From 567253d147132d94b7c1a9cf22bbc6fc970b1f9e Mon Sep 17 00:00:00 2001
From: chylex <contact@chylex.com>
Date: Sun, 24 Dec 2023 13:26:11 +0100
Subject: [PATCH] Use multiple threads to download attachments

---
 app/Desktop/Main/MainWindow.axaml.cs          |   6 +-
 app/Desktop/Main/MainWindowModel.cs           |  12 +-
 app/Desktop/Main/Pages/AttachmentsPage.axaml  |   2 +-
 .../Main/Pages/AttachmentsPageModel.cs        |  62 +++++----
 .../Main/Screens/MainContentScreenModel.cs    |   8 +-
 app/Server/Data/DownloadStatus.cs             |   1 +
 app/Server/Database/DummyDatabaseFile.cs      |   2 +-
 app/Server/Database/IDatabaseFile.cs          |   2 +-
 app/Server/Database/Sqlite/Schema.cs          |   8 +-
 .../Database/Sqlite/SqliteDatabaseFile.cs     |  52 ++++---
 .../Database/Sqlite/Utils/SqliteExtensions.cs |   4 +
 .../Download/BackgroundDownloadThread.cs      | 130 ------------------
 app/Server/Download/BackgroundDownloader.cs   | 107 ++++++++++++++
 app/Utils/Tasks/TaskExtensions.cs             |  12 ++
 14 files changed, 213 insertions(+), 195 deletions(-)
 delete mode 100644 app/Server/Download/BackgroundDownloadThread.cs
 create mode 100644 app/Server/Download/BackgroundDownloader.cs
 create mode 100644 app/Utils/Tasks/TaskExtensions.cs

diff --git a/app/Desktop/Main/MainWindow.axaml.cs b/app/Desktop/Main/MainWindow.axaml.cs
index 742c2d4..5f417e1 100644
--- a/app/Desktop/Main/MainWindow.axaml.cs
+++ b/app/Desktop/Main/MainWindow.axaml.cs
@@ -20,9 +20,9 @@ public sealed partial class MainWindow : Window {
 		DataContext = new MainWindowModel(this, args);
 	}
 
-	public void OnClosed(object? sender, EventArgs e) {
-		if (DataContext is IDisposable disposable) {
-			disposable.Dispose();
+	public async void OnClosed(object? sender, EventArgs e) {
+		if (DataContext is MainWindowModel model) {
+			await model.DisposeAsync();
 		}
 
 		foreach (var temporaryFile in ViewerPageModel.TemporaryFiles) {
diff --git a/app/Desktop/Main/MainWindowModel.cs b/app/Desktop/Main/MainWindowModel.cs
index 80783a7..d555c9a 100644
--- a/app/Desktop/Main/MainWindowModel.cs
+++ b/app/Desktop/Main/MainWindowModel.cs
@@ -12,7 +12,7 @@ using DHT.Utils.Models;
 
 namespace DHT.Desktop.Main;
 
-sealed class MainWindowModel : BaseModel, IDisposable {
+sealed class MainWindowModel : BaseModel, IAsyncDisposable {
 	private const string DefaultTitle = "Discord History Tracker";
 
 	public string Title { get; private set; } = DefaultTitle;
@@ -75,7 +75,7 @@ sealed class MainWindowModel : BaseModel, IDisposable {
 		if (e.PropertyName == nameof(welcomeScreenModel.Db)) {
 			if (mainContentScreenModel != null) {
 				mainContentScreenModel.DatabaseClosed -= MainContentScreenModelOnDatabaseClosed;
-				mainContentScreenModel.Dispose();
+				await mainContentScreenModel.DisposeAsync();
 			}
 
 			db?.Dispose();
@@ -107,9 +107,13 @@ sealed class MainWindowModel : BaseModel, IDisposable {
 		welcomeScreenModel.CloseDatabase();
 	}
 
-	public void Dispose() {
+	public async ValueTask DisposeAsync() {
 		welcomeScreenModel.Dispose();
-		mainContentScreenModel?.Dispose();
+		
+		if (mainContentScreenModel != null) {
+			await mainContentScreenModel.DisposeAsync();
+		}
+		
 		db?.Dispose();
 		db = null;
 	}
diff --git a/app/Desktop/Main/Pages/AttachmentsPage.axaml b/app/Desktop/Main/Pages/AttachmentsPage.axaml
index 6d82546..b4aaf07 100644
--- a/app/Desktop/Main/Pages/AttachmentsPage.axaml
+++ b/app/Desktop/Main/Pages/AttachmentsPage.axaml
@@ -33,7 +33,7 @@
     <StackPanel Orientation="Vertical" Spacing="20">
         <DockPanel>
             <Button Command="{Binding OnClickToggleDownload}" Content="{Binding ToggleDownloadButtonText}" IsEnabled="{Binding IsToggleDownloadButtonEnabled}" DockPanel.Dock="Left" />
-            <TextBlock Text="{Binding DownloadMessage}" Margin="10 0 0 0" VerticalAlignment="Center" DockPanel.Dock="Left" />
+            <TextBlock Text="{Binding DownloadMessage}" MinWidth="100" Margin="10 0 0 0" VerticalAlignment="Center" TextAlignment="Right" DockPanel.Dock="Left" />
             <ProgressBar Value="{Binding DownloadProgress}" IsVisible="{Binding IsDownloading}" Margin="15 0" VerticalAlignment="Center" DockPanel.Dock="Right" />
         </DockPanel>
         <controls:AttachmentFilterPanel DataContext="{Binding FilterModel}" IsEnabled="{Binding !IsDownloading, RelativeSource={RelativeSource AncestorType=pages:AttachmentsPageModel}}" />
diff --git a/app/Desktop/Main/Pages/AttachmentsPageModel.cs b/app/Desktop/Main/Pages/AttachmentsPageModel.cs
index f2b7911..3612bf9 100644
--- a/app/Desktop/Main/Pages/AttachmentsPageModel.cs
+++ b/app/Desktop/Main/Pages/AttachmentsPageModel.cs
@@ -2,6 +2,7 @@ using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Threading;
+using System.Threading.Tasks;
 using Avalonia.Threading;
 using DHT.Desktop.Common;
 using DHT.Desktop.Main.Controls;
@@ -15,16 +16,17 @@ using DHT.Utils.Tasks;
 
 namespace DHT.Desktop.Main.Pages;
 
-sealed class AttachmentsPageModel : BaseModel, IDisposable {
+sealed class AttachmentsPageModel : BaseModel, IAsyncDisposable {
 	private static readonly DownloadItemFilter EnqueuedItemFilter = new() {
 		IncludeStatuses = new HashSet<DownloadStatus> {
-			DownloadStatus.Enqueued
+			DownloadStatus.Enqueued,
+			DownloadStatus.Downloading
 		}
 	};
 
 	private bool isThreadDownloadButtonEnabled = true;
 
-	public string ToggleDownloadButtonText => downloadThread == null ? "Start Downloading" : "Stop Downloading";
+	public string ToggleDownloadButtonText => downloader == null ? "Start Downloading" : "Stop Downloading";
 
 	public bool IsToggleDownloadButtonEnabled {
 		get => isThreadDownloadButtonEnabled;
@@ -32,7 +34,7 @@ sealed class AttachmentsPageModel : BaseModel, IDisposable {
 	}
 
 	public string DownloadMessage { get; set; } = "";
-	public double DownloadProgress => allItemsCount is null or 0 ? 0.0 : 100.0 * doneItemsCount / allItemsCount.Value;
+	public double DownloadProgress => totalItemsToDownloadCount is null or 0 ? 0.0 : 100.0 * doneItemsCount / totalItemsToDownloadCount.Value;
 
 	public AttachmentFilterPanelModel FilterModel { get; }
 
@@ -52,15 +54,16 @@ sealed class AttachmentsPageModel : BaseModel, IDisposable {
 		}
 	}
 
-	public bool IsDownloading => downloadThread != null;
+	public bool IsDownloading => downloader != null;
 	public bool HasFailedDownloads => statisticsFailed.Items > 0;
 
 	private readonly IDatabaseFile db;
 	private readonly AsyncValueComputer<DownloadStatusStatistics>.Single downloadStatisticsComputer;
-	private BackgroundDownloadThread? downloadThread;
+	private BackgroundDownloader? downloader;
 
 	private int doneItemsCount;
-	private int? allItemsCount;
+	private int initialFinishedCount;
+	private int? totalItemsToDownloadCount;
 
 	public AttachmentsPageModel() : this(DummyDatabaseFile.Instance) {}
 
@@ -74,11 +77,11 @@ sealed class AttachmentsPageModel : BaseModel, IDisposable {
 		db.Statistics.PropertyChanged += OnDbStatisticsChanged;
 	}
 
-	public void Dispose() {
+	public async ValueTask DisposeAsync() {
 		db.Statistics.PropertyChanged -= OnDbStatisticsChanged;
 
 		FilterModel.Dispose();
-		DisposeDownloadThread();
+		await DisposeDownloader();
 	}
 
 	private void OnDbStatisticsChanged(object? sender, PropertyChangedEventArgs e) {
@@ -124,44 +127,42 @@ sealed class AttachmentsPageModel : BaseModel, IDisposable {
 			OnPropertyChanged(nameof(HasFailedDownloads));
 		}
 
-		allItemsCount = doneItemsCount + statisticsEnqueued.Items;
+		totalItemsToDownloadCount = statisticsEnqueued.Items + statisticsDownloaded.Items + statisticsFailed.Items - initialFinishedCount;
 		UpdateDownloadMessage();
 	}
 
 	private void UpdateDownloadMessage() {
-		DownloadMessage = IsDownloading ? doneItemsCount.Format() + " / " + (allItemsCount?.Format() ?? "?") : "";
+		DownloadMessage = IsDownloading ? doneItemsCount.Format() + " / " + (totalItemsToDownloadCount?.Format() ?? "?") : "";
 
 		OnPropertyChanged(nameof(DownloadMessage));
 		OnPropertyChanged(nameof(DownloadProgress));
 	}
 
-	private void DownloadThreadOnOnItemFinished(object? sender, DownloadItem e) {
+	private void DownloaderOnOnItemFinished(object? sender, DownloadItem e) {
 		Interlocked.Increment(ref doneItemsCount);
-
+		
 		Dispatcher.UIThread.Invoke(UpdateDownloadMessage);
 		downloadStatisticsComputer.Recompute();
 	}
 
-	private void DownloadThreadOnOnServerStopped(object? sender, EventArgs e) {
-		downloadStatisticsComputer.Recompute();
-		IsToggleDownloadButtonEnabled = true;
-	}
-
-	public void OnClickToggleDownload() {
-		if (downloadThread == null) {
+	public async Task OnClickToggleDownload() {
+		if (downloader == null) {
+			initialFinishedCount = statisticsDownloaded.Items + statisticsFailed.Items;
 			EnqueueDownloadItems();
-			downloadThread = new BackgroundDownloadThread(db);
-			downloadThread.OnItemFinished += DownloadThreadOnOnItemFinished;
-			downloadThread.OnServerStopped += DownloadThreadOnOnServerStopped;
+			downloader = new BackgroundDownloader(db);
+			downloader.OnItemFinished += DownloaderOnOnItemFinished;
 		}
 		else {
 			IsToggleDownloadButtonEnabled = false;
-			DisposeDownloadThread();
+			await DisposeDownloader();
+			downloadStatisticsComputer.Recompute();
+			IsToggleDownloadButtonEnabled = true;
 
 			db.RemoveDownloadItems(EnqueuedItemFilter, FilterRemovalMode.RemoveMatching);
 
 			doneItemsCount = 0;
-			allItemsCount = null;
+			initialFinishedCount = 0;
+			totalItemsToDownloadCount = null;
 			UpdateDownloadMessage();
 		}
 
@@ -173,6 +174,7 @@ sealed class AttachmentsPageModel : BaseModel, IDisposable {
 		var allExceptFailedFilter = new DownloadItemFilter {
 			IncludeStatuses = new HashSet<DownloadStatus> {
 				DownloadStatus.Enqueued,
+				DownloadStatus.Downloading,
 				DownloadStatus.Success
 			}
 		};
@@ -184,13 +186,13 @@ sealed class AttachmentsPageModel : BaseModel, IDisposable {
 		}
 	}
 
-	private void DisposeDownloadThread() {
-		if (downloadThread != null) {
-			downloadThread.OnItemFinished -= DownloadThreadOnOnItemFinished;
-			downloadThread.StopThread();
+	private async Task DisposeDownloader() {
+		if (downloader != null) {
+			downloader.OnItemFinished -= DownloaderOnOnItemFinished;
+			await downloader.Stop();
 		}
 
-		downloadThread = null;
+		downloader = null;
 	}
 
 	public sealed class StatisticsRow {
diff --git a/app/Desktop/Main/Screens/MainContentScreenModel.cs b/app/Desktop/Main/Screens/MainContentScreenModel.cs
index 69ce26d..20e46a4 100644
--- a/app/Desktop/Main/Screens/MainContentScreenModel.cs
+++ b/app/Desktop/Main/Screens/MainContentScreenModel.cs
@@ -11,7 +11,7 @@ using DHT.Utils.Logging;
 
 namespace DHT.Desktop.Main.Screens;
 
-sealed class MainContentScreenModel : IDisposable {
+sealed class MainContentScreenModel : IAsyncDisposable {
 	private static readonly Log Log = Log.ForType<MainContentScreenModel>();
 
 	public DatabasePage DatabasePage { get; }
@@ -35,7 +35,7 @@ sealed class MainContentScreenModel : IDisposable {
 	public bool HasDebugPage => true;
 	private DebugPageModel DebugPageModel { get; }
 	#else
-		public bool HasDebugPage => false;
+	public bool HasDebugPage => false;
 	#endif
 
 	public StatusBarModel StatusBarModel { get; }
@@ -97,9 +97,9 @@ sealed class MainContentScreenModel : IDisposable {
 		serverManager.Launch();
 	}
 
-	public void Dispose() {
+	public async ValueTask DisposeAsync() {
 		ServerLauncher.ServerManagementExceptionCaught -= ServerLauncherOnServerManagementExceptionCaught;
-		AttachmentsPageModel.Dispose();
+		await AttachmentsPageModel.DisposeAsync();
 		ViewerPageModel.Dispose();
 		serverManager.Dispose();
 	}
diff --git a/app/Server/Data/DownloadStatus.cs b/app/Server/Data/DownloadStatus.cs
index 7c83576..a220808 100644
--- a/app/Server/Data/DownloadStatus.cs
+++ b/app/Server/Data/DownloadStatus.cs
@@ -8,5 +8,6 @@ namespace DHT.Server.Data;
 public enum DownloadStatus {
 	Enqueued = 0,
 	GenericError = 1,
+	Downloading = 2,
 	Success = HttpStatusCode.OK
 }
diff --git a/app/Server/Database/DummyDatabaseFile.cs b/app/Server/Database/DummyDatabaseFile.cs
index 87deaf0..a983cd6 100644
--- a/app/Server/Database/DummyDatabaseFile.cs
+++ b/app/Server/Database/DummyDatabaseFile.cs
@@ -74,7 +74,7 @@ public sealed class DummyDatabaseFile : IDatabaseFile {
 
 	public void EnqueueDownloadItems(AttachmentFilter? filter = null) {}
 
-	public List<DownloadItem> GetEnqueuedDownloadItems(int count) {
+	public List<DownloadItem> PullEnqueuedDownloadItems(int count) {
 		return new();
 	}
 
diff --git a/app/Server/Database/IDatabaseFile.cs b/app/Server/Database/IDatabaseFile.cs
index dac2613..97a752a 100644
--- a/app/Server/Database/IDatabaseFile.cs
+++ b/app/Server/Database/IDatabaseFile.cs
@@ -35,7 +35,7 @@ public interface IDatabaseFile : IDisposable {
 	DownloadedAttachment? GetDownloadedAttachment(string url);
 
 	void EnqueueDownloadItems(AttachmentFilter? filter = null);
-	List<DownloadItem> GetEnqueuedDownloadItems(int count);
+	List<DownloadItem> PullEnqueuedDownloadItems(int count);
 	void RemoveDownloadItems(DownloadItemFilter? filter, FilterRemovalMode mode);
 	DownloadStatusStatistics GetDownloadStatusStatistics();
 
diff --git a/app/Server/Database/Sqlite/Schema.cs b/app/Server/Database/Sqlite/Schema.cs
index b9c1ec5..d5be3fd 100644
--- a/app/Server/Database/Sqlite/Schema.cs
+++ b/app/Server/Database/Sqlite/Schema.cs
@@ -174,8 +174,8 @@ sealed class Schema {
 		int processedUrls = -1;
 
 		await using (var updateCmd = conn.Command("UPDATE attachments SET download_url = url, url = :normalized_url WHERE attachment_id = :attachment_id")) {
-			updateCmd.Parameters.Add(":attachment_id", SqliteType.Integer);
-			updateCmd.Parameters.Add(":normalized_url", SqliteType.Text);
+			updateCmd.Add(":attachment_id", SqliteType.Integer);
+			updateCmd.Add(":normalized_url", SqliteType.Text);
 				
 			foreach (var (attachmentId, normalizedUrl) in normalizedUrls) {
 				if (++processedUrls % 1000 == 0) {
@@ -235,8 +235,8 @@ sealed class Schema {
 		tx = conn.BeginTransaction();
 		
 		await using (var updateCmd = conn.Command("UPDATE downloads SET download_url = :download_url, url = :normalized_url WHERE url = :download_url")) {
-			updateCmd.Parameters.Add(":normalized_url", SqliteType.Text);
-			updateCmd.Parameters.Add(":download_url", SqliteType.Text);
+			updateCmd.Add(":normalized_url", SqliteType.Text);
+			updateCmd.Add(":download_url", SqliteType.Text);
 			
 			foreach (var (normalizedUrl, downloadUrl) in normalizedUrlsToOriginalUrls) {
 				if (++processedUrls % 100 == 0) {
diff --git a/app/Server/Database/Sqlite/SqliteDatabaseFile.cs b/app/Server/Database/Sqlite/SqliteDatabaseFile.cs
index fb21b17..7ff6780 100644
--- a/app/Server/Database/Sqlite/SqliteDatabaseFile.cs
+++ b/app/Server/Database/Sqlite/SqliteDatabaseFile.cs
@@ -522,25 +522,42 @@ public sealed class SqliteDatabaseFile : IDatabaseFile {
 		cmd.ExecuteNonQuery();
 	}
 
-	public List<DownloadItem> GetEnqueuedDownloadItems(int count) {
-		var list = new List<DownloadItem>();
+	public List<DownloadItem> PullEnqueuedDownloadItems(int count) {
+		var found = new List<DownloadItem>();
+		var pulled = new List<DownloadItem>();
 
 		using var conn = pool.Take();
-		using var cmd = conn.Command("SELECT normalized_url, download_url, size FROM downloads WHERE status = :enqueued LIMIT :limit");
-		cmd.AddAndSet(":enqueued", SqliteType.Integer, (int) DownloadStatus.Enqueued);
-		cmd.AddAndSet(":limit", SqliteType.Integer, Math.Max(0, count));
+		using (var cmd = conn.Command("SELECT normalized_url, download_url, size FROM downloads WHERE status = :enqueued LIMIT :limit")) {
+			cmd.AddAndSet(":enqueued", SqliteType.Integer, (int) DownloadStatus.Enqueued);
+			cmd.AddAndSet(":limit", SqliteType.Integer, Math.Max(0, count));
 
-		using var reader = cmd.ExecuteReader();
+			using var reader = cmd.ExecuteReader();
 
-		while (reader.Read()) {
-			list.Add(new DownloadItem {
-				NormalizedUrl = reader.GetString(0),
-				DownloadUrl = reader.GetString(1),
-				Size = reader.GetUint64(2),
-			});
+			while (reader.Read()) {
+				found.Add(new DownloadItem {
+					NormalizedUrl = reader.GetString(0),
+					DownloadUrl = reader.GetString(1),
+					Size = reader.GetUint64(2),
+				});
+			}
 		}
 
-		return list;
+		if (found.Count != 0) {
+			using var cmd = conn.Command("UPDATE downloads SET status = :downloading WHERE normalized_url = :normalized_url AND status = :enqueued");
+			cmd.AddAndSet(":enqueued", SqliteType.Integer, (int) DownloadStatus.Enqueued);
+			cmd.AddAndSet(":downloading", SqliteType.Integer, (int) DownloadStatus.Downloading);
+			cmd.Add(":normalized_url", SqliteType.Text);
+
+			foreach (var item in found) {
+				cmd.Set(":normalized_url", item.NormalizedUrl);
+
+				if (cmd.ExecuteNonQuery() == 1) {
+					pulled.Add(item);
+				}
+			}
+		}
+
+		return pulled;
 	}
 
 	public void RemoveDownloadItems(DownloadItemFilter? filter, FilterRemovalMode mode) {
@@ -562,15 +579,16 @@ public sealed class SqliteDatabaseFile : IDatabaseFile {
 		static void LoadSuccessStatistics(ISqliteConnection conn, DownloadStatusStatistics result) {
 			using var cmd = conn.Command("""
 			                             SELECT
-			                             IFNULL(SUM(CASE WHEN status = :enqueued THEN 1 ELSE 0 END), 0),
-			                             IFNULL(SUM(CASE WHEN status = :enqueued THEN size ELSE 0 END), 0),
+			                             IFNULL(SUM(CASE WHEN status IN (:enqueued, :downloading) THEN 1 ELSE 0 END), 0),
+			                             IFNULL(SUM(CASE WHEN status IN (:enqueued, :downloading) THEN size ELSE 0 END), 0),
 			                             IFNULL(SUM(CASE WHEN status = :success THEN 1 ELSE 0 END), 0),
 			                             IFNULL(SUM(CASE WHEN status = :success THEN size ELSE 0 END), 0),
-			                             IFNULL(SUM(CASE WHEN status != :enqueued AND status != :success THEN 1 ELSE 0 END), 0),
-			                             IFNULL(SUM(CASE WHEN status != :enqueued AND status != :success THEN size ELSE 0 END), 0)
+			                             IFNULL(SUM(CASE WHEN status NOT IN (:enqueued, :downloading) AND status != :success THEN 1 ELSE 0 END), 0),
+			                             IFNULL(SUM(CASE WHEN status NOT IN (:enqueued, :downloading) AND status != :success THEN size ELSE 0 END), 0)
 			                             FROM downloads
 			                             """);
 			cmd.AddAndSet(":enqueued", SqliteType.Integer, (int) DownloadStatus.Enqueued);
+			cmd.AddAndSet(":downloading", SqliteType.Integer, (int) DownloadStatus.Downloading);
 			cmd.AddAndSet(":success", SqliteType.Integer, (int) DownloadStatus.Success);
 
 			using var reader = cmd.ExecuteReader();
diff --git a/app/Server/Database/Sqlite/Utils/SqliteExtensions.cs b/app/Server/Database/Sqlite/Utils/SqliteExtensions.cs
index bcb5548..2769f22 100644
--- a/app/Server/Database/Sqlite/Utils/SqliteExtensions.cs
+++ b/app/Server/Database/Sqlite/Utils/SqliteExtensions.cs
@@ -62,6 +62,10 @@ static class SqliteExtensions {
 		}
 	}
 
+	public static void Add(this SqliteCommand cmd, string key, SqliteType type) {
+		cmd.Parameters.Add(key, type);
+	}
+
 	public static void AddAndSet(this SqliteCommand cmd, string key, SqliteType type, object? value) {
 		cmd.Parameters.Add(key, type).Value = value ?? DBNull.Value;
 	}
diff --git a/app/Server/Download/BackgroundDownloadThread.cs b/app/Server/Download/BackgroundDownloadThread.cs
deleted file mode 100644
index f75eae3..0000000
--- a/app/Server/Download/BackgroundDownloadThread.cs
+++ /dev/null
@@ -1,130 +0,0 @@
-using System;
-using System.Collections.Concurrent;
-using System.Net.Http;
-using System.Threading;
-using DHT.Server.Database;
-using DHT.Utils.Logging;
-using DHT.Utils.Models;
-
-namespace DHT.Server.Download;
-
-public sealed class BackgroundDownloadThread : BaseModel {
-	private static readonly Log Log = Log.ForType<BackgroundDownloadThread>();
-
-	public event EventHandler<DownloadItem>? OnItemFinished {
-		add => parameters.OnItemFinished += value;
-		remove => parameters.OnItemFinished -= value;
-	}
-
-	public event EventHandler? OnServerStopped {
-		add => parameters.OnServerStopped += value;
-		remove => parameters.OnServerStopped -= value;
-	}
-
-	private readonly CancellationTokenSource cancellationTokenSource;
-	private readonly ThreadInstance.Parameters parameters;
-
-	public BackgroundDownloadThread(IDatabaseFile db) {
-		this.cancellationTokenSource = new CancellationTokenSource();
-		this.parameters = new ThreadInstance.Parameters(db, cancellationTokenSource);
-
-		var thread = new Thread(new ThreadInstance().Work) {
-			Name = "DHT download thread"
-		};
-
-		thread.Start(parameters);
-	}
-
-	public void StopThread() {
-		try {
-			cancellationTokenSource.Cancel();
-		} catch (ObjectDisposedException) {
-			Log.Warn("Attempted to stop background download thread after the cancellation token has been disposed.");
-		}
-	}
-
-	private sealed class ThreadInstance {
-		private const int QueueSize = 32;
-
-		public sealed class Parameters {
-			public event EventHandler<DownloadItem>? OnItemFinished;
-			public event EventHandler? OnServerStopped;
-
-			public IDatabaseFile Db { get; }
-			public CancellationTokenSource CancellationTokenSource { get; }
-
-			public Parameters(IDatabaseFile db, CancellationTokenSource cancellationTokenSource) {
-				Db = db;
-				CancellationTokenSource = cancellationTokenSource;
-			}
-
-			public void FireOnItemFinished(DownloadItem item) {
-				OnItemFinished?.Invoke(null, item);
-			}
-
-			public void FireOnServerStopped() {
-				OnServerStopped?.Invoke(null, EventArgs.Empty);
-			}
-		}
-
-		private readonly HttpClient client = new ();
-
-		public ThreadInstance() {
-			client.DefaultRequestHeaders.UserAgent.ParseAdd("Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/99.0.4844.51 Safari/537.36");
-		}
-
-		public async void Work(object? obj) {
-			var parameters = (Parameters) obj!;
-
-			var cancellationTokenSource = parameters.CancellationTokenSource;
-			var cancellationToken = cancellationTokenSource.Token;
-
-			var db = parameters.Db;
-			var queue = new ConcurrentQueue<DownloadItem>();
-
-			try {
-				while (!cancellationToken.IsCancellationRequested) {
-					FillQueue(db, queue, cancellationToken);
-
-					while (!cancellationToken.IsCancellationRequested && queue.TryDequeue(out var item)) {
-						var downloadUrl = item.DownloadUrl;
-						Log.Debug("Downloading " + downloadUrl + "...");
-
-						try {
-							db.AddDownload(Data.Download.NewSuccess(item, await client.GetByteArrayAsync(downloadUrl, cancellationToken)));
-						} catch (HttpRequestException e) {
-							db.AddDownload(Data.Download.NewFailure(item, e.StatusCode, item.Size));
-							Log.Error(e);
-						} catch (Exception e) {
-							db.AddDownload(Data.Download.NewFailure(item, null, item.Size));
-							Log.Error(e);
-						} finally {
-							parameters.FireOnItemFinished(item);
-						}
-					}
-				}
-			} catch (OperationCanceledException) {
-				//
-			} catch (ObjectDisposedException) {
-				//
-			} finally {
-				cancellationTokenSource.Dispose();
-				parameters.FireOnServerStopped();
-			}
-		}
-
-		private static void FillQueue(IDatabaseFile db, ConcurrentQueue<DownloadItem> queue, CancellationToken cancellationToken) {
-			while (!cancellationToken.IsCancellationRequested && queue.IsEmpty) {
-				var newItems = db.GetEnqueuedDownloadItems(QueueSize);
-				if (newItems.Count == 0) {
-					Thread.Sleep(TimeSpan.FromMilliseconds(50));
-				}
-				else {
-					foreach (var item in newItems) {
-						queue.Enqueue(item);
-					}
-				}
-			}
-		}
-	}
-}
diff --git a/app/Server/Download/BackgroundDownloader.cs b/app/Server/Download/BackgroundDownloader.cs
new file mode 100644
index 0000000..60e97d1
--- /dev/null
+++ b/app/Server/Download/BackgroundDownloader.cs
@@ -0,0 +1,107 @@
+using System;
+using System.Linq;
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Channels;
+using System.Threading.Tasks;
+using DHT.Server.Database;
+using DHT.Utils.Logging;
+using DHT.Utils.Models;
+using DHT.Utils.Tasks;
+
+namespace DHT.Server.Download;
+
+public sealed class BackgroundDownloader : BaseModel {
+	private static readonly Log Log = Log.ForType<BackgroundDownloader>();
+
+	private const int DownloadTasks = 4;
+	private const int QueueSize = 25;
+	private const string UserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/99.0.4844.51 Safari/537.36";
+
+	public event EventHandler<DownloadItem>? OnItemFinished;
+
+	private readonly Channel<DownloadItem> downloadQueue = Channel.CreateBounded<DownloadItem>(new BoundedChannelOptions(QueueSize) {
+		SingleReader = false,
+		SingleWriter = true,
+		AllowSynchronousContinuations = false,
+		FullMode = BoundedChannelFullMode.Wait
+	});
+
+	private readonly CancellationTokenSource cancellationTokenSource = new ();
+	private readonly CancellationToken cancellationToken;
+
+	private readonly IDatabaseFile db;
+	private readonly Task queueWriterTask;
+	private readonly Task[] downloadTasks;
+
+	public BackgroundDownloader(IDatabaseFile db) {
+		this.cancellationToken = cancellationTokenSource.Token;
+		this.db = db;
+		this.queueWriterTask = Task.Run(RunQueueWriterTask);
+		this.downloadTasks = Enumerable.Range(1, DownloadTasks).Select(taskIndex => Task.Run(() => RunDownloadTask(taskIndex))).ToArray();
+	}
+
+	private async Task RunQueueWriterTask() {
+		while (await downloadQueue.Writer.WaitToWriteAsync(cancellationToken)) {
+			var newItems = db.PullEnqueuedDownloadItems(QueueSize);
+			if (newItems.Count == 0) {
+				await Task.Delay(TimeSpan.FromMilliseconds(50), cancellationToken);
+				continue;
+			}
+
+			foreach (var newItem in newItems) {
+				await downloadQueue.Writer.WriteAsync(newItem, cancellationToken);
+			}
+		}
+	}
+
+	private async Task RunDownloadTask(int taskIndex) {
+		var log = Log.ForType<BackgroundDownloader>("Task " + taskIndex);
+
+		var client = new HttpClient();
+		client.DefaultRequestHeaders.UserAgent.ParseAdd(UserAgent);
+		client.Timeout = TimeSpan.FromSeconds(30);
+
+		while (!cancellationToken.IsCancellationRequested) {
+			var item = await downloadQueue.Reader.ReadAsync(cancellationToken);
+			log.Debug("Downloading " + item.DownloadUrl + "...");
+
+			try {
+				var downloadedBytes = await client.GetByteArrayAsync(item.DownloadUrl, cancellationToken);
+				db.AddDownload(Data.Download.NewSuccess(item, downloadedBytes));
+			} catch (OperationCanceledException) {
+				// Ignore.
+			} catch (HttpRequestException e) {
+				db.AddDownload(Data.Download.NewFailure(item, e.StatusCode, item.Size));
+				log.Error(e);
+			} catch (Exception e) {
+				db.AddDownload(Data.Download.NewFailure(item, null, item.Size));
+				log.Error(e);
+			} finally {
+				try {
+					OnItemFinished?.Invoke(this, item);
+				} catch (Exception e) {
+					log.Error("Caught exception in event handler: " + e);
+				}
+			}
+		}
+	}
+
+	public async Task Stop() {
+		try {
+			await cancellationTokenSource.CancelAsync();
+		} catch (Exception) {
+			Log.Warn("Attempted to stop background download twice.");
+			return;
+		}
+
+		downloadQueue.Writer.Complete();
+
+		try {
+			await queueWriterTask.WaitIgnoringCancellation();
+			await Task.WhenAll(downloadTasks).WaitIgnoringCancellation();
+		} finally {
+			cancellationTokenSource.Dispose();
+		}
+	}
+}
diff --git a/app/Utils/Tasks/TaskExtensions.cs b/app/Utils/Tasks/TaskExtensions.cs
new file mode 100644
index 0000000..b3a9b4b
--- /dev/null
+++ b/app/Utils/Tasks/TaskExtensions.cs
@@ -0,0 +1,12 @@
+using System;
+using System.Threading.Tasks;
+
+namespace DHT.Utils.Tasks; 
+
+public static class TaskExtensions {
+	public static async Task WaitIgnoringCancellation(this Task task) {
+		try {
+			await task;
+		} catch (OperationCanceledException) {}
+	}
+}