Use native query to avoid huge overheads when cleaning up realm files

Author: peppy

osu.Game/Database/RealmFileStore.cs

@@ -2,6 +2,7 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Diagnostics;
 using System.IO;
 using System.Linq;
 using osu.Framework.Extensions;
@@ -98,15 +99,11 @@ public void Cleanup()
             // can potentially be run asynchronously, although we will need to consider operation order for disk deletion vs realm removal.
             realm.Write(r =>
             {
-                // TODO: consider using a realm native query to avoid iterating all files (https://github.com/realm/realm-dotnet/issues/2659#issuecomment-927823707)
-                var files = r.All<RealmFile>().ToList();
-
-                foreach (var file in files)
+                foreach (var file in r.All<RealmFile>().Filter("Usages.@count = 0"))
                 {
                     totalFiles++;
 
-                    if (file.BacklinksCount > 0)
-                        continue;
+                    Debug.Assert(file.BacklinksCount == 0);
 
                     try
                     {

osu.Game/Models/RealmFile.cs

@@ -1,6 +1,7 @@
 // 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.Linq;
 using osu.Game.IO;
 using Realms;
 
@@ -11,5 +12,8 @@ public class RealmFile : RealmObject, IFileInfo
     {
         [PrimaryKey]
         public string Hash { get; set; } = string.Empty;
+
+        [Backlink(nameof(RealmNamedFileUsage.File))]
+        public IQueryable<RealmNamedFileUsage> Usages { get; } = null!;
     }
 }