[3.9] bpo-43710: Rollback the 3.9 bpo-42500 fix, it broke the ABI in 3.9.3 (#25179)

This reverts commit 8b795ab554.

It changed the PyThreadState structure size, breaking the ABI in 3.9.3.

Python/ceval.c

@@ -793,22 +793,23 @@ _Py_CheckRecursiveCall(PyThreadState *tstate, const char *where)
         _Py_CheckRecursionLimit = recursion_limit;
     }
 #endif
-    if (tstate->recursion_headroom) {
+    if (tstate->recursion_critical)
+        /* Somebody asked that we don't check for recursion. */
+        return 0;
+    if (tstate->overflowed) {
         if (tstate->recursion_depth > recursion_limit + 50) {
             /* Overflowing while handling an overflow. Give up. */
             Py_FatalError("Cannot recover from stack overflow.");
         }
+        return 0;
     }
-    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;
-        }
+    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;
     }
     return 0;
 }

Python/errors.c

@@ -290,14 +290,12 @@ _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;
     }
 
@@ -349,7 +347,6 @@ _PyErr_NormalizeException(PyThreadState *tstate, PyObject **exc,
     }
     *exc = type;
     *val = value;
-    tstate->recursion_headroom--;
     return;
 
   error:

Python/pystate.c

@@ -576,7 +576,7 @@ new_threadstate(PyInterpreterState *interp, int init)
 
     tstate->frame = NULL;
     tstate->recursion_depth = 0;
-    tstate->recursion_headroom = 0;
+    tstate->overflowed = 0;
     tstate->recursion_critical = 0;
     tstate->stackcheck_counter = 0;
     tstate->tracing = 0;

Python/sysmodule.c

@@ -1160,6 +1160,7 @@ 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) {
@@ -1177,7 +1178,8 @@ 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. */
-    if (tstate->recursion_depth >= new_limit) {
+    mark = _Py_RecursionLimitLowerWaterMark(new_limit);
+    if (tstate->recursion_depth >= mark) {
         _PyErr_Format(tstate, PyExc_RecursionError,
                       "cannot set the recursion limit to %i at "
                       "the recursion depth %i: the limit is too low",