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.

4 files changed, 96 insertions, 26 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)
 
diff --git a/Lib/traceback.py b/Lib/traceback.py
index afab0a4..4e7605d 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -310,6 +310,8 @@ def walk_tb(tb):
         tb = tb.tb_next
 
 
+_RECURSIVE_CUTOFF = 3 # Also hardcoded in traceback.c.
+
 class StackSummary(list):
     """A stack of frames."""
 
@@ -398,18 +400,21 @@ class StackSummary(list):
         last_name = None
         count = 0
         for frame in self:
-            if (last_file is not None and last_file == frame.filename and
-                last_line is not None and last_line == frame.lineno and
-                last_name is not None and last_name == frame.name):
-                count += 1
-            else:
-                if count > 3:
-                    result.append(f'  [Previous line repeated {count-3} more times]\n')
+            if (last_file is None or last_file != frame.filename or
+                last_line is None or last_line != frame.lineno or
+                last_name is None or last_name != frame.name):
+                if count > _RECURSIVE_CUTOFF:
+                    count -= _RECURSIVE_CUTOFF
+                    result.append(
+                        f'  [Previous line repeated {count} more '
+                        f'time{"s" if count > 1 else ""}]\n'
+                    )
                 last_file = frame.filename
                 last_line = frame.lineno
                 last_name = frame.name
                 count = 0
-            if count >= 3:
+            count += 1
+            if count > _RECURSIVE_CUTOFF:
                 continue
             row = []
             row.append('  File "{}", line {}, in {}\n'.format(
@@ -420,8 +425,12 @@ class StackSummary(list):
                 for name, value in sorted(frame.locals.items()):
                     row.append('    {name} = {value}\n'.format(name=name, value=value))
             result.append(''.join(row))
-        if count > 3:
-            result.append(f'  [Previous line repeated {count-3} more times]\n')
+        if count > _RECURSIVE_CUTOFF:
+            count -= _RECURSIVE_CUTOFF
+            result.append(
+                f'  [Previous line repeated {count} more '
+                f'time{"s" if count > 1 else ""}]\n'
+            )
         return result
 
 
diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-09-05-22-56-52.bpo-34588.UIuPmL.rst b/Misc/NEWS.d/next/Core and Builtins/2018-09-05-22-56-52.bpo-34588.UIuPmL.rst
new file mode 100644
index 0000000..ec7a57f
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2018-09-05-22-56-52.bpo-34588.UIuPmL.rst
@@ -0,0 +1,2 @@
+Fix an off-by-one in the recursive call pruning feature of traceback
+formatting.
diff --git a/Python/traceback.c b/Python/traceback.c
index 21fb034..5b5c715 100644
--- a/Python/traceback.c
+++ b/Python/traceback.c
@@ -511,16 +511,21 @@ tb_displayline(PyObject *f, PyObject *filename, int lineno, PyObject *name)
     return err;
 }
 
+static const int TB_RECURSIVE_CUTOFF = 3; // Also hardcoded in traceback.py.
+
 static int
 tb_print_line_repeated(PyObject *f, long cnt)
 {
-    int err;
+    cnt -= TB_RECURSIVE_CUTOFF;
     PyObject *line = PyUnicode_FromFormat(
-            "  [Previous line repeated %ld more times]\n", cnt-3);
+        (cnt > 1)
+          ? "  [Previous line repeated %ld more times]\n"
+          : "  [Previous line repeated %ld more time]\n",
+        cnt);
     if (line == NULL) {
         return -1;
     }
-    err = PyFile_WriteObject(line, f, Py_PRINT_RAW);
+    int err = PyFile_WriteObject(line, f, Py_PRINT_RAW);
     Py_DECREF(line);
     return err;
 }
@@ -544,15 +549,11 @@ tb_printinternal(PyTracebackObject *tb, PyObject *f, long limit)
         tb = tb->tb_next;
     }
     while (tb != NULL && err == 0) {
-        if (last_file != NULL &&
-            tb->tb_frame->f_code->co_filename == last_file &&
-            last_line != -1 && tb->tb_lineno == last_line &&
-            last_name != NULL && tb->tb_frame->f_code->co_name == last_name)
-        {
-            cnt++;
-        }
-        else {
-            if (cnt > 3) {
+        if (last_file == NULL ||
+            tb->tb_frame->f_code->co_filename != last_file ||
+            last_line == -1 || tb->tb_lineno != last_line ||
+            last_name == NULL || tb->tb_frame->f_code->co_name != last_name) {
+            if (cnt > TB_RECURSIVE_CUTOFF) {
                 err = tb_print_line_repeated(f, cnt);
             }
             last_file = tb->tb_frame->f_code->co_filename;
@@ -560,7 +561,8 @@ tb_printinternal(PyTracebackObject *tb, PyObject *f, long limit)
             last_name = tb->tb_frame->f_code->co_name;
             cnt = 0;
         }
-        if (err == 0 && cnt < 3) {
+        cnt++;
+        if (err == 0 && cnt <= TB_RECURSIVE_CUTOFF) {
             err = tb_displayline(f,
                                  tb->tb_frame->f_code->co_filename,
                                  tb->tb_lineno,
@@ -571,7 +573,7 @@ tb_printinternal(PyTracebackObject *tb, PyObject *f, long limit)
         }
         tb = tb->tb_next;
     }
-    if (err == 0 && cnt > 3) {
+    if (err == 0 && cnt > TB_RECURSIVE_CUTOFF) {
         err = tb_print_line_repeated(f, cnt);
     }
     return err;