Enforce sequential access to DbContext.

Source/ApplicationServices/DbContexts.cs

@@ -9,7 +9,7 @@ namespace ApplicationServices
 	{
 		/// <summary>Use for fully functional context, incl. SaveChanges(). For query-only, use the other method</summary>
 		public static LibationContext GetContext()
-			=> LibationContext.Create(SqliteStorage.ConnectionString);
+			=> InstanceQueue<LibationContext>.WaitToCreateInstance(() => LibationContext.Create(SqliteStorage.ConnectionString));
 
 		/// <summary>Use for full library querying. No lazy loading</summary>
 		public static List<LibraryBook> GetLibrary_Flat_NoTracking(bool includeParents = false)

Source/DataLayer/InstanceQueue.cs

@@ -0,0 +1,103 @@
+using System;
+using System.Diagnostics;
+using System.Threading;
+
+#nullable enable
+namespace DataLayer;
+
+/// <summary> Notifies clients that the object is being disposed. </summary>
+public interface INotifyDisposed : IDisposable
+{
+	/// <summary> Event raised when the object is disposed. </summary>
+	event EventHandler? ObjectDisposed;	
+}
+
+/// <summary> Creates a single instance of <typeparamref name="TDisposable"/> at a time, blocking subsequent creations until the previous creations are disposed. </summary>
+public static class InstanceQueue<TDisposable> where TDisposable : INotifyDisposed
+{
+	/// <summary> Synchronization object for access to <see cref="LastInLine"/>"/> </summary>
+	private static Lock Locker { get; } = new();
+	/// <summary> Ticket holder for the last instance creator in line. </summary>
+	private static Ticket? LastInLine { get; set; }
+
+	/// <summary> Waits for all previously created instances of <typeparamref name="TDisposable"/> to be disposed, then creates and returns a new instance of <typeparamref name="TDisposable"/> using the provided <paramref name="creator"/> factory. </summary>
+	public static TDisposable WaitToCreateInstance(Func<TDisposable> creator)
+	{
+		Ticket ticket;
+		lock (Locker)
+		{
+			ticket = LastInLine = new Ticket(creator, LastInLine);
+		}
+
+		return ticket.Fulfill();
+	}
+
+	/// <summary> A queue ticket for an instance creator. </summary>
+	/// <param name="creator">Factory to create a new instance of <typeparamref name="TDisposable"/></param>
+	/// <param name="inFront">The ticket immediately in preceding this new ticket. This new ticket must wait for <paramref name="inFront"/> to signal its instance has been disposed before creating a new instance of <typeparamref name="TDisposable"/></param>
+	private class Ticket(Func<TDisposable> creator, Ticket? inFront) : IDisposable
+	{
+		/// <summary> Factory to create a new instance of <typeparamref name="TDisposable"/> </summary>
+		private Func<TDisposable> Creator { get; } = creator;
+		/// <summary> Ticket immediately in front of this one. This instance must wait for <see cref="InFront"/> to signal its instance has been disposed before creating a new instance of <typeparamref name="TDisposable"/></summary>
+		private Ticket? InFront { get; } = inFront;
+		/// <summary> Wait handle to signal when this ticket's created instance is disposed </summary>
+		private EventWaitHandle WaitHandle { get; } = new(false, EventResetMode.ManualReset);
+		/// <summary> This ticket's created instance of <typeparamref name="TDisposable"/> </summary>
+		private TDisposable? CreatedInstance { get; set; }
+
+		/// <summary> Disposes of this ticket and every ticket queued in front of it. </summary>
+		public void Dispose()
+		{
+			WaitHandle.Dispose();
+			InFront?.Dispose();
+		}
+
+		/// <summary>
+		/// Waits for the <see cref="InFront"/> ticket's instance to be disposed, then creates and returns a new instance of <typeparamref name="TDisposable"/> using the <see cref="Creator"/> factory.
+		/// </summary>
+		public TDisposable Fulfill()
+		{
+#if DEBUG
+			var sw = Stopwatch.StartNew();
+#endif
+			//Wait for the previous ticket's instance to be disposed, then dispose of the previous ticket.
+			InFront?.WaitHandle.WaitOne(Timeout.Infinite);
+			InFront?.Dispose();
+#if DEBUG
+			sw.Stop();
+			Debug.WriteLine($"Waited {sw.ElapsedMilliseconds}ms to create instance of {typeof(TDisposable).Name}");
+#endif
+			CreatedInstance = Creator();
+			CreatedInstance.ObjectDisposed += CreatedInstance_ObjectDisposed;
+			return CreatedInstance;
+		}
+
+		private void CreatedInstance_ObjectDisposed(object? sender, EventArgs e)
+		{
+			Debug.WriteLine($"{typeof(TDisposable).Name} Disposed");
+			if (CreatedInstance is not null)
+			{
+				CreatedInstance.ObjectDisposed -= CreatedInstance_ObjectDisposed;
+				CreatedInstance = default;
+			}
+
+			lock (Locker)
+			{
+				if (this == LastInLine)
+				{
+					//There are no ticket holders waiting after this one.
+					//This ticket is fulfilled and will never be waited on.
+					LastInLine = null;
+					Dispose();
+				}
+				else
+				{
+					//Signal the that this ticket has been fulfilled so that
+					//the next ticket in line may proceed.
+					WaitHandle.Set();
+				}
+			}
+		}
+	}
+}

Source/DataLayer/LibationContext.cs

@@ -1,9 +1,11 @@
 using DataLayer.Configurations;
 using Microsoft.EntityFrameworkCore;
+using System;
+using System.Threading.Tasks;
 
 namespace DataLayer
 {
-    public class LibationContext : DbContext
+    public class LibationContext : DbContext, INotifyDisposed
     {
         // IMPORTANT: USING DbSet<>
         // ========================
@@ -25,6 +27,18 @@ namespace DataLayer
         public DbSet<Category> Categories { get; private set; }
         public DbSet<CategoryLadder> CategoryLadders { get; private set; }
 
+        public event EventHandler ObjectDisposed;
+        public override void Dispose()
+        {
+            base.Dispose();
+            ObjectDisposed?.Invoke(this, EventArgs.Empty);
+        }
+        public override async ValueTask DisposeAsync()
+        {
+            await base.DisposeAsync();
+            ObjectDisposed?.Invoke(this, EventArgs.Empty);
+        }
+
         public static LibationContext Create(string connectionString)
         {
             var factory = new LibationContextFactory();