ersatztv/ErsatzTV.Application/MediaCollections/Commands/AddItemsToCollectionHandler.cs

using System.Threading.Channels;
using ErsatzTV.Application.Playouts;
using ErsatzTV.Application.Search;
using ErsatzTV.Core;
using ErsatzTV.Core.Domain;
using ErsatzTV.Core.Errors;
using ErsatzTV.Core.Interfaces.Repositories;
using ErsatzTV.Core.Scheduling;
using ErsatzTV.Infrastructure.Data;
using ErsatzTV.Infrastructure.Extensions;
using Microsoft.EntityFrameworkCore;

namespace ErsatzTV.Application.MediaCollections;

public class AddItemsToCollectionHandler :
    IRequestHandler<AddItemsToCollection, Either<BaseError, Unit>>
{
    private readonly ChannelWriter<IBackgroundServiceRequest> _channel;
    private readonly IDbContextFactory<TvContext> _dbContextFactory;
    private readonly IMediaCollectionRepository _mediaCollectionRepository;
    private readonly IMovieRepository _movieRepository;
    private readonly ChannelWriter<ISearchIndexBackgroundServiceRequest> _searchChannel;
    private readonly ITelevisionRepository _televisionRepository;

    public AddItemsToCollectionHandler(
        IDbContextFactory<TvContext> dbContextFactory,
        IMediaCollectionRepository mediaCollectionRepository,
        IMovieRepository movieRepository,
        ITelevisionRepository televisionRepository,
        ChannelWriter<IBackgroundServiceRequest> channel,
        ChannelWriter<ISearchIndexBackgroundServiceRequest> searchChannel)
    {
        _dbContextFactory = dbContextFactory;
        _mediaCollectionRepository = mediaCollectionRepository;
        _movieRepository = movieRepository;
        _televisionRepository = televisionRepository;
        _channel = channel;
        _searchChannel = searchChannel;
    }

    // A duplicate-key race can roll back the whole batch (#308); recompute membership from a fresh
    // context and retry with only the still-missing items. Bounded to avoid a livelock — the common
    // no-collision path runs the loop body exactly once.
    private const int MaxDuplicateRetries = 5;

    public async Task<Either<BaseError, Unit>> Handle(
        AddItemsToCollection request,
        CancellationToken cancellationToken)
    {
        for (var attempt = 0; ; attempt++)
        {
            await using TvContext dbContext = await _dbContextFactory.CreateDbContextAsync(cancellationToken);
            Option<Collection> maybeCollection = await CollectionMustExist(dbContext, request, cancellationToken);

            // true = terminal (nothing to add, or our batch committed); false = a duplicate-key race
            // rolled the batch back, recompute membership and retry.
            Either<BaseError, bool> attemptResult = await maybeCollection.Match(
                Some: async collection =>
                {
                    Validation<BaseError, Collection> validation = await Validate(dbContext, request, collection, cancellationToken);
                    return await validation.Apply(c => ApplyAddItemsRequest(dbContext, c, request, cancellationToken));
                },
                None: () => Task.FromResult<Either<BaseError, bool>>(
                    new NotFoundError($"Collection {request.CollectionId} does not exist.")));

            if (attemptResult.IsLeft)
            {
                return attemptResult.Map(_ => Unit.Default);
            }

            bool committed = attemptResult.Match(Left: _ => false, Right: done => done);
            if (committed)
            {
                return Unit.Default;
            }

            // A concurrent add inserted one+ of our items first; recompute against fresh membership.
            if (attempt >= MaxDuplicateRetries)
            {
                return BaseError.New(
                    "Concurrent modification while adding items to the collection; please retry.");
            }
        }
    }

    private async Task<bool> ApplyAddItemsRequest(
        TvContext dbContext,
        Collection collection,
        AddItemsToCollection request,
        CancellationToken cancellationToken)
    {
        var allItems = request.MovieIds
            .Append(request.ShowIds)
            .Append(request.SeasonIds)
            .Append(request.EpisodeIds)
            .Append(request.ArtistIds)
            .Append(request.MusicVideoIds)
            .Append(request.OtherVideoIds)
            .Append(request.SongIds)
            .Append(request.ImageIds)
            .Append(request.RemoteStreamIds)
            .ToList();

        var toAddIds = allItems.Where(item => collection.MediaItems.All(mi => mi.Id != item)).ToList();

        // No-op when every requested item is already a member: don't rotate the ETag or fan out
        // rebuilds for an idempotent re-add — #269. Terminal success (no retry).
        if (toAddIds.Count == 0)
        {
            return true;
        }

        List<MediaItem> toAdd = await dbContext.MediaItems
            .Filter(mi => toAddIds.Contains(mi.Id))
            .ToListAsync(cancellationToken);

        collection.MediaItems.AddRange(toAdd);

        // Rotate the collection ETag so an open custom-order editor's If-Match invalidates (#269);
        // force-write past a concurrent replace-all Version bump — this add takes no If-Match, so a
        // benign race must not 500 (#253/#269 §7a).
        collection.Version++;

        // A concurrent add of an overlapping item won the composite-PK race and rolled back this whole
        // batch. Unlike the single-item handlers (idempotent no-op), a bulk add must NOT drop the items
        // that did NOT collide — signal the caller to recompute membership and retry the still-missing
        // ones. #308
        if (!await dbContext.TrySaveChangesForcingVersion(cancellationToken))
        {
            return false;
        }

        // post-commit side effect runs on CancellationToken.None so a late request cancellation
        // can't abort it after the commit landed (#254)
        await _searchChannel.WriteAsync(new ReindexMediaItems(toAddIds.ToArray()), CancellationToken.None);

        // refresh all playouts that use this collection
        foreach (int playoutId in await _mediaCollectionRepository
                     .PlayoutIdsUsingCollection(request.CollectionId))
        {
            await _channel.WriteAsync(new BuildPlayout(playoutId, PlayoutBuildMode.Refresh), CancellationToken.None);
        }

        return true;
    }

    private async Task<Validation<BaseError, Collection>> Validate(
        TvContext dbContext,
        AddItemsToCollection request,
        Collection collection,
        CancellationToken cancellationToken) =>
        (await ValidateMovies(request),
            await ValidateShows(request),
            await ValidateSeasons(request),
            await ValidateEpisodes(request),
            await ValidateMediaItems(dbContext, request, cancellationToken))
        .Apply((_, _, _, _, _) => collection);

    private static Task<Option<Collection>> CollectionMustExist(
        TvContext dbContext,
        AddItemsToCollection request,
        CancellationToken cancellationToken) =>
        dbContext.Collections
            .Include(c => c.MediaItems)
            .SelectOneAsync(c => c.Id, c => c.Id == request.CollectionId, cancellationToken)
            .Map(identity);

    private Task<Validation<BaseError, Unit>> ValidateMovies(AddItemsToCollection request) =>
        _movieRepository.AllMoviesExist(request.MovieIds)
            .Map(Optional)
            .Filter(v => v == true)
            .MapT(_ => Unit.Default)
            .Map(v => v.ToValidation<BaseError>("Movie does not exist"));

    private Task<Validation<BaseError, Unit>> ValidateShows(AddItemsToCollection request) =>
        _televisionRepository.AllShowsExist(request.ShowIds)
            .Map(Optional)
            .Filter(v => v == true)
            .MapT(_ => Unit.Default)
            .Map(v => v.ToValidation<BaseError>("Show does not exist"));

    private Task<Validation<BaseError, Unit>> ValidateSeasons(AddItemsToCollection request) =>
        _televisionRepository.AllSeasonsExist(request.SeasonIds)
            .Map(Optional)
            .Filter(v => v == true)
            .MapT(_ => Unit.Default)
            .Map(v => v.ToValidation<BaseError>("Season does not exist"));

    private Task<Validation<BaseError, Unit>> ValidateEpisodes(AddItemsToCollection request) =>
        _televisionRepository.AllEpisodesExist(request.EpisodeIds)
            .Map(Optional)
            .Filter(v => v == true)
            .MapT(_ => Unit.Default)
            .Map(v => v.ToValidation<BaseError>("Episode does not exist"));

    private static async Task<Validation<BaseError, Unit>> ValidateMediaItems(
        TvContext dbContext,
        AddItemsToCollection request,
        CancellationToken cancellationToken)
    {
        List<int> ids = GetRequestedMediaItemIds(request).Distinct().ToList();
        int existingCount = await dbContext.MediaItems
            .CountAsync(mi => ids.Contains(mi.Id), cancellationToken);

        return existingCount == ids.Count
            ? Unit.Default
            : BaseError.New("Media item does not exist");
    }

    private static IEnumerable<int> GetRequestedMediaItemIds(AddItemsToCollection request) =>
        request.MovieIds
            .Append(request.ShowIds)
            .Append(request.SeasonIds)
            .Append(request.EpisodeIds)
            .Append(request.ArtistIds)
            .Append(request.MusicVideoIds)
            .Append(request.OtherVideoIds)
            .Append(request.SongIds)
            .Append(request.ImageIds)
            .Append(request.RemoteStreamIds);
}