ersatztv/ErsatzTV.Tests/Integration/LibraryFolderConcurrencyTests.cs

using ErsatzTV.Core;
using ErsatzTV.Core.Domain;
using ErsatzTV.Infrastructure.Data;
using ErsatzTV.Infrastructure.Data.Repositories;
using ErsatzTV.Infrastructure.Sqlite.Data;
using ErsatzTV.Tests.Support;
using LanguageExt;
using Microsoft.Data.Sqlite;
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Diagnostics;
using NSubstitute;
using NUnit.Framework;
using Shouldly;
using IFileSystem = System.IO.Abstractions.IFileSystem;

namespace ErsatzTV.Tests.Integration;

/// <summary>
///     ersatztv#491: <c>ILibraryRepository.GetOrAddFolder</c> is a check-then-insert, so two callers
///     racing the same <c>(LibraryPathId, Path)</c> both miss the lookup and both insert. The fix is a
///     unique index on <c>(LibraryPathId, PathHash)</c> plus a catch-and-re-read in the repository, so
///     the loser adopts the winner's row instead of creating a duplicate.
/// </summary>
[TestFixture]
public class LibraryFolderConcurrencyTests
{
    private const string LibraryPathValue = "/data/music";
    private const string FolderPath = "/data/music/artist1";

    private static LibraryRepository Repository(IDbContextFactory<TvContext> factory) =>
        new(Substitute.For<IFileSystem>(), factory);

    private static async Task<int> SeedLibraryPath(Func<TvContext> createContext)
    {
        await using TvContext context = createContext();
        var libraryPath = new LibraryPath { Path = LibraryPathValue };
        await context.LibraryPaths.AddAsync(libraryPath);
        await context.SaveChangesAsync();
        return libraryPath.Id;
    }

    private static async Task<int> FolderCount(Func<TvContext> createContext, int libraryPathId, string path)
    {
        await using TvContext context = createContext();
        return await context.LibraryFolders.CountAsync(f => f.LibraryPathId == libraryPathId && f.Path == path);
    }

    private static async Task InsertFolderRaw(
        SharedCacheTvContext db,
        int libraryPathId,
        string path,
        string pathHash,
        CancellationToken cancellationToken = default)
    {
        await using SqliteConnection connection = db.OpenConnection();
        await using SqliteCommand command = connection.CreateCommand();
        command.CommandText =
            "INSERT INTO \"LibraryFolder\" (\"LibraryPathId\", \"Path\", \"PathHash\", \"Etag\", \"ParentId\") " +
            "VALUES ($libraryPathId, $path, $pathHash, NULL, NULL)";
        command.Parameters.AddWithValue("$libraryPathId", libraryPathId);
        command.Parameters.AddWithValue("$path", path);
        command.Parameters.AddWithValue("$pathHash", (object?)pathHash ?? DBNull.Value);
        await command.ExecuteNonQueryAsync(cancellationToken);
    }

    /// <summary>
    ///     Simulates the concurrent "winner": exactly once, on a SEPARATE connection, insert the same
    ///     folder and commit — AFTER the intercepted context read the (stale) absent lookup but BEFORE its
    ///     own INSERT runs. This interposes the race deterministically instead of hoping for a timing
    ///     window. <see cref="Fired" /> proves the race actually happened (non-vacuity).
    /// </summary>
    private sealed class InsertConflictingFolderOnce(SharedCacheTvContext db, int libraryPathId, string path)
        : SaveChangesInterceptor
    {
        private int _fired;

        public int Fired => _fired;

        public override async ValueTask<InterceptionResult<int>> SavingChangesAsync(
            DbContextEventData eventData,
            InterceptionResult<int> result,
            CancellationToken cancellationToken = default)
        {
            if (Interlocked.Exchange(ref _fired, 1) == 0)
            {
                await InsertFolderRaw(db, libraryPathId, path, PathUtils.GetPathHash(path), cancellationToken);
            }

            return result;
        }
    }

    /// <summary>Counts insert attempts so the multi-threaded test can prove it really raced.</summary>
    private sealed class CountSaveAttempts : SaveChangesInterceptor
    {
        private int _attempts;

        public int Attempts => _attempts;

        public override ValueTask<InterceptionResult<int>> SavingChangesAsync(
            DbContextEventData eventData,
            InterceptionResult<int> result,
            CancellationToken cancellationToken = default)
        {
            Interlocked.Increment(ref _attempts);
            return ValueTask.FromResult(result);
        }
    }

    // ----- Negative control #1: the index itself. Without the new unique index this test fails, because
    // the second insert simply succeeds and there is no violation to classify. -----

    [Test]
    public async Task Duplicate_LibraryFolder_Insert_Throws_A_Classified_UniqueViolation()
    {
        await using SharedCacheTvContext db = await SharedCacheTvContext.CreateAsync("etv491-index");
        int libraryPathId = await SeedLibraryPath(db.CreateContext);
        string hash = PathUtils.GetPathHash(FolderPath);

        await InsertFolderRaw(db, libraryPathId, FolderPath, hash);

        await using TvContext context = db.CreateContext();
        await context.LibraryFolders.AddAsync(
            new LibraryFolder { LibraryPathId = libraryPathId, Path = FolderPath, PathHash = hash });

        DbUpdateException ex = await Should.ThrowAsync<DbUpdateException>(() => context.SaveChangesAsync());
        SqliteErrorClassifier.IsUniqueConstraintViolation(ex).ShouldBeTrue();

        // and the classifier is not a blanket "true"
        SqliteErrorClassifier.IsUniqueConstraintViolation(
            new DbUpdateException("nope", new InvalidOperationException())).ShouldBeFalse();
    }

    // The migration leaves pre-#491 rows with a null hash; a unique index treats nulls as distinct, so
    // applying the index to an existing database can never fail on them. Documents that premise.
    [Test]
    public async Task Legacy_Null_PathHash_Rows_Do_Not_Collide()
    {
        await using SharedCacheTvContext db = await SharedCacheTvContext.CreateAsync("etv491-nulls");
        int libraryPathId = await SeedLibraryPath(db.CreateContext);

        await InsertFolderRaw(db, libraryPathId, "/data/music/a", null);
        await InsertFolderRaw(db, libraryPathId, "/data/music/b", null);

        await using TvContext context = db.CreateContext();
        (await context.LibraryFolders.CountAsync(f => f.LibraryPathId == libraryPathId)).ShouldBe(2);
    }

    // ----- The deterministic cross-connection race through the real repository -----

    [Test]
    public async Task GetOrAddFolder_Losing_The_Race_Adopts_The_Winner_Instead_Of_Duplicating()
    {
        await using SharedCacheTvContext db = await SharedCacheTvContext.CreateAsync("etv491-race");
        int libraryPathId = await SeedLibraryPath(db.CreateContext);
        var libraryPath = new LibraryPath { Id = libraryPathId, Path = LibraryPathValue, LibraryFolders = null };

        var racer = new InsertConflictingFolderOnce(db, libraryPathId, FolderPath);
        LibraryRepository repository = Repository(db.Factory(racer));

        LibraryFolder result = await repository.GetOrAddFolder(libraryPath, Option<int>.None, FolderPath);

        racer.Fired.ShouldBe(1); // the race genuinely occurred — this assertion is the vacuity guard
        result.ShouldNotBeNull();
        result.Id.ShouldBeGreaterThan(0);
        result.Path.ShouldBe(FolderPath);
        (await FolderCount(db.CreateContext, libraryPathId, FolderPath)).ShouldBe(1);

        // the returned row is the winner's persisted row, not a phantom
        await using TvContext context = db.CreateContext();
        LibraryFolder persisted = await context.LibraryFolders.SingleAsync(f => f.LibraryPathId == libraryPathId);
        persisted.Id.ShouldBe(result.Id);
        persisted.PathHash.ShouldBe(PathUtils.GetPathHash(FolderPath));
    }

    // The loser must still apply the parent id it was asked to set — to the WINNER's row.
    [Test]
    public async Task GetOrAddFolder_Losing_The_Race_Still_Persists_The_ParentId()
    {
        await using SharedCacheTvContext db = await SharedCacheTvContext.CreateAsync("etv491-race-parent");
        int libraryPathId = await SeedLibraryPath(db.CreateContext);
        var libraryPath = new LibraryPath { Id = libraryPathId, Path = LibraryPathValue, LibraryFolders = null };

        LibraryFolder parent = await Repository(db.Factory())
            .GetOrAddFolder(libraryPath, Option<int>.None, LibraryPathValue);

        var racer = new InsertConflictingFolderOnce(db, libraryPathId, FolderPath);
        LibraryFolder result = await Repository(db.Factory(racer))
            .GetOrAddFolder(libraryPath, Option<int>.Some(parent.Id), FolderPath);

        racer.Fired.ShouldBe(1);
        result.ParentId.ShouldBe(parent.Id);

        await using TvContext context = db.CreateContext();
        LibraryFolder persisted = await context.LibraryFolders.SingleAsync(f => f.Path == FolderPath);
        persisted.Id.ShouldBe(result.Id);
        persisted.ParentId.ShouldBe(parent.Id);
    }

    // ----- Negative control #2: invert the real condition (the provider classifier) and the SAME race
    // must blow up, proving the catch in GetOrAddFolder is load-bearing rather than decorative. -----

    [Test]
    public async Task GetOrAddFolder_Rethrows_When_The_Provider_Does_Not_Classify_The_Violation()
    {
        Func<DbUpdateException, bool> original = TvContext.IsUniqueConstraintViolation;
        try
        {
            await using SharedCacheTvContext db = await SharedCacheTvContext.CreateAsync("etv491-negctl");
            int libraryPathId = await SeedLibraryPath(db.CreateContext);
            var libraryPath = new LibraryPath { Id = libraryPathId, Path = LibraryPathValue, LibraryFolders = null };

            var racer = new InsertConflictingFolderOnce(db, libraryPathId, FolderPath);
            LibraryRepository repository = Repository(db.Factory(racer));

            TvContext.IsUniqueConstraintViolation = _ => false;
            await Should.ThrowAsync<DbUpdateException>(
                () => repository.GetOrAddFolder(libraryPath, Option<int>.None, FolderPath));

            racer.Fired.ShouldBe(1);
        }
        finally
        {
            TvContext.IsUniqueConstraintViolation = original;
        }
    }

    // ----- N threads x rounds over a single (LibraryPathId, Path) -----

    [Test]
    public async Task Concurrent_GetOrAddFolder_Never_Produces_Duplicate_Rows()
    {
        const int threads = 8;
        const int rounds = 10;

        await using SharedCacheTvContext db = await SharedCacheTvContext.CreateAsync("etv491-threads");
        int libraryPathId = await SeedLibraryPath(db.CreateContext);

        var counter = new CountSaveAttempts();
        IDbContextFactory<TvContext> factory = db.Factory(counter);

        for (var round = 0; round < rounds; round++)
        {
            string path = $"{LibraryPathValue}/round{round}";
            using var gate = new Barrier(threads);

            var tasks = new Task<LibraryFolder>[threads];
            for (var thread = 0; thread < threads; thread++)
            {
                tasks[thread] = Task.Run(async () =>
                {
                    // every thread carries its own detached LibraryPath, as the scanners do
                    var libraryPath = new LibraryPath
                    {
                        Id = libraryPathId, Path = LibraryPathValue, LibraryFolders = null
                    };

                    gate.SignalAndWait();
                    return await Repository(factory).GetOrAddFolder(libraryPath, Option<int>.None, path);
                });
            }

            LibraryFolder[] results = await Task.WhenAll(tasks);

            // every caller got the same single row back...
            results.Select(f => f.Id).Distinct().Count().ShouldBe(1);
            results[0].Id.ShouldBeGreaterThan(0);

            // ...and exactly one row exists for it
            (await FolderCount(db.CreateContext, libraryPathId, path)).ShouldBe(1);
        }

        await using TvContext context = db.CreateContext();
        (await context.LibraryFolders.CountAsync(f => f.LibraryPathId == libraryPathId)).ShouldBe(rounds);

        // Vacuity guard: one insert attempt per round would mean the threads never actually collided and
        // the test proved nothing. More attempts than rounds means at least one caller lost the race and
        // was rescued by the index + catch.
        counter.Attempts.ShouldBeGreaterThan(rounds);
    }

    // ----- SetEtag is the repository's other check-then-insert on LibraryFolder -----

    [Test]
    public async Task SetEtag_Losing_The_Race_Updates_The_Winner_Instead_Of_Duplicating()
    {
        await using SharedCacheTvContext db = await SharedCacheTvContext.CreateAsync("etv491-setetag");
        int libraryPathId = await SeedLibraryPath(db.CreateContext);
        var libraryPath = new LibraryPath { Id = libraryPathId, Path = LibraryPathValue, LibraryFolders = null };

        var racer = new InsertConflictingFolderOnce(db, libraryPathId, FolderPath);
        LibraryRepository repository = Repository(db.Factory(racer));

        await repository.SetEtag(libraryPath, Option<LibraryFolder>.None, FolderPath, "etag-1");

        racer.Fired.ShouldBe(1);

        await using TvContext context = db.CreateContext();
        LibraryFolder persisted = await context.LibraryFolders.SingleAsync(f => f.Path == FolderPath);
        persisted.Etag.ShouldBe("etag-1");
    }
}