commit 49020174305ca3dc90a811b03a05f44873297c61
author Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com> Mon Sep 10 09:00:08 2018 -0700
committer GitHub <noreply@github.com> Mon Sep 10 09:00:08 2018 -0700
tree fd2e3a946864286ffff75559a8a5d93ab4f36d68
parent d53f1cabe8837697df4acb70c9c6537461b5eeda

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.
(cherry picked from commit d545869d084e70d4838310e79b52a25a72a1ca56)

Co-authored-by: Benjamin Peterson <benjamin@python.org>

Python/traceback.c

diff --git a/Python/traceback.c b/Python/traceback.c
index b00864b..95bef64 100644
--- a/Python/traceback.c
+++ b/Python/traceback.c
@@ -511,16 +511,21 @@
     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 = 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 @@
             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 = tb->tb_next;
     }
-    if (err == 0 && cnt > 3) {
+    if (err == 0 && cnt > TB_RECURSIVE_CUTOFF) {
         err = tb_print_line_repeated(f, cnt);
     }
     return err;