Workaround for blocked PyObject_GenericSetAttr in metatypes

Python 3.14 introduced a new assertion that prevents us from using
PyObject_GenericSetAttr directly in our meta type. To work around
this, we manipulate the type dict directly.

This workaround is a simplified variant of Cython's workaround from
cython/cython#6325.

The relevant Python change is in
python/cpython#118454

(cherry picked from commit 08550d0)

Author: filmor

src/runtime/Native/PyIdentifier_.cs

@@ -25,6 +25,8 @@ static class PyIdentifier
          public static BorrowedReference __self__ => new(f__self__);
          static IntPtr f__annotations__;
          public static BorrowedReference __annotations__ => new(f__annotations__);
+         static IntPtr f__dictoffset__;
+         public static BorrowedReference __dictoffset__ => new(f__dictoffset__);
          static IntPtr f__init__;
          public static BorrowedReference __init__ => new(f__init__);
          static IntPtr f__repr__;
@@ -57,6 +59,7 @@ static partial class InternString
             "__slots__",
             "__self__",
             "__annotations__",
+            "__dictoffset__",
             "__init__",
             "__repr__",
             "__import__",

src/runtime/Native/PyIdentifier_.tt

@@ -13,6 +13,7 @@
         "__slots__",
         "__self__",
         "__annotations__",
+        "__dictoffset__",
 
         "__init__",
         "__repr__",

src/runtime/Types/MetaType.cs

@@ -18,6 +18,7 @@ internal sealed class MetaType : ManagedType
         // set in Initialize
         private static PyType PyCLRMetaType;
         private static SlotsHolder _metaSlotsHodler;
+        private static int TypeDictOffset = -1;
 #pragma warning restore CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
 
         internal static readonly string[] CustomMethods = new string[]
@@ -32,6 +33,25 @@ internal sealed class MetaType : ManagedType
         public static PyType Initialize()
         {
             PyCLRMetaType = TypeManager.CreateMetaType(typeof(MetaType), out _metaSlotsHodler);
+
+            // Retrieve the offset of the type's dictionary from PyType_Type for
+            // use in the tp_setattro implementation.
+            using (NewReference dictOffset = Runtime.PyObject_GetAttr(Runtime.PyTypeType, PyIdentifier.__dictoffset__))
+            {
+                if (dictOffset.IsNull())
+                {
+                    throw new InvalidOperationException("Could not get __dictoffset__ from PyType_Type");
+                }
+
+                nint dictOffsetVal = Runtime.PyLong_AsSignedSize_t(dictOffset.Borrow());
+                if (dictOffsetVal <= 0)
+                {
+                    throw new InvalidOperationException("Could not get __dictoffset__ from PyType_Type");
+                }
+
+                TypeDictOffset = checked((int)dictOffsetVal);
+            }
+
             return PyCLRMetaType;
         }
 
@@ -41,6 +61,7 @@ public static void Release()
             {
                 _metaSlotsHodler.ResetSlots();
             }
+            TypeDictOffset = -1;
             PyCLRMetaType.Dispose();
         }
 
@@ -246,7 +267,28 @@ public static int tp_setattro(BorrowedReference tp, BorrowedReference name, Borr
                 }
             }
 
-            int res = Runtime.PyObject_GenericSetAttr(tp, name, value);
+            // Access the type's dictionary directly
+            //
+            // We can not use the PyObject_GenericSetAttr because since Python
+            // 3.14 as https://github.com/python/cpython/pull/118454 intrdoduced
+            // an assertion to prevent it from being called from metatypes.
+            //
+            // The direct dictionary access is equivalent to what Cython does
+            // to work around the same issue: https://github.com/cython/cython/pull/6325
+            BorrowedReference typeDict = new(Util.ReadIntPtr(tp, TypeDictOffset));
+            int res;
+            if (value.IsNull)
+            {
+                res = Runtime.PyDict_DelItem(typeDict, name);
+                if (res != 0)
+                {
+                    Exceptions.SetError(Exceptions.AttributeError, "attribute not found");
+                }
+            }
+            else
+            {
+                res = Runtime.PyDict_SetItem(typeDict, name, value);
+            }
             Runtime.PyType_Modified(tp);
 
             return res;