python · xdegaye · Dec 20, 2017
diff --git a/Doc/whatsnew/3.6.rst b/Doc/whatsnew/3.6.rst
@@ -1852,10 +1852,10 @@ Build and C API Changes
 * The :c:func:`PyUnicode_FSConverter` and :c:func:`PyUnicode_FSDecoder`
   functions will now accept :term:`path-like objects <path-like object>`.
 
-* The ``PyExc_RecursionErrorInst`` singleton that was part of the public API
-  has been removed as its members being never cleared may cause a segfault
-  during finalization of the interpreter. Contributed by Xavier de Gaye in
-  :issue:`22898` and :issue:`30697`.
+* The ``PyExc_RecursionErrorInst`` singleton is not used anymore as its members
+  being never cleared may cause a segfault during finalization of the
+  interpreter. Contributed by Xavier de Gaye in :issue:`22898` and
+  :issue:`30697`.
 
 
 Other Improvements

diff --git a/Include/pyerrors.h b/Include/pyerrors.h
@@ -219,6 +219,8 @@ PyAPI_DATA(PyObject *) PyExc_IOError;
 PyAPI_DATA(PyObject *) PyExc_WindowsError;
 #endif
 
+PyAPI_DATA(PyObject *) PyExc_RecursionErrorInst;
+
 /* Predefined warning categories */
 PyAPI_DATA(PyObject *) PyExc_Warning;
 PyAPI_DATA(PyObject *) PyExc_UserWarning;

diff --git a/Misc/NEWS.d/next/C API/2017-12-20-15-23-06.bpo-30697.v9FmgG.rst b/Misc/NEWS.d/next/C API/2017-12-20-15-23-06.bpo-30697.v9FmgG.rst
@@ -0,0 +1 @@
+Restore PyExc_RecursionErrorInst in 3.6
diff --git a/Objects/exceptions.c b/Objects/exceptions.c
@@ -2430,6 +2430,12 @@ SimpleExtendsException(PyExc_Warning, ResourceWarning,
 
 
 
+/* Pre-computed RecursionError instance for when recursion depth is reached.
+   Meant to be used when normalizing the exception for exceeding the recursion
+   depth will cause its own infinite recursion.
+*/
+PyObject *PyExc_RecursionErrorInst = NULL;
+
 #define PRE_INIT(TYPE) \
     if (!(_PyExc_ ## TYPE.tp_flags & Py_TPFLAGS_READY)) { \
         if (PyType_Ready(&_PyExc_ ## TYPE) < 0) \
@@ -2691,11 +2697,37 @@ _PyExc_Init(PyObject *bltinmod)
     ADD_ERRNO(TimeoutError, ETIMEDOUT);
 
     preallocate_memerrors();
+
+    if (!PyExc_RecursionErrorInst) {
+        PyExc_RecursionErrorInst = BaseException_new(&_PyExc_RecursionError, NULL, NULL);
+        if (!PyExc_RecursionErrorInst)
+            Py_FatalError("Cannot pre-allocate RecursionError instance for "
+                            "recursion errors");
+        else {
+            PyBaseExceptionObject *err_inst =
+                (PyBaseExceptionObject *)PyExc_RecursionErrorInst;
+            PyObject *args_tuple;
+            PyObject *exc_message;
+            exc_message = PyUnicode_FromString("maximum recursion depth exceeded");
+            if (!exc_message)
+                Py_FatalError("cannot allocate argument for RecursionError "
+                                "pre-allocation");
+            args_tuple = PyTuple_Pack(1, exc_message);
+            if (!args_tuple)
+                Py_FatalError("cannot allocate tuple for RecursionError "
+                                "pre-allocation");
+            Py_DECREF(exc_message);
+            if (BaseException_init(err_inst, args_tuple, NULL))
+                Py_FatalError("init of pre-allocated RecursionError failed");
+            Py_DECREF(args_tuple);
+        }
+    }
 }
 
 void
 _PyExc_Fini(void)
 {
+    Py_CLEAR(PyExc_RecursionErrorInst);
     free_preallocated_memerrors();
     Py_CLEAR(errnomap);
 }

diff --git a/PC/python3.def b/PC/python3.def
@@ -224,6 +224,7 @@ EXPORTS
   PyExc_PermissionError=python36.PyExc_PermissionError DATA
   PyExc_ProcessLookupError=python36.PyExc_ProcessLookupError DATA
   PyExc_RecursionError=python36.PyExc_RecursionError DATA
+  PyExc_RecursionErrorInst=python36.PyExc_RecursionErrorInst DATA
   PyExc_ReferenceError=python36.PyExc_ReferenceError DATA
   PyExc_ResourceWarning=python36.PyExc_ResourceWarning DATA
   PyExc_RuntimeError=python36.PyExc_RuntimeError DATA