using Microsoft.Data.SqlClient;
using Microsoft.EntityFrameworkCore;

namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Tests.Migrations;

/// <summary>
/// Per-test-class MSSQL fixture for the Bundle C integration tests (#23 M1).
///
/// Creates a fresh, uniquely-named test database on the running infra/mssql
/// container, applies the EF migrations against it, and drops it on dispose.
/// When MSSQL is not reachable (CI without the container), <see cref="Available"/>
/// is set to false and <see cref="SkipReason"/> describes why — tests pair
/// <c>[SkippableFact]</c> with <c>Skip.IfNot(_fixture.Available, _fixture.SkipReason)</c>
/// so the runner reports them as Skipped (not silently Passed).
/// </summary>
/// <remarks>
/// <para>
/// xunit 2.9.x has no native <c>Assert.Skip</c>/<c>Assert.SkipUnless</c> (those
/// are v3); the project uses the Xunit.SkippableFact package as the canonical
/// equivalent. The fixture attempts connect + create-db + migrate once at
/// construct time. Only connect-failure exceptions (SqlException, plus the
/// InvalidOperationException SqlClient raises from OpenAsync) flip Available to
/// false — every other exception bubbles up so a real bug is not silently
/// swallowed.
/// </para>
/// <para>
/// <b>Cross-process throttling (deferred-work register row 82).</b> Database
/// <i>naming</i> was already isolated (a guid per fixture instance), but nothing
/// bounded how many fixtures ran <c>CREATE DATABASE</c> + the full migration
/// chain against the single dev SQL Server at the same time. xunit runs
/// processor-count test collections in parallel <i>per process</i>, and a
/// full-solution run starts several MSSQL-backed test processes at once — so the
/// server saw dozens of concurrent fresh-database migrations. That is pure
/// resource saturation (CPU / log IO / tempdb on one container), not object-level
/// locking, and it pushed individual migration DDL statements past EF's default
/// 30-second command timeout. Every observed failure had the identical
/// fingerprint: <c>MsSqlMigrationFixture..ctor</c> → <c>Migrator.MigrateAsync</c>
/// → <c>SqlException: Execution Timeout Expired</c> (error 258), which xunit
/// reports as "Class fixture type … threw in its constructor" against every test
/// in the class — a false red that vanished on a re-run in isolation.
/// </para>
/// <para>
/// Two changes close it. First, <see cref="MigrationGate"/> bounds the number of
/// simultaneous create+migrate windows <i>across every process on the machine</i>
/// using SQL Server application locks (<c>sp_getapplock</c>) on <c>master</c> as a
/// counting semaphore — an xunit collection attribute cannot do this, because the
/// contention is between separate test processes. Second, the create/migrate
/// commands get timeouts sized for a queued-but-progressing server rather than an
/// idle one, so a slot holder waits instead of failing. The work always completed;
/// it just did not complete within 30 seconds.
/// </para>
/// </remarks>
public sealed class MsSqlMigrationFixture : IDisposable
{
    // Same credentials infra/mssql/setup.sql + docker-compose use. Not a committed
    // production secret — this is a local dev container connection string.
    // Connect Timeout=3 makes the connectivity probe fail fast (~3s per attempt) in a
    // no-container environment rather than hanging on SqlClient's default 30s.
    private const string DefaultAdminConnectionString =
        "Server=localhost,1433;User Id=sa;Password=ScadaBridge_Dev1#;TrustServerCertificate=true;Encrypt=false;Connect Timeout=3";

    private const string AdminEnvVar = "SCADABRIDGE_MSSQL_TEST_CONN";

    /// <summary>
    /// Command timeout for the migration chain. Deliberately far above EF's 30s
    /// default: under a bounded queue the server is busy but progressing, and the
    /// correct response to a slow DDL statement is to wait for it, not to fail the
    /// whole class. See the register-row-82 note in the type remarks.
    /// </summary>
    private const int MigrationCommandTimeoutSeconds = 300;

    /// <summary>
    /// Command timeout for the CREATE / DROP DATABASE statements. Lower than the
    /// migration chain (one statement, not fifty-odd) but still well clear of the
    /// 30s default, because CREATE DATABASE zeroes a fresh log file and that is the
    /// single most IO-bound step in the fixture.
    /// </summary>
    private const int DatabaseDdlCommandTimeoutSeconds = 120;

    /// <summary>
    /// Number of connectivity-probe attempts before declaring MSSQL unavailable.
    /// A single 3s attempt is not proof of absence once the server is under load —
    /// treating a busy server as an absent one would silently turn every MSSQL test
    /// into a Skip, which reads as green and is worse than a red.
    /// </summary>
    private const int ConnectProbeAttempts = 3;

    /// <summary>
    /// Databases whose name matches the fixture prefix and that are older than this
    /// are assumed to be strays from a crashed run and are dropped at process start.
    /// Comfortably longer than any legitimate fixture lifetime, so a live fixture in
    /// a concurrently-running process is never targeted.
    /// </summary>
    private static readonly TimeSpan OrphanDatabaseMaxAge = TimeSpan.FromHours(2);

    private const string DatabaseNamePrefix = "ScadaBridgeAuditMigTest_";

    /// <summary>Guards the once-per-process orphan sweep.</summary>
    private static int _orphanSweepStarted;

    public string DatabaseName { get; }

    public string ConnectionString { get; }

    /// <summary>
    /// True when the MSSQL container was reachable at fixture construction
    /// time AND the per-fixture test database was successfully created. When
    /// false, the integration tests using this fixture must early-return.
    /// </summary>
    public bool Available { get; }

    /// <summary>
    /// Populated when <see cref="Available"/> is false; describes why the
    /// fixture chose to skip (env var unset, connect failed, etc.).
    /// </summary>
    public string SkipReason { get; }

    private readonly string _adminConnectionString;

    public MsSqlMigrationFixture()
    {
        // Short, lowercase guid suffix keeps the database identifier under SQL Server's
        // 128-char limit and safe for raw concatenation (no quoting required).
        DatabaseName = $"{DatabaseNamePrefix}{Guid.NewGuid():N}".Substring(0, 38);

        // Env var lets CI / power users override the admin endpoint; absent
        // defaults to the local docker dev container's sa connection.
        var fromEnv = Environment.GetEnvironmentVariable(AdminEnvVar);
        _adminConnectionString = string.IsNullOrWhiteSpace(fromEnv)
            ? DefaultAdminConnectionString
            : fromEnv;

        // Step 1: probe connectivity. This is the only step that may legitimately
        // fail when MSSQL is absent; SqlException + the rare InvalidOperationException
        // from Open are the connect-failure surfaces we tolerate. Everything else
        // (CREATE DATABASE, MigrateAsync) is treated as a hard fixture failure once we
        // *have* a connection.
        var probeFailure = ProbeConnectivity(_adminConnectionString);
        if (probeFailure is not null)
        {
            ConnectionString = string.Empty;
            Available = false;
            SkipReason = probeFailure;
            return;
        }

        // Step 2: best-effort, once per process — drop databases stranded by a run
        // that crashed before Dispose. The guid suffix means they never collide, but
        // they do accumulate and consume server resources, which feeds the very
        // saturation this fixture now throttles against.
        SweepOrphanDatabasesOnce(_adminConnectionString);

        try
        {
            // Step 3: take a create+migrate slot. Held across BOTH the CREATE DATABASE
            // and the migration chain, then released — this is the machine-wide bound
            // on how much fresh-database DDL the dev server is asked to do at once.
            using (MigrationGate.Acquire(_adminConnectionString))
            {
                using (var connection = new SqlConnection(_adminConnectionString))
                {
                    connection.Open();
                    using var createCmd = connection.CreateCommand();
                    createCmd.CommandTimeout = DatabaseDdlCommandTimeoutSeconds;
                    createCmd.CommandText = $"CREATE DATABASE [{DatabaseName}];";
                    createCmd.ExecuteNonQuery();
                }

                ConnectionString = BuildPerDbConnectionString(_adminConnectionString, DatabaseName);

                // Apply the EF migrations once at fixture construction so each test
                // can read from a fully-migrated database without per-test setup.
                // Failures here are real bugs — let them bubble.
                ApplyMigrationsCore(ConnectionString, CancellationToken.None).GetAwaiter().GetResult();
            }

            Available = true;
            SkipReason = string.Empty;
        }
        catch
        {
            // Best-effort cleanup if we created the database but failed before
            // setting Available — otherwise Dispose() would skip the drop.
            TryDropOrphanDatabase();
            throw;
        }
    }

    /// <summary>
    /// Opens the admin connection up to <see cref="ConnectProbeAttempts"/> times.
    /// Returns null when a connection was established, or the skip reason when every
    /// attempt failed.
    /// </summary>
    private static string? ProbeConnectivity(string adminConnectionString)
    {
        string lastFailure = "MSSQL unavailable (no connect attempt was made)";

        for (var attempt = 1; attempt <= ConnectProbeAttempts; attempt++)
        {
            try
            {
                using var connection = new SqlConnection(adminConnectionString);
                connection.Open();
                return null;
            }
            catch (SqlException ex)
            {
                lastFailure =
                    $"MSSQL unavailable (connect failed after {attempt} attempt(s): SqlException {ex.Number}: {ex.Message})";
            }
            catch (InvalidOperationException ex)
            {
                lastFailure =
                    $"MSSQL unavailable (Open threw after {attempt} attempt(s): {ex.Message})";
            }

            if (attempt < ConnectProbeAttempts)
            {
                Thread.Sleep(TimeSpan.FromMilliseconds(250 * attempt));
            }
        }

        return lastFailure;
    }

    /// <summary>
    /// Drops fixture databases left behind by a run that died before
    /// <see cref="Dispose"/>. Runs at most once per test process and swallows every
    /// failure — an uncleaned stray is untidy, not fatal, and must never turn into a
    /// fixture error.
    /// </summary>
    private static void SweepOrphanDatabasesOnce(string adminConnectionString)
    {
        if (Interlocked.Exchange(ref _orphanSweepStarted, 1) != 0)
        {
            return;
        }

        try
        {
            using var connection = new SqlConnection(adminConnectionString);
            connection.Open();

            var stale = new List<string>();
            using (var listCmd = connection.CreateCommand())
            {
                listCmd.CommandTimeout = DatabaseDdlCommandTimeoutSeconds;
                listCmd.CommandText =
                    "SELECT name FROM sys.databases " +
                    "WHERE name LIKE @Prefix + '%' AND create_date < DATEADD(minute, -@AgeMinutes, GETDATE());";
                listCmd.Parameters.AddWithValue("@Prefix", DatabaseNamePrefix);
                listCmd.Parameters.AddWithValue("@AgeMinutes", (int)OrphanDatabaseMaxAge.TotalMinutes);

                using var reader = listCmd.ExecuteReader();
                while (reader.Read())
                {
                    stale.Add(reader.GetString(0));
                }
            }

            foreach (var name in stale)
            {
                try
                {
                    using var dropCmd = connection.CreateCommand();
                    dropCmd.CommandTimeout = DatabaseDdlCommandTimeoutSeconds;
                    // Name comes from sys.databases filtered to our own guid-suffixed
                    // prefix, so it cannot carry an injection payload.
                    dropCmd.CommandText =
                        $"ALTER DATABASE [{name}] SET SINGLE_USER WITH ROLLBACK IMMEDIATE; " +
                        $"DROP DATABASE [{name}];";
                    dropCmd.ExecuteNonQuery();
                }
                catch
                {
                    // Another process may be mid-drop, or the database may be in use.
                    // Best-effort by design.
                }
            }
        }
        catch
        {
            // Best-effort by design.
        }
    }

    private void TryDropOrphanDatabase()
    {
        if (string.IsNullOrEmpty(ConnectionString))
        {
            return;
        }

        DropSelf();
    }

    /// <summary>
    /// Applies the EF migrations to the per-fixture test database via a freshly
    /// constructed <see cref="ScadaBridgeDbContext"/> pointed at it. Uses the
    /// schema-only single-argument constructor — the AuditLog migration does
    /// not write secret-bearing columns at apply time. Called once from the
    /// constructor; tests do not invoke this directly.
    /// </summary>
    private static async Task ApplyMigrationsCore(string connectionString, CancellationToken cancellationToken)
    {
        var options = new DbContextOptionsBuilder<ScadaBridgeDbContext>()
            // The explicit command timeout is the register-row-82 fix: EF's 30s default
            // is what actually failed under cross-process load, and every migration
            // statement in the chain inherits this value.
            .UseSqlServer(connectionString, sql => sql.CommandTimeout(MigrationCommandTimeoutSeconds))
            .Options;

        await using var context = new ScadaBridgeDbContext(options);
        await context.Database.MigrateAsync(cancellationToken);
    }

    /// <summary>
    /// Convenience for opening a fresh <see cref="SqlConnection"/> to the test
    /// database. Caller is responsible for disposal.
    /// </summary>
    public SqlConnection OpenConnection()
    {
        ThrowIfUnavailable();

        var connection = new SqlConnection(ConnectionString);
        connection.Open();
        return connection;
    }

    public void Dispose()
    {
        if (!Available)
        {
            return;
        }

        DropSelf();
    }

    /// <summary>
    /// Best-effort drop of this fixture's database — never let a teardown failure
    /// pollute later runs. SINGLE_USER WITH ROLLBACK IMMEDIATE detaches lingering
    /// pooled connections so the DROP DATABASE doesn't fail with "database is in use".
    /// Deliberately NOT gated by <see cref="MigrationGate"/>: holding a create+migrate
    /// slot at teardown would starve the fixtures still trying to start.
    /// </summary>
    private void DropSelf()
    {
        try
        {
            // Connection-pool cleanup is necessary because EF's MigrateAsync leaves
            // pooled connections behind; SqlConnection.ClearAllPools() forces them
            // closed so the SINGLE_USER + DROP sequence below can complete.
            SqlConnection.ClearAllPools();

            using var connection = new SqlConnection(_adminConnectionString);
            connection.Open();

            using var cmd = connection.CreateCommand();
            cmd.CommandTimeout = DatabaseDdlCommandTimeoutSeconds;
            cmd.CommandText =
                $"IF DB_ID(N'{DatabaseName}') IS NOT NULL " +
                $"BEGIN " +
                $"  ALTER DATABASE [{DatabaseName}] SET SINGLE_USER WITH ROLLBACK IMMEDIATE; " +
                $"  DROP DATABASE [{DatabaseName}]; " +
                $"END";
            cmd.ExecuteNonQuery();
        }
        catch
        {
            // Swallow — the database name carries a random guid suffix so a
            // stranded test database does not collide with future runs, and the
            // next process's orphan sweep will clear it.
        }
    }

    /// <summary>
    /// Throws an <see cref="InvalidOperationException"/> when invoked on an
    /// unavailable fixture; tests should branch on <see cref="Available"/>
    /// before reaching this code path.
    /// </summary>
    private void ThrowIfUnavailable()
    {
        if (!Available)
        {
            throw new InvalidOperationException(
                $"MsSqlMigrationFixture is not Available: {SkipReason}");
        }
    }

    private static string BuildPerDbConnectionString(string adminConnectionString, string databaseName)
    {
        var builder = new SqlConnectionStringBuilder(adminConnectionString)
        {
            InitialCatalog = databaseName,
        };
        return builder.ConnectionString;
    }

    /// <summary>
    /// A machine-wide counting semaphore over the create+migrate window, built from
    /// SQL Server session-scoped application locks on <c>master</c>
    /// (deferred-work register row 82).
    /// </summary>
    /// <remarks>
    /// <para>
    /// <b>Why not an xunit collection?</b> xunit's parallelism controls only bound
    /// the collections inside ONE test assembly running in ONE process. The
    /// contention here is between processes — <c>AuditLog.Tests</c>,
    /// <c>ConfigurationDatabase.Tests</c> and anything else a solution-level
    /// <c>dotnet test</c> starts in parallel all hammer the same container. The
    /// arbiter therefore has to live on the shared resource itself, which is the
    /// SQL Server instance.
    /// </para>
    /// <para>
    /// <c>sp_getapplock</c> is a binary mutex, so a counting semaphore is emulated
    /// with <see cref="SlotCount"/> distinctly-named lock resources: a caller walks
    /// the slots with a short per-slot timeout and takes the first free one, looping
    /// until it wins or the overall deadline passes. Application locks are
    /// <b>database-scoped</b>, so every gate connection targets <c>master</c> to make
    /// sure all processes contend over the same namespace regardless of which test
    /// database they are about to build.
    /// </para>
    /// <para>
    /// Pooling is disabled on the gate connection. A session-scoped application lock
    /// is released by <c>sp_reset_connection</c>, which a pooled connection only
    /// issues when it is next <i>reused</i> — so a pooled gate connection could keep
    /// a slot locked long after Dispose. Closing an unpooled connection ends the
    /// session outright, and the explicit <c>sp_releaseapplock</c> makes the normal
    /// path deterministic rather than relying on that. A crashed process drops its
    /// connection, and SQL Server releases the lock with it, so the gate cannot
    /// wedge the suite.
    /// </para>
    /// <para>
    /// The gate never fails a test: if the deadline passes without a slot, it returns
    /// an ungated instance and lets the (now generous) command timeouts carry the
    /// run. A throttle that could itself become a source of red is not worth having.
    /// </para>
    /// </remarks>
    private sealed class MigrationGate : IDisposable
    {
        /// <summary>
        /// How many fixtures may build a database at once, machine-wide. Sized to keep
        /// the dev container busy without saturating it; override with
        /// <c>SCADABRIDGE_MSSQL_TEST_MIGRATE_SLOTS</c>.
        /// </summary>
        private const int DefaultSlotCount = 4;

        private const string SlotEnvVar = "SCADABRIDGE_MSSQL_TEST_MIGRATE_SLOTS";

        private const string ResourcePrefix = "ScadaBridgeTestMigrate/";

        /// <summary>Per-slot wait before moving on to try the next slot.</summary>
        private const int PerSlotLockTimeoutMs = 500;

        /// <summary>
        /// Overall wait for a slot. Long enough to outlast a full queue of migration
        /// chains on a loaded server; past it the gate gives up and lets the caller
        /// proceed ungated rather than failing.
        /// </summary>
        private static readonly TimeSpan AcquireDeadline = TimeSpan.FromMinutes(10);

        private static int SlotCount
        {
            get
            {
                var raw = Environment.GetEnvironmentVariable(SlotEnvVar);
                return int.TryParse(raw, out var parsed) && parsed > 0 ? parsed : DefaultSlotCount;
            }
        }

        private readonly SqlConnection? _connection;
        private readonly string? _resource;

        private MigrationGate(SqlConnection? connection, string? resource)
        {
            _connection = connection;
            _resource = resource;
        }

        public static MigrationGate Acquire(string adminConnectionString)
        {
            var slots = SlotCount;
            SqlConnection? connection = null;

            try
            {
                connection = new SqlConnection(BuildGateConnectionString(adminConnectionString));
                connection.Open();

                var deadline = DateTime.UtcNow + AcquireDeadline;
                // Start each caller at a different slot so they spread out instead of
                // all queueing behind slot 0.
                var offset = Random.Shared.Next(slots);

                while (DateTime.UtcNow < deadline)
                {
                    for (var i = 0; i < slots; i++)
                    {
                        var resource = ResourcePrefix + ((offset + i) % slots);
                        if (TryAcquireSlot(connection, resource))
                        {
                            return new MigrationGate(connection, resource);
                        }
                    }
                }
            }
            catch
            {
                // The gate is an optimisation, not a correctness requirement — if it
                // cannot be established (permissions, an older server, a transient
                // fault) the caller proceeds ungated.
            }

            connection?.Dispose();
            return new MigrationGate(null, null);
        }

        private static bool TryAcquireSlot(SqlConnection connection, string resource)
        {
            using var cmd = connection.CreateCommand();
            cmd.CommandType = System.Data.CommandType.StoredProcedure;
            cmd.CommandText = "sp_getapplock";
            cmd.CommandTimeout = (PerSlotLockTimeoutMs / 1000) + 30;
            cmd.Parameters.AddWithValue("@Resource", resource);
            cmd.Parameters.AddWithValue("@LockMode", "Exclusive");
            cmd.Parameters.AddWithValue("@LockOwner", "Session");
            cmd.Parameters.AddWithValue("@LockTimeout", PerSlotLockTimeoutMs);

            var result = cmd.Parameters.Add("@Result", System.Data.SqlDbType.Int);
            result.Direction = System.Data.ParameterDirection.ReturnValue;

            cmd.ExecuteNonQuery();

            // 0 = granted immediately, 1 = granted after waiting; everything else
            // (-1 timeout, -2 cancelled, -3 deadlock victim, -999 error) means "not ours".
            var code = result.Value as int? ?? -999;
            return code >= 0;
        }

        private static string BuildGateConnectionString(string adminConnectionString)
        {
            return new SqlConnectionStringBuilder(adminConnectionString)
            {
                // Application locks are database-scoped: every process must contend in
                // the same database for the slots to mean anything.
                InitialCatalog = "master",
                // See the remarks — a pooled connection defers the implicit release of a
                // session-scoped app lock until the connection is reused.
                Pooling = false,
            }.ConnectionString;
        }

        public void Dispose()
        {
            if (_connection is null)
            {
                return;
            }

            try
            {
                if (_resource is not null)
                {
                    using var cmd = _connection.CreateCommand();
                    cmd.CommandType = System.Data.CommandType.StoredProcedure;
                    cmd.CommandText = "sp_releaseapplock";
                    cmd.Parameters.AddWithValue("@Resource", _resource);
                    cmd.Parameters.AddWithValue("@LockOwner", "Session");
                    cmd.ExecuteNonQuery();
                }
            }
            catch
            {
                // Closing the unpooled connection below ends the session, which releases
                // the lock regardless.
            }
            finally
            {
                _connection.Dispose();
            }
        }
    }
}