namespace ScadaLink.Host;

/// <summary>
/// Bounded retry-with-backoff for startup preconditions.
///
/// Host-010 / REQ-HOST-4a: a Central node applies/validates database migrations
/// before the host begins serving traffic. In container orchestration the database
/// and the app frequently start together, so the database may be briefly
/// unreachable. Rather than crashing the process on the first connection failure,
/// the migration step is wrapped in this bounded exponential backoff: it tolerates a
/// short outage and only fails fatally once attempts are exhausted.
///
/// Host-015: only <em>transient</em> faults are retried. The optional
/// <c>isTransient</c> predicate classifies each exception; a permanent failure
/// (e.g. a database schema-version mismatch — which no amount of waiting can fix)
/// is rethrown immediately rather than being retried for minutes before the
/// inevitable fatal exit.
/// </summary>
public static class StartupRetry
{
    public static async Task ExecuteWithRetryAsync(
        string operationName,
        Func<Task> operation,
        int maxAttempts,
        TimeSpan initialDelay,
        ILogger logger,
        Func<Exception, bool>? isTransient = null,
        CancellationToken cancellationToken = default)
    {
        // Default: treat every exception as transient (preserves the pre-Host-015
        // behaviour for callers that do not classify faults).
        isTransient ??= static _ => true;

        var delay = initialDelay;
        for (var attempt = 1; ; attempt++)
        {
            cancellationToken.ThrowIfCancellationRequested();
            try
            {
                await operation();
                if (attempt > 1)
                    logger.LogInformation(
                        "Startup operation '{Operation}' succeeded on attempt {Attempt}.",
                        operationName, attempt);
                return;
            }
            catch (Exception ex) when (attempt < maxAttempts && isTransient(ex))
            {
                logger.LogWarning(ex,
                    "Startup operation '{Operation}' failed (transient) on attempt {Attempt}/{MaxAttempts}; " +
                    "retrying in {Delay}.",
                    operationName, attempt, maxAttempts, delay);
                await Task.Delay(delay, cancellationToken);
                // Exponential backoff, capped so the total wait stays bounded.
                delay = TimeSpan.FromTicks(Math.Min(delay.Ticks * 2, TimeSpan.FromSeconds(30).Ticks));
            }
        }
    }

    /// <summary>
    /// Transient-fault classifier for the database-migration startup step (Host-015).
    /// Returns <c>true</c> only for connection-class faults that a brief wait can
    /// resolve — a SQL connection/transport error or a timeout — and <c>false</c>
    /// for everything else (notably schema-validation <see cref="InvalidOperationException"/>s
    /// raised by <c>MigrationHelper.ApplyOrValidateMigrationsAsync</c>, which are
    /// permanent and must fail fast).
    /// </summary>
    public static bool IsTransientDatabaseFault(Exception ex)
    {
        // Unwrap a single layer of aggregation so a faulted Task surfaces correctly.
        if (ex is AggregateException agg && agg.InnerException != null)
            ex = agg.InnerException;

        if (ex is TimeoutException)
            return true;

        // Socket / network errors raised while opening the connection.
        if (ex is System.Net.Sockets.SocketException)
            return true;

        // Microsoft.Data.SqlClient throws SqlException; matching by type name keeps
        // the Host free of a direct SqlClient package reference. A SqlException at
        // the migration stage is, in practice, a connection failure (the server is
        // not yet reachable) rather than a schema fault — schema mismatches surface
        // as InvalidOperationException from the migration helper.
        var typeName = ex.GetType().FullName;
        if (typeName != null &&
            (typeName.EndsWith("SqlException", StringComparison.Ordinal) ||
             typeName.EndsWith("DbException", StringComparison.Ordinal)))
            return true;

        return false;
    }
}