Fix dodgy threading

The fact that the stuff "just worked" previously due to one load-bearing
detach in a random location is really scary because a lot of this was
just not written the way it is supposed to be.

osu.Game/Database/RealmAccess.cs

@@ -543,6 +543,44 @@ namespace osu.Game.Database
             return writeTask;
         }
 
+        /// <summary>
+        /// Write changes to realm asynchronously, guaranteeing order of execution.
+        /// </summary>
+        /// <param name="action">The work to run.</param>
+        public Task<T> WriteAsync<T>(Func<Realm, T> action)
+        {
+            ObjectDisposedException.ThrowIf(isDisposed, this);
+
+            // Required to ensure the write is tracked and accounted for before disposal.
+            // Can potentially be avoided if we have a need to do so in the future.
+            if (!ThreadSafety.IsUpdateThread)
+                throw new InvalidOperationException(@$"{nameof(WriteAsync)} must be called from the update thread.");
+
+            // CountdownEvent will fail if already at zero.
+            if (!pendingAsyncWrites.TryAddCount())
+                pendingAsyncWrites.Reset(1);
+
+            // Regardless of calling Realm.GetInstance or Realm.GetInstanceAsync, there is a blocking overhead on retrieval.
+            // Adding a forced Task.Run resolves this.
+            var writeTask = Task.Run(async () =>
+            {
+                T result;
+                total_writes_async.Value++;
+
+                // Not attempting to use Realm.GetInstanceAsync as there's seemingly no benefit to us (for now) and it adds complexity due to locking
+                // concerns in getRealmInstance(). On a quick check, it looks to be more suited to cases where realm is connecting to an online sync
+                // server, which we don't use. May want to report upstream or revisit in the future.
+                using (var realm = getRealmInstance())
+                    // ReSharper disable once AccessToDisposedClosure (WriteAsync should be marked as [InstantHandle]).
+                    result = await realm.WriteAsync(() => action(realm)).ConfigureAwait(false);
+
+                pendingAsyncWrites.Signal();
+                return result;
+            });
+
+            return writeTask;
+        }
+
         /// <summary>
         /// Subscribe to a realm collection and begin watching for asynchronous changes.
         /// </summary>

osu.Game/Database/RealmArchiveModelImporter.cs

@@ -205,9 +205,7 @@ namespace osu.Game.Database
 
             Directory.CreateDirectory(mountedPath);
 
-            // Detach files from the model to avoid realm contention when copying to the external location.
-            // This is safe as we are not modifying the model in any way.
-            foreach (var realmFile in model.Files.Detach())
+            foreach (var realmFile in model.Files)
             {
                 string sourcePath = Files.Storage.GetFullPath(realmFile.File.GetStoragePath());
                 string destinationPath = Path.Join(mountedPath, realmFile.Filename);

osu.Game/Database/RealmObjectExtensions.cs

@@ -14,6 +14,7 @@ using osu.Game.Input.Bindings;
 using osu.Game.Models;
 using osu.Game.Rulesets;
 using osu.Game.Scoring;
+using osu.Game.Skinning;
 using Realms;
 
 namespace osu.Game.Database
@@ -177,6 +178,7 @@ namespace osu.Game.Database
             c.CreateMap<RealmUser, RealmUser>();
             c.CreateMap<RealmFile, RealmFile>();
             c.CreateMap<RealmNamedFileUsage, RealmNamedFileUsage>();
+            c.CreateMap<SkinInfo, SkinInfo>();
         }
 
         /// <summary>

osu.Game/Skinning/SkinImporter.cs

@@ -53,12 +53,11 @@ namespace osu.Game.Skinning
         /// <param name="task">The <see cref="ImportTask"/> to update the <paramref name="original"/> with</param>
         /// <param name="original">The <see cref="SkinInfo"/> to update</param>
         /// <returns></returns>
-        public override Task<Live<SkinInfo>?> ImportAsUpdate(ProgressNotification notification, ImportTask task, SkinInfo original)
+        public override async Task<Live<SkinInfo>?> ImportAsUpdate(ProgressNotification notification, ImportTask task, SkinInfo original)
         {
-            var skinInfoLive = original.ToLive(Realm);
-
-            skinInfoLive.PerformWrite(skinInfo =>
+            return await Realm.WriteAsync<Live<SkinInfo>?>(r =>
             {
+                var skinInfo = r.Find<SkinInfo>(original.ID)!;
                 skinInfo.Files.Clear();
 
                 string[] filesInMountedDirectory = Directory.EnumerateFiles(task.Path, "*.*", SearchOption.AllDirectories).Select(f => Path.GetRelativePath(task.Path, f)).ToArray();
@@ -67,28 +66,28 @@ namespace osu.Game.Skinning
                 {
                     using var stream = File.OpenRead(Path.Combine(task.Path, file));
 
-                    modelManager.AddFile(original, stream, file);
+                    modelManager.AddFile(skinInfo, stream, file, r);
                 }
 
                 string skinIniPath = Path.Combine(task.Path, "skin.ini");
 
-                if (!File.Exists(skinIniPath))
-                    return;
-
-                using (var stream = File.OpenRead(skinIniPath))
-                using (var lineReader = new LineBufferedReader(stream))
+                if (File.Exists(skinIniPath))
                 {
-                    var decodedSkinIni = new LegacySkinDecoder().Decode(lineReader);
+                    using (var stream = File.OpenRead(skinIniPath))
+                    using (var lineReader = new LineBufferedReader(stream))
+                    {
+                        var decodedSkinIni = new LegacySkinDecoder().Decode(lineReader);
 
-                    if (!string.IsNullOrEmpty(decodedSkinIni.SkinInfo.Name))
-                        skinInfo.Name = decodedSkinIni.SkinInfo.Name;
+                        if (!string.IsNullOrEmpty(decodedSkinIni.SkinInfo.Name))
+                            skinInfo.Name = decodedSkinIni.SkinInfo.Name;
 
-                    if (!string.IsNullOrEmpty(decodedSkinIni.SkinInfo.Creator))
-                        skinInfo.Creator = decodedSkinIni.SkinInfo.Creator;
+                        if (!string.IsNullOrEmpty(decodedSkinIni.SkinInfo.Creator))
+                            skinInfo.Creator = decodedSkinIni.SkinInfo.Creator;
+                    }
                 }
-            });
 
-            return Task.FromResult(skinInfoLive)!;
+                return skinInfo.ToLive(Realm);
+            }).ConfigureAwait(false);
         }
 
         protected override void Populate(SkinInfo model, ArchiveReader? archive, Realm realm, CancellationToken cancellationToken = default)