bpo-45753: Make recursion checks more efficient. (GH-29524)

* Uses recursion remaining, instead of recursion depth to speed up check against recursion limit.

1 files changed, 5 insertions, 11 deletions

diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 27937a0..3e2091e 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -1187,20 +1187,14 @@ sys_setrecursionlimit_impl(PyObject *module, int new_limit)
         return NULL;
     }
 
-    /* Issue #25274: When the recursion depth hits the recursion limit in
-       _Py_CheckRecursiveCall(), the overflowed flag of the thread state is
-       set to 1 and a RecursionError is raised. The overflowed flag is reset
-       to 0 when the recursion depth goes below the low-water mark: see
-       Py_LeaveRecursiveCall().
-
-       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) {
+    /* Reject too low new limit if the current recursion depth is higher than
+       the new low-water mark. */
+    int depth = tstate->recursion_limit - tstate->recursion_remaining;
+    if (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",
-                      new_limit, tstate->recursion_depth);
+                      new_limit, depth);
         return NULL;
     }