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

…ect (python#115433)

This adds `Py_XBEGIN_CRITICAL_SECTION` and
`Py_XEND_CRITICAL_SECTION`, 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_XBEGIN_CRITICAL_SECTION(op) \
+ { \
+ _PyCriticalSection _cs_opt = {0}; \
+ _PyCriticalSection_XBegin(&_cs_opt, _PyObject_CAST(op))
+
+# define Py_XEND_CRITICAL_SECTION() \
+ _PyCriticalSection_XEnd(&_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_XBEGIN_CRITICAL_SECTION(op)
+# define Py_XEND_CRITICAL_SECTION()
 # 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_XBegin(_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_XEnd(_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 +49,15 @@ 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_XBEGIN_CRITICAL_SECTION(d1);
+ assert_nogil(PyMutex_IsLocked(&d1->ob_mutex));
+ Py_XEND_CRITICAL_SECTION();
+
+ // No-op
+ Py_XBEGIN_CRITICAL_SECTION(NULL);
+ Py_XEND_CRITICAL_SECTION();
+
  Py_DECREF(d2);
  Py_DECREF(d1);
  Py_RETURN_NONE;