Fix a strange race condition with interop delegates

src/NexusMods.MnemonicDB.Storage/DatomStore.cs

@@ -136,16 +136,7 @@ public async Task<StoreResult> Transact(IndexSegment datoms, HashSet<ITxFunction
     /// <inheritdoc />
     public async Task<StoreResult> Sync()
     {
-        var pending = new PendingTransaction
-        {
-            Data = new IndexSegment(),
-            TxFunctions = null,
-            DatabaseFactory = null
-        };
-        if (!_txChannel.Writer.TryWrite(pending))
-            throw new InvalidOperationException("Failed to write to the transaction channel");
-
-        return await pending.CompletionSource.Task;
+        return await Transact(new IndexSegment());
     }
 
     public IObservable<(TxId TxId, ISnapshot Snapshot)> TxLog => _updatesSubject;

src/NexusMods.MnemonicDB.Storage/RocksDbBackend/IndexStore.cs

@@ -1,5 +1,7 @@
 using System;
+using System.Collections.Generic;
 using System.Runtime.InteropServices;
+using DynamicData;
 using NexusMods.MnemonicDB.Abstractions;
 using NexusMods.MnemonicDB.Abstractions.DatomComparators;
 using NexusMods.MnemonicDB.Abstractions.DatomIterators;
@@ -21,6 +23,15 @@ public class IndexStore<TComparator> : IRocksDBIndexStore
     private IntPtr _namePtr;
     private ColumnFamilyOptions _options = null!;
 
+    /// <summary>
+    /// This is a bit of a hack, but we throw all our interop delegates in here, and then they
+    /// live for the entire life of the application. It seems that RocksDB will occasionally call
+    /// delegates after we think we've disposed of the handles. It really doesn't matter as these
+    /// things will never amount to more than a few dozen objects.
+    /// </summary>
+    /// <returns></returns>
+    private static List<object> _roots = new ();
+
     public IndexStore(string handleName, IndexType type, AttributeRegistry registry)
     {
         Type = type;
@@ -47,6 +58,9 @@ public void SetupColumnFamily(IIndex index, ColumnFamilies columnFamilies)
             }
         };
 
+        // Save these as roots so they never get GC'd
+        _roots.Add((_nameDelegate, _destructorDelegate, _comparatorDelegate));
+
         _comparator =
             Native.Instance.rocksdb_comparator_create(IntPtr.Zero, _destructorDelegate, _comparatorDelegate,
                 _nameDelegate);