Implement StableStorage class.

osu.Desktop/OsuGameDesktop.cs

@@ -18,6 +18,7 @@ using osu.Framework.Screens;
 using osu.Game.Screens.Menu;
 using osu.Game.Updater;
 using osu.Desktop.Windows;
+using osu.Game.IO;
 
 namespace osu.Desktop
 {
@@ -40,7 +41,7 @@ namespace osu.Desktop
                 {
                     string stablePath = getStableInstallPath();
                     if (!string.IsNullOrEmpty(stablePath))
-                        return new DesktopStorage(stablePath, desktopHost);
+                        return new StableStorage(stablePath, desktopHost);
                 }
             }
             catch (Exception)

osu.Game/IO/StableStorage.cs

@@ -0,0 +1,64 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.IO;
+using System.Linq;
+using osu.Framework.Platform;
+
+namespace osu.Game.IO
+{
+    /// <summary>
+    /// A storage pointing to an osu-stable installation.
+    /// Provides methods for handling installations with a custom Song folder location.
+    /// </summary>
+    public class StableStorage : DesktopStorage
+    {
+        private const string stable_songs_path = "Songs";
+
+        private readonly DesktopGameHost host;
+        private string songs_path;
+
+        public StableStorage(string path, DesktopGameHost host)
+            : base(path, host)
+        {
+            this.host = host;
+            songs_path = locateSongsDirectory();
+        }
+
+        /// <summary>
+        /// Returns a <see cref="Storage"/> pointing to the osu-stable Songs directory.
+        /// </summary>
+        public Storage GetSongStorage()
+        {
+            if (songs_path.Equals(stable_songs_path, StringComparison.OrdinalIgnoreCase))
+                return GetStorageForDirectory(stable_songs_path);
+            else
+                return new DesktopStorage(songs_path, host);
+        }
+
+        private string locateSongsDirectory()
+        {
+            var configFile = GetStream(GetFiles(".", "osu!.*.cfg").First());
+            var textReader = new StreamReader(configFile);
+
+            var songs_directory_path = stable_songs_path;
+
+            while (!textReader.EndOfStream)
+            {
+                var line = textReader.ReadLine();
+
+                if (line?.StartsWith("BeatmapDirectory", StringComparison.OrdinalIgnoreCase) == true)
+                {
+                    var directory = line.Split('=')[1].TrimStart();
+                    if (Path.IsPathFullyQualified(directory) && !directory.Equals(stable_songs_path, StringComparison.OrdinalIgnoreCase))
+                        songs_directory_path = directory;
+
+                    break;
+                }
+            }
+
+            return songs_directory_path;
+        }
+    }
+}