Issue #5392: when a very low recursion limit was set, the interpreter would

abort with a fatal error after the recursion limit was hit twice.

Include/ceval.h

@@ -92,11 +92,10 @@ PyAPI_DATA(int) _Py_CheckRecursionLimit;
 #  define _Py_MakeRecCheck(x)  (++(x) > _Py_CheckRecursionLimit)
 #endif
 
-#ifdef USE_STACKCHECK
-#  define _Py_MakeEndRecCheck(x)  (--(x) < _Py_CheckRecursionLimit - 50)
-#else
-#  define _Py_MakeEndRecCheck(x)  (--(x) < _Py_CheckRecursionLimit - 50)
-#endif
+#define _Py_MakeEndRecCheck(x) \
+	(--(x) < ((_Py_CheckRecursionLimit > 100) \
+		? (_Py_CheckRecursionLimit - 50) \
+		: (3 * (_Py_CheckRecursionLimit >> 2))))
 
 #define Py_ALLOW_RECURSION \
   do { unsigned char _old = PyThreadState_GET()->recursion_critical;\

Lib/test/test_sys.py

@@ -2,6 +2,8 @@
 import unittest, test.support
 import sys, io, os
 import struct
+import subprocess
+import textwrap
 
 class SysModuleTest(unittest.TestCase):
 
@@ -155,6 +157,46 @@ class SysModuleTest(unittest.TestCase):
         self.assertEqual(sys.getrecursionlimit(), 10000)
         sys.setrecursionlimit(oldlimit)
 
+    def test_recursionlimit_recovery(self):
+        # NOTE: this test is slightly fragile in that it depends on the current
+        # recursion count when executing the test being low enough so as to
+        # trigger the recursion recovery detection in the _Py_MakeEndRecCheck
+        # macro (see ceval.h).
+        oldlimit = sys.getrecursionlimit()
+        def f():
+            f()
+        try:
+            for i in (50, 1000):
+                # Issue #5392: stack overflow after hitting recursion limit twice
+                sys.setrecursionlimit(i)
+                self.assertRaises(RuntimeError, f)
+                self.assertRaises(RuntimeError, f)
+        finally:
+            sys.setrecursionlimit(oldlimit)
+
+    def test_recursionlimit_fatalerror(self):
+        # A fatal error occurs if a second recursion limit is hit when recovering
+        # from a first one.
+        code = textwrap.dedent("""
+            import sys
+
+            def f():
+                try:
+                    f()
+                except RuntimeError:
+                    f()
+
+            sys.setrecursionlimit(%d)
+            f()""")
+        for i in (50, 1000):
+            sub = subprocess.Popen([sys.executable, '-c', code % i],
+                stderr=subprocess.PIPE)
+            err = sub.communicate()[1]
+            self.assertTrue(sub.returncode, sub.returncode)
+            self.assertTrue(
+                b"Fatal Python error: Cannot recover from stack overflow" in err,
+                err)
+
     def test_getwindowsversion(self):
         if hasattr(sys, "getwindowsversion"):
             v = sys.getwindowsversion()

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 3.1 alpha 2?
 Core and Builtins
 -----------------
 
+- Issue #5392: when a very low recursion limit was set, the interpreter would
+  abort with a fatal error after the recursion limit was hit twice.
+
 Library
 -------
 
@@ -24,8 +27,6 @@ What's New in Python 3.1 alpha 1
 Core and Builtins
 -----------------
 
-=======
-
 - The io module has been reimplemented in C for speed.
 
 - Give dict views an informative __repr__.