screenjob/service_host/ScreenJob.WindowsServiceHost/BackendProcessService.cs

using System.Diagnostics;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;

namespace ScreenJob.WindowsServiceHost;

internal sealed class BackendProcessService : BackgroundService
{
    private readonly ILogger<BackendProcessService> _logger;
    private readonly ServiceOptions _options;
    private readonly object _logLock = new();

    private Process? _backendProcess;
    private string _stdoutLogPath = string.Empty;
    private string _stderrLogPath = string.Empty;

    public BackendProcessService(ILogger<BackendProcessService> logger, ServiceOptions options)
    {
        _logger = logger;
        _options = options;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        Directory.CreateDirectory(_options.LogDirectory);
        _stdoutLogPath = Path.Combine(_options.LogDirectory, "backend-service.stdout.log");
        _stderrLogPath = Path.Combine(_options.LogDirectory, "backend-service.stderr.log");

        LogStdOut("Service host starting backend process.");
        LogStdOut($"Script: {_options.BackendScriptPath}");
        LogStdOut($"Working directory: {_options.WorkingDirectory}");

        var powershellPath = Path.Combine(
            Environment.GetFolderPath(Environment.SpecialFolder.Windows),
            "System32",
            "WindowsPowerShell",
            "v1.0",
            "powershell.exe");

        var startInfo = new ProcessStartInfo
        {
            FileName = powershellPath,
            Arguments = $"-NoProfile -ExecutionPolicy Bypass -File \"{_options.BackendScriptPath}\"",
            WorkingDirectory = _options.WorkingDirectory,
            RedirectStandardOutput = true,
            RedirectStandardError = true,
            UseShellExecute = false,
            CreateNoWindow = true
        };

        _backendProcess = new Process { StartInfo = startInfo };
        if (!_backendProcess.Start())
        {
            throw new InvalidOperationException("Failed to start backend process.");
        }

        LogStdOut($"Backend process started with PID {_backendProcess.Id}.");
        _logger.LogInformation("Backend process started with PID {Pid}.", _backendProcess.Id);

        var stdoutPump = PumpStreamAsync(_backendProcess.StandardOutput, LogStdOut, stoppingToken);
        var stderrPump = PumpStreamAsync(_backendProcess.StandardError, LogStdErr, stoppingToken);

        try
        {
            await _backendProcess.WaitForExitAsync(stoppingToken);
            LogStdOut($"Backend process exited with code {_backendProcess.ExitCode}.");
            _logger.LogWarning("Backend process exited with code {ExitCode}.", _backendProcess.ExitCode);
        }
        catch (OperationCanceledException)
        {
            LogStdOut("Service stop requested.");
        }
        finally
        {
            await Task.WhenAll(stdoutPump, stderrPump);
        }
    }

    public override async Task StopAsync(CancellationToken cancellationToken)
    {
        if (_backendProcess is { HasExited: false })
        {
            try
            {
                LogStdOut("Stopping backend process.");
                _backendProcess.Kill(entireProcessTree: true);
            }
            catch (Exception ex)
            {
                LogStdErr($"Failed to stop backend process cleanly: {ex.Message}");
                _logger.LogError(ex, "Failed to stop backend process cleanly.");
            }
        }

        await base.StopAsync(cancellationToken);
    }

    private async Task PumpStreamAsync(
        StreamReader reader,
        Action<string> sink,
        CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            var line = await reader.ReadLineAsync();
            if (line is null)
            {
                break;
            }

            sink(line);
        }
    }

    private void LogStdOut(string message)
    {
        WriteLog(_stdoutLogPath, message);
    }

    private void LogStdErr(string message)
    {
        WriteLog(_stderrLogPath, message);
    }

    private void WriteLog(string path, string message)
    {
        var stamp = DateTimeOffset.Now.ToString("yyyy-MM-dd HH:mm:ss");
        var line = $"[{stamp}] {message}{Environment.NewLine}";
        lock (_logLock)
        {
            File.AppendAllText(path, line);
        }
    }
}