feat: add thread safety helpers

Dalamud/Dalamud.cs

@@ -99,6 +99,8 @@ namespace Dalamud
         {
             try
             {
+                ThreadSafety.MarkMainThread();
+
                 SerilogEventSink.Instance.LogLine += SerilogOnLogLine;
 
                 Service<ServiceContainer>.Set();
@@ -115,7 +117,6 @@ namespace Dalamud
 
                 // Signal the main game thread to continue
                 NativeFunctions.SetEvent(this.mainThreadContinueEvent);
-
                 Log.Information("[T1] Game thread continued!");
 
                 // Initialize FFXIVClientStructs function resolver

Dalamud/Utility/ThreadSafety.cs

@@ -0,0 +1,49 @@
+using System;
+
+namespace Dalamud.Utility;
+
+/// <summary>
+/// Helpers for working with thread safety.
+/// </summary>
+public static class ThreadSafety
+{
+    [ThreadStatic]
+    private static bool isMainThread;
+
+    /// <summary>
+    /// Gets a value indicating whether the current thread is the main thread.
+    /// </summary>
+    public static bool IsMainThread => isMainThread;
+
+    /// <summary>
+    /// Throws an exception when the current thread is not the main thread.
+    /// </summary>
+    /// <exception cref="InvalidOperationException">Thrown when the current thread is not the main thread.</exception>
+    public static void AssertMainThread()
+    {
+        if (!isMainThread)
+        {
+            throw new InvalidOperationException("Not on main thread!");
+        }
+    }
+
+    /// <summary>
+    /// Throws an exception when the current thread is the main thread.
+    /// </summary>
+    /// <exception cref="InvalidOperationException">Thrown when the current thread is the main thread.</exception>
+    public static void AssertNotMainThread()
+    {
+        if (isMainThread)
+        {
+            throw new InvalidOperationException("On main thread!");
+        }
+    }
+
+    /// <summary>
+    /// Marks a thread as the main thread.
+    /// </summary>
+    internal static void MarkMainThread()
+    {
+        isMainThread = true;
+    }
+}