Beam/Beam.Stealth/Strategies/WaitingDownloadStrategy.cs

using System.Diagnostics;
using Beam.Abstractions;
using Beam.Models;
using Microsoft.Extensions.Logging;
using File = System.IO.File;

namespace Beam.Stealth.Strategies;

public class WaitingDownloadStrategy : IDownloadStrategy {
    public async Task DownloadToStream(string url, int bufferSize, Stream destinationStream, IProgress<IDownloadReport> progress, StealthConfig config,
        ILogger? logger, CancellationToken ct) {
        await using var stream = await WaitForDownloadAsync(url, progress, Stopwatch.StartNew(), config, logger, ct);
        await (stream?.CopyToAsync(destinationStream, ct) ?? Task.CompletedTask);
    }
    
    
        private async Task<Stream?> WaitForDownloadAsync(
            string link, IProgress<IDownloadReport> progress, Stopwatch sw, StealthConfig config, ILogger? logger, CancellationToken ct) {
            const int PollDelayMs = 250;   // how often we look
            const int StableDelayMs = 1000;  // size-unchanged window

            string dir = config.DownloadsDirectory;
            string? finalPath = null;
            long lastSize = -1;
            DateTime lastChange = DateTime.UtcNow;

            bool IsTemp(string p) =>
                p.EndsWith(".crdownload", StringComparison.OrdinalIgnoreCase) ||
                p.EndsWith(".part", StringComparison.OrdinalIgnoreCase);

            logger?.LogDebug("Polling {Dir} for download files", dir);

            while (sw.Elapsed < config.TimeOut && !ct.IsCancellationRequested) {
                // current files in the directory
                var files = Directory.EnumerateFiles(dir, "*", SearchOption.TopDirectoryOnly).ToArray();

                // ignore temp names; pick (or re-pick) the first real candidate
                finalPath ??= files.FirstOrDefault(f => !IsTemp(f));

                // still nothing but temps – keep waiting
                if (finalPath is null) {
                    await Task.Delay(PollDelayMs, ct);
                    continue;
                }

                // track growth
                long size = new FileInfo(finalPath).Length;
                if (size == 0 || size != lastSize) {
                    progress?.Report(new DownloadReport() {
                        BytesDownloaded = size - lastSize,
                    });
                    lastSize = size;
                    lastChange = DateTime.UtcNow;
                    await Task.Delay(PollDelayMs, ct);
                    continue;
                }

                // size stable long enough *and* no temp files left?
                bool tempsRemain = files.Any(IsTemp);
                if ((DateTime.UtcNow - lastChange).TotalMilliseconds < StableDelayMs || tempsRemain) {
                    await Task.Delay(PollDelayMs, ct);
                    continue;
                }

                // wait until writer releases lock
                while (true) {
                    try {
                        using FileStream _ =
                            File.Open(finalPath, FileMode.Open, FileAccess.Read, FileShare.None);
                        break;
                    } catch (IOException) {
                        await Task.Delay(200, ct);
                    }
                }

                return File.OpenRead(finalPath);
            }

            logger?.LogWarning("Download timed out after {Elapsed}", sw.Elapsed);
            return null;
        }

}