bpo-34588: Fix an off-by-one error in traceback formatting. (GH-9077)

The recursive frame pruning code always undercounted the number of elided frames
by one. That is, in the "[Previous line repeated N more times]" message, N would
always be one too few. Near the recursive pruning cutoff, one frame could be
silently dropped. That situation is demonstrated in the OP of the bug report.

The fix is to start the identical frame counter at 1.

1 files changed, 59 insertions, 2 deletions

diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index bffc03e..8a3aa8a 100644
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -373,7 +373,7 @@ class TracebackFormatTests(unittest.TestCase):
             '    return g(count-1)\n'
             f'  File "{__file__}", line {lineno_g+2}, in g\n'
             '    return g(count-1)\n'
-            '  [Previous line repeated 6 more times]\n'
+            '  [Previous line repeated 7 more times]\n'
             f'  File "{__file__}", line {lineno_g+3}, in g\n'
             '    raise ValueError\n'
             'ValueError\n'
@@ -412,7 +412,7 @@ class TracebackFormatTests(unittest.TestCase):
             '    return h(count-1)\n'
             f'  File "{__file__}", line {lineno_h+2}, in h\n'
             '    return h(count-1)\n'
-            '  [Previous line repeated 6 more times]\n'
+            '  [Previous line repeated 7 more times]\n'
             f'  File "{__file__}", line {lineno_h+3}, in h\n'
             '    g()\n'
         )
@@ -420,6 +420,63 @@ class TracebackFormatTests(unittest.TestCase):
         actual = stderr_h.getvalue().splitlines()
         self.assertEqual(actual, expected)
 
+        # Check the boundary conditions. First, test just below the cutoff.
+        with captured_output("stderr") as stderr_g:
+            try:
+                g(traceback._RECURSIVE_CUTOFF)
+            except ValueError as exc:
+                render_exc()
+            else:
+                self.fail("no error raised")
+        result_g = (
+            f'  File "{__file__}", line {lineno_g+2}, in g\n'
+            '    return g(count-1)\n'
+            f'  File "{__file__}", line {lineno_g+2}, in g\n'
+            '    return g(count-1)\n'
+            f'  File "{__file__}", line {lineno_g+2}, in g\n'
+            '    return g(count-1)\n'
+            f'  File "{__file__}", line {lineno_g+3}, in g\n'
+            '    raise ValueError\n'
+            'ValueError\n'
+        )
+        tb_line = (
+            'Traceback (most recent call last):\n'
+            f'  File "{__file__}", line {lineno_g+71}, in _check_recursive_traceback_display\n'
+            '    g(traceback._RECURSIVE_CUTOFF)\n'
+        )
+        expected = (tb_line + result_g).splitlines()
+        actual = stderr_g.getvalue().splitlines()
+        self.assertEqual(actual, expected)
+
+        # Second, test just above the cutoff.
+        with captured_output("stderr") as stderr_g:
+            try:
+                g(traceback._RECURSIVE_CUTOFF + 1)
+            except ValueError as exc:
+                render_exc()
+            else:
+                self.fail("no error raised")
+        result_g = (
+            f'  File "{__file__}", line {lineno_g+2}, in g\n'
+            '    return g(count-1)\n'
+            f'  File "{__file__}", line {lineno_g+2}, in g\n'
+            '    return g(count-1)\n'
+            f'  File "{__file__}", line {lineno_g+2}, in g\n'
+            '    return g(count-1)\n'
+            '  [Previous line repeated 1 more time]\n'
+            f'  File "{__file__}", line {lineno_g+3}, in g\n'
+            '    raise ValueError\n'
+            'ValueError\n'
+        )
+        tb_line = (
+            'Traceback (most recent call last):\n'
+            f'  File "{__file__}", line {lineno_g+99}, in _check_recursive_traceback_display\n'
+            '    g(traceback._RECURSIVE_CUTOFF + 1)\n'
+        )
+        expected = (tb_line + result_g).splitlines()
+        actual = stderr_g.getvalue().splitlines()
+        self.assertEqual(actual, expected)
+
     def test_recursive_traceback_python(self):
         self._check_recursive_traceback_display(traceback.print_exc)