namespace ZB.MOM.WW.OtOpcUa.Driver.S7;

/// <summary>
///     Bounds a single S7 wire operation with a wall-clock deadline, surfacing an overrun as a
///     <see cref="TimeoutException"/>.
/// </summary>
/// <remarks>
///     <para>
///         S7.Net's <c>ReadTimeout</c> / <c>WriteTimeout</c> set the underlying
///         <see cref="System.Net.Sockets.TcpClient"/>'s <c>ReceiveTimeout</c> / <c>SendTimeout</c>,
///         which govern only <b>synchronous</b> socket calls — the <c>async</c> read/write paths
///         S7.Net actually uses ignore them entirely. Without an explicit ceiling, a read on an
///         established-but-frozen connection (a frozen PLC, a firewall DROP, or a cable pulled
///         mid-flow — where the TCP session stays <i>open</i> but no reply ever arrives) blocks until
///         the OS TCP stack gives up, which can be minutes. That silently wedges the driver's poll
///         loop: no Bad tick, no reconnect — the STAB-14 "silently killed poll loop" failure, on the
///         READ leg that the connect-timeout fix (<c>S7Driver.EnsureConnectedAsync</c>'s
///         <c>CancelAfter</c>) never covered.
///     </para>
///     <para>
///         The timed-out operation surfaces as a <see cref="TimeoutException"/>, which
///         <c>S7Driver.IsS7ConnectionFatal</c> classifies connection-fatal → the handle is marked
///         dead → the next <c>EnsureConnectedAsync</c> reopens. The end-to-end proof is the live gate
///         <c>S7_1500ConnectTimeoutOutageTests</c> (a real snap7 sim frozen with <c>docker pause</c>).
///     </para>
/// </remarks>
internal static class S7OperationDeadline
{
    /// <summary>Runs <paramref name="operation"/> under a <paramref name="timeout"/> wall-clock ceiling.</summary>
    /// <typeparam name="T">The operation result type.</typeparam>
    /// <param name="operation">The wire operation; receives a token cancelled at the deadline.</param>
    /// <param name="timeout">The wall-clock ceiling for the operation.</param>
    /// <param name="what">A short description of the operation for the timeout message (e.g. <c>read of 'DB1.DBW0'</c>).</param>
    /// <param name="ct">The caller's cancellation token — its cancellation propagates as an <see cref="OperationCanceledException"/>, not a timeout.</param>
    /// <returns>The operation result when it completes within the deadline.</returns>
    /// <exception cref="TimeoutException">The operation did not complete within <paramref name="timeout"/>.</exception>
    public static async Task<T> RunAsync<T>(
        Func<CancellationToken, Task<T>> operation, TimeSpan timeout, string what, CancellationToken ct)
    {
        ArgumentNullException.ThrowIfNull(operation);
        using var opCts = CancellationTokenSource.CreateLinkedTokenSource(ct);
        opCts.CancelAfter(timeout);
        try
        {
            // WaitAsync guarantees control returns at the deadline even if the underlying op ignores
            // its token; passing opCts.Token additionally lets an op that DOES honour cancellation
            // unwind cleanly instead of leaking a wedged socket read behind an abandoned task.
            return await operation(opCts.Token).WaitAsync(timeout, ct).ConfigureAwait(false);
        }
        catch (TimeoutException)
        {
            // WaitAsync's own deadline fired (the op ignored the token).
            throw new TimeoutException(FormatMessage(what, timeout));
        }
        catch (OperationCanceledException) when (opCts.IsCancellationRequested && !ct.IsCancellationRequested)
        {
            // OUR deadline fired and the op honoured opCts.Token — not the caller's cancellation.
            throw new TimeoutException(FormatMessage(what, timeout));
        }
    }

    /// <summary>Runs a result-less <paramref name="operation"/> under a <paramref name="timeout"/> wall-clock ceiling.</summary>
    /// <param name="operation">The wire operation; receives a token cancelled at the deadline.</param>
    /// <param name="timeout">The wall-clock ceiling for the operation.</param>
    /// <param name="what">A short description of the operation for the timeout message.</param>
    /// <param name="ct">The caller's cancellation token.</param>
    /// <returns>A task that completes when the operation completes within the deadline.</returns>
    /// <exception cref="TimeoutException">The operation did not complete within <paramref name="timeout"/>.</exception>
    public static Task RunAsync(
        Func<CancellationToken, Task> operation, TimeSpan timeout, string what, CancellationToken ct)
    {
        ArgumentNullException.ThrowIfNull(operation);
        return RunAsync(
            async token => { await operation(token).ConfigureAwait(false); return true; },
            timeout, what, ct);
    }

    private static string FormatMessage(string what, TimeSpan timeout) =>
        $"S7 {what} did not complete within {(int)timeout.TotalMilliseconds} ms.";
}