fix(401): null Mirror-only fields for non-Mirror channels (repair dead else-branch)

Closes review finding: stray MirrorSourceChannelId/PlayoutOffset could persist onto a Generated channel.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ErsatzTV.Application/Channels/Commands/UpdateChannelHandler.cs

@@ -52,16 +52,6 @@ public class UpdateChannelHandler(
         UpdateChannel update,
         CancellationToken cancellationToken)
     {
-        // don't save mirror when playout exists
-        if (c.Playouts.Count > 0)
-        {
-            update = update with
-            {
-                PlayoutSource = ChannelPlayoutSource.Generated,
-                MirrorSourceChannelId = null
-            };
-        }
-
         bool hasEpgChange = c.PlayoutSource != update.PlayoutSource || c.ShowInEpg != update.ShowInEpg;
 
         c.Name = update.Name;
@@ -140,6 +130,8 @@ public class UpdateChannelHandler(
             c.PlayoutMode = ChannelPlayoutMode.Continuous;
             hasEpgChange |= c.MirrorSourceChannelId != update.MirrorSourceChannelId;
             hasEpgChange |= c.PlayoutOffset != update.PlayoutOffset;
+            c.MirrorSourceChannelId = update.MirrorSourceChannelId;
+            c.PlayoutOffset = update.PlayoutOffset;
         }
         else
         {
@@ -147,8 +139,6 @@ public class UpdateChannelHandler(
             c.PlayoutOffset = null;
         }
 
-        c.MirrorSourceChannelId = update.MirrorSourceChannelId;
-        c.PlayoutOffset = update.PlayoutOffset;
         c.StreamingMode = update.StreamingMode;
         c.WatermarkId = update.WatermarkId;
         c.FallbackFillerId = update.FallbackFillerId;
@@ -194,7 +184,7 @@ public class UpdateChannelHandler(
     {
         Validation<BaseError, Channel> channelValidation = (ValidateName(request),
             await ValidateNumber(dbContext, request, cancellationToken),
-            await MirrorSourceMustBeValid(dbContext, request, cancellationToken),
+            await MirrorSourceMustBeValid(dbContext, request, channel, cancellationToken),
             ValidateShowInEpg(request.IsEnabled, request.ShowInEpg),
             ValidateLogo(request.Logo?.Path))
         .Apply((_, _, _, _, _) => channel);
@@ -269,6 +259,7 @@ public class UpdateChannelHandler(
     private static async Task<Validation<BaseError, Unit>> MirrorSourceMustBeValid(
         TvContext dbContext,
         UpdateChannel request,
+        Channel channel,
         CancellationToken cancellationToken)
     {
         if (request.PlayoutSource is not ChannelPlayoutSource.Mirror)
@@ -276,6 +267,18 @@ public class UpdateChannelHandler(
             return Unit.Default;
         }
 
+        // a channel with its own playout already built (Generated mode) cannot become a Mirror —
+        // Mirror channels relay another channel's playout and never build one of their own, so
+        // switching this transition on would strand the existing playout. This used to be
+        // silently coerced back to Generated (issue #401); reject the transition instead so the
+        // caller sees why the requested Mirror source was not applied. A round-trip that keeps
+        // PlayoutSource as Generated never reaches this check.
+        if (channel.Playouts.Count > 0)
+        {
+            return BaseError.New(
+                "Channel cannot switch to Mirror playout source while it has a playout; reset or delete the existing playout first.");
+        }
+
         Option<Channel> maybeMirrorSource = await dbContext.Channels
             .AsNoTracking()
             .SelectOneAsync(

ErsatzTV.Tests/Application/Channels/UpdateChannelHandlerTests.cs

@@ -1,6 +1,7 @@
 using ErsatzTV.Application.Channels;
 using ErsatzTV.Core;
 using LanguageExt;
+using ErsatzTV.Core.Domain;
 using ErsatzTV.Core.Errors;
 using ErsatzTV.Infrastructure.Data;
 using ErsatzTV.Tests.Support;
@@ -114,6 +115,77 @@ public class UpdateChannelHandlerTests : ChannelHandlerTestBase
         error.Value.ShouldContain("FFmpegProfile");
     }
 
+    [Test]
+    public async Task Should_Reject_Mirror_Transition_When_Channel_Has_Playout()
+    {
+        await SeedFFmpegProfile();
+        await SeedChannel(1, "5"); // has a playout below
+        await SeedChannel(2, "6"); // valid mirror source (Generated, no playouts of its own)
+        await SeedPlayout(1, channelId: 1);
+
+        UpdateChannel update = MakeUpdate(1, number: "5") with
+        {
+            PlayoutSource = ChannelPlayoutSource.Mirror,
+            MirrorSourceChannelId = 2
+        };
+
+        Either<BaseError, ChannelViewModel> result = await MakeHandler().Handle(update, CancellationToken.None);
+
+        BaseError error = LeftOf(result);
+        error.ShouldNotBeOfType<NotFoundError>();
+        error.Value.ShouldContain("Mirror");
+
+        // the channel must NOT have been silently coerced/saved as Generated (issue #401: no
+        // silent 200, the caller's requested transition is rejected outright)
+        await using TvContext context = Db.CreateContext();
+        var channel = await context.Channels.SingleAsync(c => c.Id == 1);
+        channel.PlayoutSource.ShouldBe(ChannelPlayoutSource.Generated);
+        channel.MirrorSourceChannelId.ShouldBeNull();
+    }
+
+    [Test]
+    public async Task Should_Allow_GetPut_Roundtrip_Of_Generated_Channel_With_Playout()
+    {
+        await SeedFFmpegProfile();
+        await SeedChannel(1, "5");
+        await SeedPlayout(1, channelId: 1);
+
+        // client GETs the channel (PlayoutSource: Generated) and PUTs the same value back
+        // unchanged; this must still succeed even though the channel has a playout.
+        Either<BaseError, ChannelViewModel> result =
+            await MakeHandler().Handle(MakeUpdate(1, number: "5", name: "Renamed"), CancellationToken.None);
+
+        result.IsRight.ShouldBeTrue();
+    }
+
+    [Test]
+    public async Task Should_Null_Mirror_Only_Fields_When_Saving_Generated_Channel_With_Playout()
+    {
+        await SeedFFmpegProfile();
+        await SeedChannel(1, "5"); // has a playout below
+        await SeedChannel(2, "6"); // stray reference target
+        await SeedPlayout(1, channelId: 1);
+
+        // request keeps PlayoutSource: Generated but carries stray Mirror-only fields — e.g. a
+        // client that never cleared the fields after flipping the UI back from Mirror. These
+        // must never persist onto a Generated channel.
+        UpdateChannel update = MakeUpdate(1, number: "5") with
+        {
+            MirrorSourceChannelId = 2,
+            PlayoutOffset = TimeSpan.FromHours(1)
+        };
+
+        Either<BaseError, ChannelViewModel> result = await MakeHandler().Handle(update, CancellationToken.None);
+
+        result.IsRight.ShouldBeTrue();
+
+        await using TvContext context = Db.CreateContext();
+        var channel = await context.Channels.SingleAsync(c => c.Id == 1);
+        channel.PlayoutSource.ShouldBe(ChannelPlayoutSource.Generated);
+        channel.MirrorSourceChannelId.ShouldBeNull();
+        channel.PlayoutOffset.ShouldBeNull();
+    }
+
     private static BaseError LeftOf<TR>(Either<BaseError, TR> either) =>
         either.Match(Left: e => e, Right: _ => throw new AssertionException("Expected a Left result"));
 }

ErsatzTV.Tests/Support/ChannelHandlerTestBase.cs

@@ -64,6 +64,19 @@ public abstract class ChannelHandlerTestBase
         await context.SaveChangesAsync();
     }
 
+    protected async Task SeedPlayout(int id, int channelId)
+    {
+        await using TvContext context = Db.CreateContext();
+        context.Playouts.Add(
+            new Playout
+            {
+                Id = id,
+                ChannelId = channelId,
+                ScheduleKind = PlayoutScheduleKind.Classic
+            });
+        await context.SaveChangesAsync();
+    }
+
     protected static CreateChannel MakeCreate(
         string number = "5",
         int ffmpegProfileId = 1,