skip rendering missing source

Author: davidism

CHANGES.rst

@@ -10,6 +10,8 @@ Unreleased
     :issue:`3065`
 -   Improve CPU usage during Watchdog reloader. :issue:`3054`
 -   `Request.json` annotation is more accurate. :issue:`3067`
+-   Traceback rendering handles when thd line number is beyond the available
+    source lines. :issue:`3044`
 
 
 Version 3.1.3

src/werkzeug/debug/tbtools.py

@@ -420,7 +420,7 @@ def render_line(line: str, cls: str) -> None:
                 f"{arrow if arrow else ''}</pre>"
             )
 
-        if lines:
+        if line_idx < len(lines):
             for line in lines[start_idx:line_idx]:
                 render_line(line, "before")
 

tests/test_debug.py

@@ -1,5 +1,7 @@
+import linecache
 import re
 import sys
+from unittest import mock
 
 import pytest
 
@@ -290,3 +292,20 @@ def test_exception_without_traceback():
         # filter_hidden_frames should skip this since it has no traceback
         e.__context__ = Exception("msg2")
         DebugTraceback(e)
+
+
+@mock.patch.object(linecache, "getlines", autospec=True)
+def test_missing_source_lines(mock_getlines: mock.Mock) -> None:
+    """Rendering doesn't fail when the line number is beyond the available
+    source lines.
+    """
+    mock_getlines.return_value = ["truncated"]
+
+    try:
+        raise ValueError()
+    except ValueError as e:
+        tb = DebugTraceback(e)
+
+    html = tb.render_traceback_html()
+    assert "test_debug.py" in html
+    assert "truncated" not in html