gh-115491: Keep some fields valid across allocations (free-threading) (#115573)

This avoids filling the memory occupied by ob_tid, ob_ref_local, and
ob_ref_shared with debug bytes (e.g., 0xDD) in mimalloc in the
free-threaded build.

Python/pystate.c

@@ -2845,9 +2845,24 @@ tstate_mimalloc_bind(PyThreadState *tstate)
     // pools to keep Python objects from different interpreters separate.
     tld->segments.abandoned = &tstate->interp->mimalloc.abandoned_pool;
 
+    // Don't fill in the first N bytes up to ob_type in debug builds. We may
+    // access ob_tid and the refcount fields in the dict and list lock-less
+    // accesses, so they must remain valid for a while after deallocation.
+    size_t base_offset = offsetof(PyObject, ob_type);
+    if (_PyMem_DebugEnabled()) {
+        // The debug allocator adds two words at the beginning of each block.
+        base_offset += 2 * sizeof(size_t);
+    }
+    size_t debug_offsets[_Py_MIMALLOC_HEAP_COUNT] = {
+        [_Py_MIMALLOC_HEAP_OBJECT] = base_offset,
+        [_Py_MIMALLOC_HEAP_GC] = base_offset,
+        [_Py_MIMALLOC_HEAP_GC_PRE] = base_offset + 2 * sizeof(PyObject *),
+    };
+
     // Initialize each heap
     for (uint8_t i = 0; i < _Py_MIMALLOC_HEAP_COUNT; i++) {
         _mi_heap_init_ex(&mts->heaps[i], tld, _mi_arena_id_none(), false, i);
+        mts->heaps[i].debug_offset = (uint8_t)debug_offsets[i];
     }
 
     // By default, object allocations use _Py_MIMALLOC_HEAP_OBJECT.