
             NetworkClient.Send(new TestMessage1());
             ProcessMessages();
             Assert.That(variant1Called, Is.EqualTo(1));
@@ -887,7 +889,9 @@ public void ClearHandler()
             Assert.That(variant1Called, Is.EqualTo(1));
 
             // clear handlers, send again, should not be called again
+            // error about remaining batches is expected
             NetworkServer.ClearHandlers();
+            LogAssert.Expect(LogType.Error, new Regex("Still had 1 batches remaining after processing.*"));
             NetworkClient.Send(new TestMessage1());
             ProcessMessages();
             Assert.That(variant1Called, Is.EqualTo(1));