pythongh-115432: Add critical section variant that handles a NULL object

This adds `Py_BEGIN_CRITICAL_SECTION_OPT` and
`Py_END_CRITICAL_SECTION_OPT`, which accept a possibly NULL object as an
argument. If the argument is NULL, then nothing is locked or unlocked.
Otherwise, they behave like `Py_BEGIN/END_CRITICAL_SECTION`.

Include/internal/pycore_critical_section.h

@@ -96,6 +96,15 @@ extern "C" {
         _PyCriticalSection_End(&_cs);                                   \
     }
 
+# define Py_BEGIN_CRITICAL_SECTION_OPT(op)                              \
+    {                                                                   \
+        _PyCriticalSection _cs_opt = {0};                               \
+        _PyCriticalSection_BeginOpt(&_cs_opt, _PyObject_CAST(op))
+
+# define Py_END_CRITICAL_SECTION_OPT()                                  \
+        _PyCriticalSection_EndOpt(&_cs_opt);                            \
+    }
+
 # define Py_BEGIN_CRITICAL_SECTION2(a, b)                               \
     {                                                                   \
         _PyCriticalSection2 _cs2;                                       \
@@ -131,6 +140,8 @@ extern "C" {
 // The critical section APIs are no-ops with the GIL.
 # define Py_BEGIN_CRITICAL_SECTION(op)
 # define Py_END_CRITICAL_SECTION()
+# define Py_BEGIN_CRITICAL_SECTION_OPT(op)
+# define Py_END_CRITICAL_SECTION_OPT()
 # define Py_BEGIN_CRITICAL_SECTION2(a, b)
 # define Py_END_CRITICAL_SECTION2()
 # define _Py_CRITICAL_SECTION_ASSERT_MUTEX_LOCKED(mutex)
@@ -187,6 +198,16 @@ _PyCriticalSection_Begin(_PyCriticalSection *c, PyMutex *m)
     }
 }
 
+static inline void
+_PyCriticalSection_BeginOpt(_PyCriticalSection *c, PyObject *op)
+{
+#ifdef Py_GIL_DISABLED
+    if (op != NULL) {
+        _PyCriticalSection_Begin(c, &_PyObject_CAST(op)->ob_mutex);
+    }
+#endif
+}
+
 // Removes the top-most critical section from the thread's stack of critical
 // sections. If the new top-most critical section is inactive, then it is
 // resumed.
@@ -209,6 +230,14 @@ _PyCriticalSection_End(_PyCriticalSection *c)
     _PyCriticalSection_Pop(c);
 }
 
+static inline void
+_PyCriticalSection_EndOpt(_PyCriticalSection *c)
+{
+    if (c->mutex) {
+        _PyCriticalSection_End(c);
+    }
+}
+
 static inline void
 _PyCriticalSection2_Begin(_PyCriticalSection2 *c, PyMutex *m1, PyMutex *m2)
 {

Modules/_testinternalcapi/test_critical_sections.c

@@ -49,6 +49,16 @@ test_critical_sections(PyObject *self, PyObject *Py_UNUSED(args))
     Py_END_CRITICAL_SECTION2();
     assert_nogil(!PyMutex_IsLocked(&d2->ob_mutex));
 
+    // Optional variant behaves the same if the object is non-NULL
+    Py_BEGIN_CRITICAL_SECTION_OPT(d1);
+    assert_nogil(PyMutex_IsLocked(&d1->ob_mutex));
+    Py_END_CRITICAL_SECTION_OPT();
+
+    // No-op
+    PyObject *null_object = NULL;
+    Py_BEGIN_CRITICAL_SECTION_OPT(null_object);
+    Py_END_CRITICAL_SECTION_OPT();
+
     Py_DECREF(d2);
     Py_DECREF(d1);
     Py_RETURN_NONE;