bpo-42500: Fix recursion in or after except (GH-23568)

* Use counter, rather boolean state when handling soft overflows.

Python/ceval.c

@@ -857,20 +857,22 @@ _Py_CheckRecursiveCall(PyThreadState *tstate, const char *where)
         return -1;
     }
 #endif
-    if (tstate->overflowed) {
+    if (tstate->recursion_headroom) {
         if (tstate->recursion_depth > recursion_limit + 50) {
             /* Overflowing while handling an overflow. Give up. */
             Py_FatalError("Cannot recover from stack overflow.");
         }
-        return 0;
     }
-    if (tstate->recursion_depth > recursion_limit) {
-        --tstate->recursion_depth;
-        tstate->overflowed = 1;
-        _PyErr_Format(tstate, PyExc_RecursionError,
-                      "maximum recursion depth exceeded%s",
-                      where);
-        return -1;
+    else {
+        if (tstate->recursion_depth > recursion_limit) {
+            tstate->recursion_headroom++;
+            _PyErr_Format(tstate, PyExc_RecursionError,
+                        "maximum recursion depth exceeded%s",
+                        where);
+            tstate->recursion_headroom--;
+            --tstate->recursion_depth;
+            return -1;
+        }
     }
     return 0;
 }

Python/errors.c

@@ -290,12 +290,14 @@ _PyErr_NormalizeException(PyThreadState *tstate, PyObject **exc,
                           PyObject **val, PyObject **tb)
 {
     int recursion_depth = 0;
+    tstate->recursion_headroom++;
     PyObject *type, *value, *initial_tb;
 
   restart:
     type = *exc;
     if (type == NULL) {
         /* There was no exception, so nothing to do. */
+        tstate->recursion_headroom--;
         return;
     }
 
@@ -347,6 +349,7 @@ _PyErr_NormalizeException(PyThreadState *tstate, PyObject **exc,
     }
     *exc = type;
     *val = value;
+    tstate->recursion_headroom--;
     return;
 
   error:

Python/pystate.c

@@ -605,7 +605,7 @@ new_threadstate(PyInterpreterState *interp, int init)
 
     tstate->frame = NULL;
     tstate->recursion_depth = 0;
-    tstate->overflowed = 0;
+    tstate->recursion_headroom = 0;
     tstate->stackcheck_counter = 0;
     tstate->tracing = 0;
     tstate->use_tracing = 0;

Python/sysmodule.c

@@ -1181,7 +1181,6 @@ static PyObject *
 sys_setrecursionlimit_impl(PyObject *module, int new_limit)
 /*[clinic end generated code: output=35e1c64754800ace input=b0f7a23393924af3]*/
 {
-    int mark;
     PyThreadState *tstate = _PyThreadState_GET();
 
     if (new_limit < 1) {
@@ -1199,8 +1198,7 @@ sys_setrecursionlimit_impl(PyObject *module, int new_limit)
        Reject too low new limit if the current recursion depth is higher than
        the new low-water mark. Otherwise it may not be possible anymore to
        reset the overflowed flag to 0. */
-    mark = _Py_RecursionLimitLowerWaterMark(new_limit);
-    if (tstate->recursion_depth >= mark) {
+    if (tstate->recursion_depth >= new_limit) {
         _PyErr_Format(tstate, PyExc_RecursionError,
                       "cannot set the recursion limit to %i at "
                       "the recursion depth %i: the limit is too low",