gh-117323: Make `cell` thread-safe in free-threaded builds (#117330)

Use critical sections to lock around accesses to cell contents. The critical sections are no-ops in the default (with GIL) build.

1 files changed, 48 insertions, 0 deletions

diff --git a/Include/internal/pycore_cell.h b/Include/internal/pycore_cell.h
new file mode 100644
index 0000000..27f67d5
--- /dev/null
+++ b/Include/internal/pycore_cell.h
@@ -0,0 +1,48 @@
+#ifndef Py_INTERNAL_CELL_H
+#define Py_INTERNAL_CELL_H
+
+#include "pycore_critical_section.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+// Sets the cell contents to `value` and return previous contents. Steals a
+// reference to `value`.
+static inline PyObject *
+PyCell_SwapTakeRef(PyCellObject *cell, PyObject *value)
+{
+    PyObject *old_value;
+    Py_BEGIN_CRITICAL_SECTION(cell);
+    old_value = cell->ob_ref;
+    cell->ob_ref = value;
+    Py_END_CRITICAL_SECTION();
+    return old_value;
+}
+
+static inline void
+PyCell_SetTakeRef(PyCellObject *cell, PyObject *value)
+{
+    PyObject *old_value = PyCell_SwapTakeRef(cell, value);
+    Py_XDECREF(old_value);
+}
+
+// Gets the cell contents. Returns a new reference.
+static inline PyObject *
+PyCell_GetRef(PyCellObject *cell)
+{
+    PyObject *res;
+    Py_BEGIN_CRITICAL_SECTION(cell);
+    res = Py_XNewRef(cell->ob_ref);
+    Py_END_CRITICAL_SECTION();
+    return res;
+}
+
+#ifdef __cplusplus
+}
+#endif
+#endif   /* !Py_INTERNAL_CELL_H */