Implement /savelog chat command (#37093)

Addresses https://github.com/ppy/osu/discussions/37090.

Filename and format almost match stable with the exception of:

- adding `chat-` prefix to export filename to distinguish from
everything else in the `exports` folder
- adding date to message timestamps as stable writes time only

osu.Game/Online/Chat/ChannelManager.cs

@@ -6,7 +6,9 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Specialized;
+using System.IO;
 using System.Linq;
+using System.Threading.Tasks;
 using JetBrains.Annotations;
 using Microsoft.AspNetCore.SignalR;
 using osu.Framework.Allocation;
@@ -15,13 +17,19 @@ using osu.Framework.Extensions;
 using osu.Framework.Extensions.ObjectExtensions;
 using osu.Framework.Graphics.Containers;
 using osu.Framework.Logging;
+using osu.Framework.Platform;
 using osu.Framework.Threading;
 using osu.Game.Database;
+using osu.Game.Extensions;
+using osu.Game.IO;
+using osu.Game.Localisation;
 using osu.Game.Online.API;
 using osu.Game.Online.API.Requests;
 using osu.Game.Online.API.Requests.Responses;
 using osu.Game.Online.Multiplayer;
+using osu.Game.Overlays;
 using osu.Game.Overlays.Chat.Listing;
+using osu.Game.Overlays.Notifications;
 
 namespace osu.Game.Online.Chat
 {
@@ -77,6 +85,14 @@ namespace osu.Game.Online.Chat
         [CanBeNull]
         private MultiplayerClient multiplayerClient { get; set; }
 
+        [Resolved(CanBeNull = true)]
+        [CanBeNull]
+        private Storage storage { get; set; }
+
+        [Resolved(CanBeNull = true)]
+        [CanBeNull]
+        private INotificationOverlay notifications { get; set; }
+
         private readonly IBindable<APIUser> localUser = new Bindable<APIUser>();
         private readonly IBindable<APIState> apiState = new Bindable<APIState>();
         private readonly IBindableList<APIRelation> localUserBlocks = new BindableList<APIRelation>();
@@ -358,8 +374,44 @@ namespace osu.Game.Online.Chat
                     });
                     break;
 
+                case @"savelog":
+                    ProgressNotification notification = new ProgressNotification
+                    {
+                        State = ProgressNotificationState.Active,
+                        Text = NotificationsStrings.LogsExportOngoing,
+                    };
+                    notifications?.Post(notification);
+
+                    exportChannelLog(target).ContinueWith(t =>
+                    {
+                        if (t.Exception != null)
+                        {
+                            Logger.Log($@"Failed to export channel log: {t.Exception}");
+                            notification.State = ProgressNotificationState.Cancelled;
+                            return;
+                        }
+
+                        string result = t.GetResultSafely();
+
+                        if (result == null)
+                        {
+                            Logger.Log("Failed to export channel log due to missing storage.");
+                            notification.State = ProgressNotificationState.Cancelled;
+                            return;
+                        }
+
+                        notification.CompletionText = NotificationsStrings.FileExportFinished(result);
+                        notification.CompletionClickAction = () =>
+                        {
+                            (storage as OsuStorage)?.GetExportStorage().PresentFileExternally(result);
+                            return true;
+                        };
+                        notification.State = ProgressNotificationState.Completed;
+                    });
+                    break;
+
                 case @"help":
-                    target.AddNewMessages(new InfoMessage("Supported commands: /help, /me [action], /join [channel], /chat [user], /np, /roll [2-100] (multiplayer only)"));
+                    target.AddNewMessages(new InfoMessage("Supported commands: /help, /me [action], /join [channel], /chat [user], /np, /savelog, /roll [2-100] (multiplayer only)"));
                     break;
 
                 default:
@@ -714,6 +766,26 @@ namespace osu.Game.Online.Chat
             }
         }
 
+        [ItemCanBeNull]
+        private async Task<string> exportChannelLog(Channel channel)
+        {
+            if (storage is not OsuStorage osuStorage)
+                return null;
+
+            string filename = string.Format($@"chat-{channel.Name}-{DateTimeOffset.Now:yyyyMMdd-hhmmss}.txt").GetValidFilename();
+            var exportStorage = osuStorage.GetExportStorage();
+
+            using (var file = exportStorage.CreateFileSafely(filename))
+            {
+                using var textWriter = new StreamWriter(file);
+
+                foreach (var message in channel.Messages)
+                    await textWriter.WriteLineAsync($@"{message.Timestamp:yyyy-MM-dd HH:mm} {message.Sender.Username}: {message.Content}").ConfigureAwait(false);
+            }
+
+            return filename;
+        }
+
         protected override void Dispose(bool isDisposing)
         {
             base.Dispose(isDisposing);