using Microsoft.Extensions.Logging.Abstractions;
using Microsoft.Extensions.Options;
using MxGateway.Server.Configuration;
using MxGateway.Server.Metrics;

namespace MxGateway.Server.Workers;

/// <summary>
///     Terminates leftover MXAccess worker processes on gateway startup.
///     <para>
///         Per <c>gateway.md</c> ("first version should terminate orphaned workers
///         on startup") and CLAUDE.md, a gateway restart does not reattach old
///         workers. After an unclean gateway crash, x86 worker processes — each
///         holding an MXAccess COM instance on an STA — survive indefinitely. This
///         terminator finds those processes by executable name/path and kills them
///         before the restarted gateway accepts sessions.
///     </para>
/// </summary>
public sealed class OrphanWorkerTerminator
{
    private readonly IRunningProcessInspector _inspector;
    private readonly GatewayMetrics _metrics;
    private readonly WorkerOptions _workerOptions;
    private readonly ILogger<OrphanWorkerTerminator> _logger;

    /// <summary>Initializes a new instance of the <see cref="OrphanWorkerTerminator"/> class.</summary>
    /// <param name="gatewayOptions">Gateway configuration options.</param>
    /// <param name="inspector">Running-process inspector.</param>
    /// <param name="metrics">Gateway metrics collector.</param>
    /// <param name="logger">Optional logger for diagnostic output.</param>
    public OrphanWorkerTerminator(
        IOptions<GatewayOptions> gatewayOptions,
        IRunningProcessInspector inspector,
        GatewayMetrics metrics,
        ILogger<OrphanWorkerTerminator>? logger = null)
    {
        ArgumentNullException.ThrowIfNull(gatewayOptions);
        _inspector = inspector ?? throw new ArgumentNullException(nameof(inspector));
        _metrics = metrics ?? throw new ArgumentNullException(nameof(metrics));
        _workerOptions = gatewayOptions.Value.Worker;
        _logger = logger ?? NullLogger<OrphanWorkerTerminator>.Instance;
    }

    /// <summary>
    ///     Finds and kills every leftover worker process. Safe to call once at
    ///     startup before any session-owned worker is launched.
    /// </summary>
    /// <returns>The number of orphan worker processes that were terminated.</returns>
    public int TerminateOrphans()
    {
        string? configuredPath = ResolveConfiguredExecutablePath();
        string processName = ResolveProcessName(configuredPath);
        int currentProcessId = Environment.ProcessId;

        int terminated = 0;
        foreach (RunningProcessInfo candidate in _inspector.GetProcessesByName(processName))
        {
            if (candidate.ProcessId == currentProcessId)
            {
                continue;
            }

            if (!IsOrphanWorker(candidate, configuredPath))
            {
                continue;
            }

            try
            {
                _inspector.Kill(candidate.ProcessId);
                _metrics.WorkerKilled("OrphanStartupCleanup");
                terminated++;
                _logger.LogWarning(
                    "Terminated orphan worker process {ProcessId} ({ExecutablePath}) left over from a previous gateway run.",
                    candidate.ProcessId,
                    candidate.ExecutablePath ?? processName);
            }
            catch (Exception exception)
            {
                // The process may have already exited, or be inaccessible.
                // A failure to kill one orphan must not block gateway startup.
                _logger.LogWarning(
                    exception,
                    "Failed to terminate orphan worker process {ProcessId}.",
                    candidate.ProcessId);
            }
        }

        if (terminated > 0)
        {
            _logger.LogInformation("Terminated {Count} orphan worker process(es) on startup.", terminated);
        }

        return terminated;
    }

    private static bool IsOrphanWorker(RunningProcessInfo candidate, string? configuredPath)
    {
        // When the executable path is readable, require an exact match against
        // the configured worker path so unrelated processes that merely share
        // the image name are never killed.
        if (candidate.ExecutablePath is { } path)
        {
            return configuredPath is not null
                && string.Equals(path, configuredPath, StringComparison.OrdinalIgnoreCase);
        }

        // A null path means the x64 gateway could not introspect the module —
        // the expected case for the x86 worker. Image-name match is the only
        // signal available; treat it as an orphan.
        return true;
    }

    private string? ResolveConfiguredExecutablePath()
    {
        try
        {
            return Path.GetFullPath(_workerOptions.ExecutablePath);
        }
        catch (Exception exception) when (exception is ArgumentException
            or NotSupportedException
            or PathTooLongException)
        {
            _logger.LogWarning(
                exception,
                "Configured worker executable path '{ExecutablePath}' is not a valid filesystem path; "
                + "orphan cleanup will match by image name only.",
                _workerOptions.ExecutablePath);
            return null;
        }
    }

    private static string ResolveProcessName(string? configuredPath)
    {
        string source = configuredPath ?? "MxGateway.Worker.exe";
        return Path.GetFileNameWithoutExtension(source);
    }
}