bpo-34588: Fix an off-by-one error in traceback formatting.

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.

diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index bffc03e663ff787f29c0bb9c8fcb9ace7586bc09..8a3aa8a8648f86c5aa1d3a42b23db1b3a413ed87 100644 (file)
--- 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 afab0a4b91f1cca93b052f2dac34e5401070092f..9f8aee1a9912c7f952e5e5e12e6222bbc8c59a4e 100644 (file)
--- 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 +425,11 @@ 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 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 (file)
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 21fb034160022ba83a6680d51b649cc5a51ee55a..5b5c715a0f4499c748a46f072462fbd6080656cb 100644 (file)
--- 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;