fix: preserve prepared workflow state

src/MBN_STOCK_WEBVIEW.Core/Playout/Scenes/LegacyPlayoutWorkflow.cs

@@ -74,10 +74,12 @@ public sealed class LegacyPlayoutWorkflow
     private readonly IPlayoutEngine _engine;
     private readonly ILegacySceneCueProvider _cueProvider;
     private readonly SemaphoreSlim _gate = new(1, 1);
+    private readonly object _engineStatusSync = new();
     private IReadOnlyList<LegacyPlaylistEntry> _playlist = [];
     private ActivePage? _prepared;
     private ActivePage? _onAir;
     private int _engineClearRequested;
+    private long _lastObservedEngineStatusSequence = long.MinValue;
 
     public LegacyPlayoutWorkflow(
         IPlayoutEngine engine,
@@ -98,13 +100,27 @@ public sealed class LegacyPlayoutWorkflow
     public void ObserveEngineStatus(PlayoutStatus status)
     {
         ArgumentNullException.ThrowIfNull(status);
-        if (!string.IsNullOrWhiteSpace(status.PreparedSceneName) ||
-            !string.IsNullOrWhiteSpace(status.OnAirSceneName))
+        var hasActiveScene = !string.IsNullOrWhiteSpace(status.PreparedSceneName) ||
+            !string.IsNullOrWhiteSpace(status.OnAirSceneName);
+        lock (_engineStatusSync)
+        {
+            if (status.Sequence <= _lastObservedEngineStatusSequence)
+            {
+                return;
+            }
+
+            _lastObservedEngineStatusSequence = status.Sequence;
+            // Availability/OnHello updates can report no scene while PREPARE still owns
+            // the workflow gate. A later authoritative PREPARED/PROGRAM status must
+            // cancel that deferred clear before the command publishes its page metadata.
+            Interlocked.Exchange(ref _engineClearRequested, hasActiveScene ? 0 : 1);
+        }
+
+        if (hasActiveScene)
         {
             return;
         }
 
-        Interlocked.Exchange(ref _engineClearRequested, 1);
         if (!_gate.Wait(0))
         {
             return;
@@ -538,15 +554,18 @@ public sealed class LegacyPlayoutWorkflow
 
     private void ApplyRequestedEngineClear()
     {
-        if (Interlocked.Exchange(ref _engineClearRequested, 0) == 0)
+        lock (_engineStatusSync)
         {
-            return;
-        }
+            if (Interlocked.Exchange(ref _engineClearRequested, 0) == 0)
+            {
+                return;
+            }
 
-        _prepared = null;
-        _onAir = null;
-        _playlist = [];
-        State = EmptyState;
+            _prepared = null;
+            _onAir = null;
+            _playlist = [];
+            State = EmptyState;
+        }
     }
 
     private static readonly LegacyPlayoutWorkflowState EmptyState = new(

tests/MBN_STOCK_WEBVIEW.Core.Tests/LegacyPlayoutWorkflowTests.cs

@@ -352,6 +352,79 @@ public sealed class LegacyPlayoutWorkflowTests
         Assert.DoesNotContain(engine.Calls, call => call.StartsWith("UpdateOnAir", StringComparison.Ordinal));
     }
 
+    [Fact]
+    public async Task Prepare_EmptyThenPreparedEngineStatusWhileGateIsHeld_PreservesPublishedPageState()
+    {
+        var engine = new RecordingEngine();
+        var provider = new RecordingProvider(new Dictionary<string, (int, ScenePageSize?)>
+        {
+            ["5074"] = (6, ScenePageSize.Five)
+        })
+        {
+            BuilderKey = "kospi-five-row",
+            PreviewFields = [new("text", "title", "KOSPI")]
+        };
+        var workflow = new LegacyPlayoutWorkflow(engine, provider);
+        engine.PrepareCallback = () =>
+        {
+            workflow.ObserveEngineStatus(engine.Status with
+            {
+                PreparedSceneName = null,
+                OnAirSceneName = null,
+                Sequence = 10
+            });
+            workflow.ObserveEngineStatus(engine.Status with
+            {
+                PreparedSceneName = "5074",
+                OnAirSceneName = null,
+                Sequence = 11
+            });
+        };
+
+        var result = await workflow.PrepareAsync([new("playlist-entry", "5074")], 0);
+
+        Assert.True(result.IsSuccess);
+        Assert.Equal("5074", workflow.State.PreparedCutCode);
+        Assert.Null(workflow.State.OnAirCutCode);
+        Assert.Equal("kospi-five-row", workflow.State.BuilderKey);
+        Assert.Equal(0, workflow.State.PageIndexZeroBased);
+        Assert.Equal(2, workflow.State.PageCount);
+        Assert.Equal(5, workflow.State.PageSize);
+        Assert.Equal(6, workflow.State.ItemCount);
+        Assert.Equal(5, workflow.State.CurrentPageItemCount);
+        Assert.Equal("playlist-entry", workflow.State.CurrentEntryId);
+        Assert.Equal(LegacyWorkflowNextKind.PageNext, workflow.State.NextKind);
+        Assert.Equal(provider.PreviewFields, workflow.State.PreviewFields);
+    }
+
+    [Fact]
+    public async Task ObserveEngineStatus_OlderEmptyStatusCannotOverrideNewerPreparedStatus()
+    {
+        var engine = new RecordingEngine();
+        var provider = new RecordingProvider(new Dictionary<string, (int, ScenePageSize?)>
+        {
+            ["5001"] = (1, null)
+        });
+        var workflow = new LegacyPlayoutWorkflow(engine, provider);
+        Assert.True((await workflow.PrepareAsync([new("one", "5001")], 0)).IsSuccess);
+
+        workflow.ObserveEngineStatus(engine.Status with
+        {
+            PreparedSceneName = "5001",
+            OnAirSceneName = null,
+            Sequence = 20
+        });
+        workflow.ObserveEngineStatus(engine.Status with
+        {
+            PreparedSceneName = null,
+            OnAirSceneName = null,
+            Sequence = 19
+        });
+
+        Assert.Equal("5001", workflow.State.PreparedCutCode);
+        Assert.Equal("one", workflow.State.CurrentEntryId);
+    }
+
     private sealed class RecordingProvider(
         IReadOnlyDictionary<string, (int Count, ScenePageSize? Size)> definitions)
         : ILegacySceneCueProvider
@@ -360,6 +433,10 @@ public sealed class LegacyPlayoutWorkflowTests
 
         public Func<string, int, string>? SceneNameOverride { get; init; }
 
+        public string? BuilderKey { get; init; }
+
+        public IReadOnlyList<LegacyScenePreviewField>? PreviewFields { get; init; }
+
         public Task<LegacySceneCuePage> CreatePageAsync(
             LegacyPlaylistEntry entry,
             int pageIndexZeroBased,
@@ -375,7 +452,9 @@ public sealed class LegacyPlayoutWorkflowTests
                     name,
                     Mutations: [new PlayoutSetValue("page", $"p{pageIndexZeroBased}")]),
                 definition.Count,
-                definition.Size));
+                definition.Size,
+                BuilderKey,
+                PreviewFields));
         }
     }
 
@@ -387,6 +466,8 @@ public sealed class LegacyPlayoutWorkflowTests
 
         public Queue<PlayoutResultCode> UpdateOnAirResults { get; } = [];
 
+        public Action? PrepareCallback { get; set; }
+
         public PlayoutStatus Status { get; } = new(
             PlayoutMode.DryRun,
             PlayoutConnectionState.DryRunReady,
@@ -417,6 +498,7 @@ public sealed class LegacyPlayoutWorkflowTests
             CancellationToken cancellationToken = default)
         {
             Calls.Add("Prepare:" + Describe(cue));
+            PrepareCallback?.Invoke();
             return Complete(PlayoutOperation.Prepare, PrepareResults);
         }