gh-112529: Implement GC for free-threaded builds (#114262)

* gh-112529: Implement GC for free-threaded builds

This implements a mark and sweep GC for the free-threaded builds of
CPython. The implementation relies on mimalloc to find GC tracked
objects (i.e., "containers").

Include/internal/pycore_object_stack.h

@@ -0,0 +1,84 @@
+#ifndef Py_INTERNAL_OBJECT_STACK_H
+#define Py_INTERNAL_OBJECT_STACK_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+// _PyObjectStack is a stack of Python objects implemented as a linked list of
+// fixed size buffers.
+
+// Chosen so that _PyObjectStackChunk is a power-of-two size.
+#define _Py_OBJECT_STACK_CHUNK_SIZE 254
+
+typedef struct _PyObjectStackChunk {
+    struct _PyObjectStackChunk *prev;
+    Py_ssize_t n;
+    PyObject *objs[_Py_OBJECT_STACK_CHUNK_SIZE];
+} _PyObjectStackChunk;
+
+typedef struct _PyObjectStack {
+    _PyObjectStackChunk *head;
+} _PyObjectStack;
+
+
+extern _PyObjectStackChunk *
+_PyObjectStackChunk_New(void);
+
+extern void
+_PyObjectStackChunk_Free(_PyObjectStackChunk *);
+
+extern void
+_PyObjectStackChunk_ClearFreeList(_PyFreeListState *state, int is_finalization);
+
+// Push an item onto the stack. Return -1 on allocation failure, 0 on success.
+static inline int
+_PyObjectStack_Push(_PyObjectStack *stack, PyObject *obj)
+{
+    _PyObjectStackChunk *buf = stack->head;
+    if (buf == NULL || buf->n == _Py_OBJECT_STACK_CHUNK_SIZE) {
+        buf = _PyObjectStackChunk_New();
+        if (buf == NULL) {
+            return -1;
+        }
+        buf->prev = stack->head;
+        buf->n = 0;
+        stack->head = buf;
+    }
+
+    assert(buf->n >= 0 && buf->n < _Py_OBJECT_STACK_CHUNK_SIZE);
+    buf->objs[buf->n] = obj;
+    buf->n++;
+    return 0;
+}
+
+// Pop the top item from the stack.  Return NULL if the stack is empty.
+static inline PyObject *
+_PyObjectStack_Pop(_PyObjectStack *stack)
+{
+    _PyObjectStackChunk *buf = stack->head;
+    if (buf == NULL) {
+        return NULL;
+    }
+    assert(buf->n > 0 && buf->n <= _Py_OBJECT_STACK_CHUNK_SIZE);
+    buf->n--;
+    PyObject *obj = buf->objs[buf->n];
+    if (buf->n == 0) {
+        stack->head = buf->prev;
+        _PyObjectStackChunk_Free(buf);
+    }
+    return obj;
+}
+
+// Remove all items from the stack
+extern void
+_PyObjectStack_Clear(_PyObjectStack *stack);
+
+#ifdef __cplusplus
+}
+#endif
+#endif  // !Py_INTERNAL_OBJECT_STACK_H