Avoid MemoryStream.ToArray overhead in LegacyByteArrayReader

osu.Game/Database/ImportTask.cs

@@ -54,7 +54,7 @@ namespace osu.Game.Database
                 if (ZipUtils.IsZipArchive(memoryStream))
                     return new ZipArchiveReader(memoryStream, Path);
 
-                return new LegacyByteArrayReader(memoryStream.ToArray(), Path);
+                return new MemoryStreamArchiveReader(memoryStream, Path);
             }
 
             // This isn't used in any current path. May need to reconsider for performance reasons (ie. if we don't expect the incoming stream to be copied out).

osu.Game/IO/Archives/MemoryStreamArchiveReader.cs

@@ -0,0 +1,30 @@
+// 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.Collections.Generic;
+using System.IO;
+
+namespace osu.Game.IO.Archives
+{
+    /// <summary>
+    /// Allows reading a single file from the provided memory stream.
+    /// </summary>
+    public class MemoryStreamArchiveReader : ArchiveReader
+    {
+        private readonly MemoryStream stream;
+
+        public MemoryStreamArchiveReader(MemoryStream stream, string filename)
+            : base(filename)
+        {
+            this.stream = stream;
+        }
+
+        public override Stream GetStream(string name) => new MemoryStream(stream.GetBuffer(), 0, (int)stream.Length);
+
+        public override void Dispose()
+        {
+        }
+
+        public override IEnumerable<string> Filenames => new[] { Name };
+    }
+}