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.

Author: colesbury

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 */

Makefile.pre.in

@@ -1130,6 +1130,7 @@ PYTHON_HEADERS= \
 		$(srcdir)/Include/internal/pycore_bytesobject.h \
 		$(srcdir)/Include/internal/pycore_call.h \
 		$(srcdir)/Include/internal/pycore_capsule.h \
+		$(srcdir)/Include/internal/pycore_cell.h \
 		$(srcdir)/Include/internal/pycore_ceval.h \
 		$(srcdir)/Include/internal/pycore_ceval_state.h \
 		$(srcdir)/Include/internal/pycore_code.h \

Objects/cellobject.c

@@ -1,6 +1,7 @@
 /* Cell object implementation */
 
 #include "Python.h"
+#include "pycore_cell.h"          // PyCell_GetRef()
 #include "pycore_modsupport.h"    // _PyArg_NoKeywords()
 #include "pycore_object.h"
 
@@ -56,8 +57,7 @@ PyCell_Get(PyObject *op)
         PyErr_BadInternalCall();
         return NULL;
     }
-    PyObject *value = PyCell_GET(op);
-    return Py_XNewRef(value);
+    return PyCell_GetRef((PyCellObject *)op);
 }
 
 int
@@ -67,9 +67,7 @@ PyCell_Set(PyObject *op, PyObject *value)
         PyErr_BadInternalCall();
         return -1;
     }
-    PyObject *old_value = PyCell_GET(op);
-    PyCell_SET(op, Py_XNewRef(value));
-    Py_XDECREF(old_value);
+    PyCell_SetTakeRef((PyCellObject *)op, Py_XNewRef(value));
     return 0;
 }
 

PCbuild/pythoncore.vcxproj

@@ -210,6 +210,7 @@
     <ClInclude Include="..\Include\internal\pycore_bytesobject.h" />
     <ClInclude Include="..\Include\internal\pycore_call.h" />
     <ClInclude Include="..\Include\internal\pycore_capsule.h" />
+    <ClInclude Include="..\Include\internal\pycore_cell.h" />
     <ClInclude Include="..\Include\internal\pycore_ceval.h" />
     <ClInclude Include="..\Include\internal\pycore_ceval_state.h" />
     <ClInclude Include="..\Include\internal\pycore_cfg.h" />

PCbuild/pythoncore.vcxproj.filters

@@ -555,6 +555,9 @@
     <ClInclude Include="..\Include\internal\pycore_capsule.h">
       <Filter>Include\internal</Filter>
     </ClInclude>
+    <ClInclude Include="..\Include\internal\pycore_cell.h">
+      <Filter>Include\internal</Filter>
+    </ClInclude>
     <ClInclude Include="..\Include\internal\pycore_ceval.h">
       <Filter>Include\internal</Filter>
     </ClInclude>

Python/bytecodes.c

@@ -8,6 +8,7 @@
 
 #include "Python.h"
 #include "pycore_abstract.h"      // _PyIndex_Check()
+#include "pycore_cell.h"          // PyCell_GetRef()
 #include "pycore_code.h"
 #include "pycore_emscripten_signal.h"  // _Py_CHECK_EMSCRIPTEN_SIGNALS
 #include "pycore_function.h"
@@ -1523,14 +1524,13 @@ dummy_func(
 
         inst(DELETE_DEREF, (--)) {
             PyObject *cell = GETLOCAL(oparg);
-            PyObject *oldobj = PyCell_GET(cell);
             // Can't use ERROR_IF here.
             // Fortunately we don't need its superpower.
+            PyObject *oldobj = PyCell_SwapTakeRef((PyCellObject *)cell, NULL);
             if (oldobj == NULL) {
                 _PyEval_FormatExcUnbound(tstate, _PyFrame_GetCode(frame), oparg);
                 ERROR_NO_POP();
             }
-            PyCell_SET(cell, NULL);
             Py_DECREF(oldobj);
         }
 
@@ -1543,32 +1543,28 @@ dummy_func(
                 ERROR_NO_POP();
             }
             if (!value) {
-                PyObject *cell = GETLOCAL(oparg);
-                value = PyCell_GET(cell);
+                PyCellObject *cell = (PyCellObject *)GETLOCAL(oparg);
+                value = PyCell_GetRef(cell);
                 if (value == NULL) {
                     _PyEval_FormatExcUnbound(tstate, _PyFrame_GetCode(frame), oparg);
                     ERROR_NO_POP();
                 }
-                Py_INCREF(value);
             }
             Py_DECREF(class_dict);
         }
 
         inst(LOAD_DEREF, ( -- value)) {
-            PyObject *cell = GETLOCAL(oparg);
-            value = PyCell_GET(cell);
+            PyCellObject *cell = (PyCellObject *)GETLOCAL(oparg);
+            value = PyCell_GetRef(cell);
             if (value == NULL) {
                 _PyEval_FormatExcUnbound(tstate, _PyFrame_GetCode(frame), oparg);
                 ERROR_IF(true, error);
             }
-            Py_INCREF(value);
         }
 
         inst(STORE_DEREF, (v --)) {
-            PyObject *cell = GETLOCAL(oparg);
-            PyObject *oldobj = PyCell_GET(cell);
-            PyCell_SET(cell, v);
-            Py_XDECREF(oldobj);
+            PyCellObject *cell = (PyCellObject *)GETLOCAL(oparg);
+            PyCell_SetTakeRef(cell, v);
         }
 
         inst(COPY_FREE_VARS, (--)) {

Python/ceval.c

@@ -5,6 +5,7 @@
 #include "Python.h"
 #include "pycore_abstract.h"      // _PyIndex_Check()
 #include "pycore_call.h"          // _PyObject_CallNoArgs()
+#include "pycore_cell.h"          // PyCell_GetRef()
 #include "pycore_ceval.h"
 #include "pycore_code.h"
 #include "pycore_emscripten_signal.h"  // _Py_CHECK_EMSCRIPTEN_SIGNALS

Python/executor_cases.c.h

@@ -520,8 +520,9 @@ def effect_depends_on_oparg_1(op: parser.InstDef) -> bool:
 def compute_properties(op: parser.InstDef) -> Properties:
     has_free = (
         variable_used(op, "PyCell_New")
-        or variable_used(op, "PyCell_GET")
-        or variable_used(op, "PyCell_SET")
+        or variable_used(op, "PyCell_GetRef")
+        or variable_used(op, "PyCell_SetTakeRef")
+        or variable_used(op, "PyCell_SwapTakeRef")
     )
     deopts_if = variable_used(op, "DEOPT_IF")
     exits_if = variable_used(op, "EXIT_IF")