Fix access violation exception on shutdown (#1977)

When nulling the GC handles on shutdown the reference count of all objects pointed to by the IntPtr in the `CLRObject.reflectedObjects` are zero. This caused an exception in some scenarios because `Runtime.PyObject_TYPE(reflectedClrObject)` is called while the reference counter is at zero. After `TypeManager.RemoveTypes();` is called in the `Runtime.Shutdown()` method, reference count decrements to zero do not invoke `ClassBase.tp_clear` for managed objects anymore which normally is responsible for removing references from `CLRObject.reflectedObjects`. Collecting objects referenced in `CLRObject.reflectedObjects` only after leads to an unstable state in which the reference count for these object addresses is zero while still maintaining them to be used for further pseudo-cleanup. In that time, the memory could have been reclaimed already which leads to the exception.
pythonnet · May 13, 2024 · d496959 · d496959
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diff --git a/AUTHORS.md b/AUTHORS.md
@@ -38,6 +38,7 @@
 -   Dmitriy Se ([@dmitriyse](https://github.com/dmitriyse))
 -   Félix Bourbonnais ([@BadSingleton](https://github.com/BadSingleton))
 -   Florian Treurniet ([@ftreurni](https://github.com/ftreurni))
+-   Frank Witscher ([@Frawak](https://github.com/Frawak))
 -   He-chien Tsai ([@t3476](https://github.com/t3476))
 -   Inna Wiesel ([@inna-w](https://github.com/inna-w))
 -   Ivan Cronyn ([@cronan](https://github.com/cronan))

diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -23,6 +23,7 @@ This document follows the conventions laid out in [Keep a CHANGELOG][].
 
 -   Fixed RecursionError for reverse operators on C# operable types from python. See #2240
 -   Fixed probing for assemblies in `sys.path` failing when a path in `sys.path` has invalid characters. See #2376
+-   Fixed possible access violation exception on shutdown. See ([#1977][i1977])
 
 ## [3.0.3](https://github.com/pythonnet/pythonnet/releases/tag/v3.0.3) - 2023-10-11
 
@@ -970,3 +971,4 @@ This version improves performance on benchmarks significantly compared to 2.3.
 [i1481]: https://github.com/pythonnet/pythonnet/issues/1481
 [i1672]: https://github.com/pythonnet/pythonnet/pull/1672
 [i2311]: https://github.com/pythonnet/pythonnet/issues/2311
+[i1977]: https://github.com/pythonnet/pythonnet/issues/1977
diff --git a/src/runtime/Runtime.cs b/src/runtime/Runtime.cs
@@ -278,6 +278,8 @@ internal static void Shutdown()
             ClearClrModules();
             RemoveClrRootModule();
 
+            TryCollectingGarbage(MaxCollectRetriesOnShutdown, forceBreakLoops: true);
+
             NullGCHandles(ExtensionType.loadedExtensions);
             ClassManager.RemoveClasses();
             TypeManager.RemoveTypes();
@@ -295,8 +297,8 @@ internal static void Shutdown()
             PyObjectConversions.Reset();
 
             PyGC_Collect();
-            bool everythingSeemsCollected = TryCollectingGarbage(MaxCollectRetriesOnShutdown,
-                                                                 forceBreakLoops: true);
+            bool everythingSeemsCollected = TryCollectingGarbage(MaxCollectRetriesOnShutdown);
+
             Debug.Assert(everythingSeemsCollected);
 
             Finalizer.Shutdown();