fix: server not tracking scene handles for pre-loaded scenes

Scenes that were already loaded when a session starts were added to
ScenesLoaded but never to the server-to-client scene handle tables, so
UnloadScene passed its ScenesLoaded check and then failed inside
RemoveServerClientSceneHandle. The scene unloaded on every peer, but an
error was logged and the ScenesLoaded entry was never removed.

The client-server branch of the NetworkSceneManager constructor now
registers those scenes the same way InitializeScenesLoaded does for
distributed authority.

NetcodeIntegrationTestHelpers registered the test runner scene in
ServerSceneHandleToClientSceneHandle unconditionally, which now collides
with the constructor already having added it, so that add is guarded.

Author: NoelStephensUnity

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -22,6 +22,8 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Fixed issue where scenes additively loaded before a session started were tracked as loaded on the server but had no scene handle entries, which caused `NetworkSceneManager.UnloadScene` to log an error and leave the scene registered as loaded even though it unloaded on all peers. (#4145)
+
 
 ### Security
 

com.unity.netcode.gameobjects/Runtime/SceneManagement/NetworkSceneManager.cs

@@ -847,7 +847,7 @@ internal NetworkSceneManager(NetworkManager networkManager)
                 for (int i = 0; i < SceneManager.sceneCount; i++)
                 {
                     var loadedScene = SceneManager.GetSceneAt(i);
-                    ScenesLoaded.Add(loadedScene.handle, loadedScene);
+                    UpdateServerClientSceneHandle(loadedScene.handle, loadedScene.handle, loadedScene);
                 }
                 SceneManagerHandler.PopulateLoadedScenes(ref ScenesLoaded, NetworkManager);
             }

com.unity.netcode.gameobjects/Tests/Runtime/TestHelpers/NetcodeIntegrationTestHelpers.cs

@@ -540,7 +540,12 @@ private static void SceneManagerValidationAndTestRunnerInitialization(NetworkMan
                     }
                     return;
                 }
-                networkManager.SceneManager.ServerSceneHandleToClientSceneHandle.Add(scene.handle, scene.handle);
+
+                // The server already registers every scene loaded prior to startup, so only add the test runner scene if it is not already there.
+                if (!networkManager.SceneManager.ServerSceneHandleToClientSceneHandle.ContainsKey(scene.handle))
+                {
+                    networkManager.SceneManager.ServerSceneHandleToClientSceneHandle.Add(scene.handle, scene.handle);
+                }
             }
         }
 

testproject/Assets/Tests/Runtime/NetworkSceneManager/NetworkSceneManagerStartupTests.cs

@@ -21,6 +21,7 @@ internal class NetworkSceneManagerStartupTests : NetcodeIntegrationTest
     {
         private const string k_ActiveScene = "SessionSynchronize";
         private const string k_AdditionalScene = "InSceneNetworkObjectMovesToDDOL";
+        private const string k_PreLoadedScene = "EmptyScene1";
 
         private readonly List<NetworkObject> m_ObjectsInScenes = new List<NetworkObject>();
         private Scene m_OriginalActiveScene;
@@ -141,6 +142,35 @@ public IEnumerator AllExistingObjectsAreSpawnedAtStartup([Values] LoadSceneMode
             AssertOnTimeout("Timed out waiting for objects to spawn on all clients!");
         }
 
+        /// <summary>
+        /// Validates that a scene additively loaded before the session started is tracked well enough
+        /// to be unloaded through <see cref="NetworkSceneManager"/> without error.
+        /// </summary>
+        [UnityTest]
+        public IEnumerator UnloadPreLoadedScene()
+        {
+            yield return PreLoadScene(k_PreLoadedScene);
+            var preLoadedScene = m_SceneLoaded;
+
+            m_CanStart = true;
+            yield return StartServerAndClients();
+
+            // Scenes loaded before the session started are registered in both the loaded scenes and the
+            // scene handle tables, otherwise unloading them fails part way through and leaks the entry.
+            var sceneManager = GetAuthorityNetworkManager().SceneManager;
+            Assert.IsTrue(sceneManager.ScenesLoaded.ContainsKey(preLoadedScene.handle), $"{k_PreLoadedScene} is not in {nameof(NetworkSceneManager.ScenesLoaded)}!");
+            Assert.IsTrue(sceneManager.ServerSceneHandleToClientSceneHandle.ContainsKey(preLoadedScene.handle), $"{k_PreLoadedScene} is not in {nameof(NetworkSceneManager.ServerSceneHandleToClientSceneHandle)}!");
+
+            var status = sceneManager.UnloadScene(preLoadedScene);
+            Assert.AreEqual(SceneEventProgressStatus.Started, status, $"{nameof(NetworkSceneManager.UnloadScene)} returned {status}!");
+
+            yield return WaitForConditionOrTimeOut(() => !preLoadedScene.isLoaded);
+            AssertOnTimeout($"Timed out waiting for {k_PreLoadedScene} to unload!");
+
+            Assert.IsFalse(sceneManager.ScenesLoaded.ContainsKey(preLoadedScene.handle), $"{k_PreLoadedScene} was unloaded but is still in {nameof(NetworkSceneManager.ScenesLoaded)}!");
+            Assert.IsFalse(sceneManager.ServerSceneHandleToClientSceneHandle.ContainsKey(preLoadedScene.handle), $"{k_PreLoadedScene} was unloaded but is still in {nameof(NetworkSceneManager.ServerSceneHandleToClientSceneHandle)}!");
+        }
+
         #region Scene loading and related methods
 
         /// <summary>