emit thread safety warning in defaultdict.__missing__

Author: Changaco

Include/internal/pycore_critical_section.h

@@ -252,6 +252,26 @@ _PyCriticalSection_AssertHeldObj(PyObject *op)
 #endif
 }
 
+static inline void
+_PyCriticalSection_WarnIfNotHeld(PyObject *op, const char *message)
+{
+#ifdef Py_GIL_DISABLED
+    PyMutex *mutex = &_PyObject_CAST(op)->ob_mutex;
+    PyThreadState *tstate = _PyThreadState_GET();
+    uintptr_t prev = tstate->critical_section;
+    if (prev & _Py_CRITICAL_SECTION_TWO_MUTEXES) {
+        PyCriticalSection2 *cs = (PyCriticalSection2 *)(prev & ~_Py_CRITICAL_SECTION_MASK);
+        if (cs == NULL || (cs->_cs_base._cs_mutex != mutex && cs->_cs_mutex2 != mutex))
+            PyErr_WarnEx(NULL, message, 2);
+    }
+    else {
+        PyCriticalSection *cs = (PyCriticalSection *)(prev & ~_Py_CRITICAL_SECTION_MASK);
+        if (cs == NULL || cs->_cs_mutex != mutex)
+            PyErr_WarnEx(NULL, message, 2);
+    }
+#endif
+}
+
 #undef Py_BEGIN_CRITICAL_SECTION
 # define Py_BEGIN_CRITICAL_SECTION(op)                                  \
     {                                                                   \

Modules/_collectionsmodule.c

@@ -2262,6 +2262,12 @@ PyDoc_STRVAR(defdict_missing_doc,
 static PyObject *
 defdict_missing(PyObject *op, PyObject *key)
 {
+    _PyCriticalSection_WarnIfNotHeld(
+        op,
+        "the defaultdict.__missing__ method should not be called directly; "
+        "use dd.pop(key, None) to safely trigger a reset to a default value "
+        "the next time key is accessed"
+    );
     defdictobject *dd = defdictobject_CAST(op);
     PyObject *factory = dd->default_factory;
     PyObject *value;