Format frame locals when captured

Before this commit, if ENABLE_STACKTRACES_LOCALS was True, each stack
frame's locals dict would be stored directly in the captured stack
trace, and only converted to a string once the stack trace was rendered.
This means that for panels, such as the SQL Panel, which do not render
the stack traces until later after they have all been captured, it is
quite possible that the values in the locals dicts at the point when the
stack trace was rendered would no longer be the same as they were at the
time when the stack trace was captured, resulting in incorrect values
being displayed to the user.

Fix by converting the locals dict to a string immediately when it is
captured (in the get_stack_trace() function as well as in the deprecated
tidy_stacktrace() function) instead of in the render_stacktrace()
function.

Author: living180

debug_toolbar/utils.py

@@ -3,7 +3,7 @@
 import os.path
 import sys
 import warnings
-from pprint import pformat
+from pprint import PrettyPrinter, pformat
 from typing import Any, Dict, List, Optional, Sequence, Tuple, Union
 
 from asgiref.local import Local
@@ -62,7 +62,7 @@ def tidy_stacktrace(stack: List[stubs.InspectStack]) -> stubs.TidyStackTrace:
             continue
         text = "".join(text).strip() if text else ""
         frame_locals = (
-            frame.f_locals
+            pformat(frame.f_locals)
             if dt_settings.get_config()["ENABLE_STACKTRACES_LOCALS"]
             else None
         )
@@ -99,7 +99,7 @@ def render_stacktrace(trace: stubs.TidyStackTrace) -> SafeString:
         if show_locals:
             html += format_html(
                 '  <pre class="djdt-locals">{}</pre>\n',
-                pformat(locals_),
+                locals_,
             )
         html += "\n"
     return mark_safe(html)
@@ -266,6 +266,8 @@ def _stack_frames(*, skip=0):
 
 
 class _StackTraceRecorder:
+    pretty_printer = PrettyPrinter()
+
     def __init__(self):
         self.filename_cache = {}
 
@@ -315,7 +317,10 @@ def get_stack_trace(
             else:
                 source_line = ""
 
-            frame_locals = frame.f_locals if include_locals else None
+            if include_locals:
+                frame_locals = self.pretty_printer.pformat(frame.f_locals)
+            else:
+                frame_locals = None
 
             trace.append((filename, line_no, func_name, source_line, frame_locals))
         trace.reverse()

docs/changes.rst

@@ -15,6 +15,11 @@ Pending
   environments (but not others). The maintainers judged that time and effort is
   better invested elsewhere.
 * Added support for psycopg3.
+* When ``ENABLE_STACKTRACE_LOCALS`` is ``True``, the stack frames' locals dicts
+  will be converted to strings when the stack trace is captured rather when it
+  is rendered, so that the correct values will be displayed in the rendered
+  stack trace, as they may have changed between the time the stack trace was
+  captured and when it is rendered.
 
 3.8.1 (2022-12-03)
 ------------------

docs/spelling_wordlist.txt

@@ -3,6 +3,7 @@ backends
 backported
 checkbox
 contrib
+dicts
 django
 fallbacks
 flamegraph

tests/test_utils.py

@@ -80,3 +80,26 @@ def test_deprecated_functions(self):
         with self.assertWarns(DeprecationWarning):
             stack_trace = tidy_stacktrace(reversed(stack))
         self.assertEqual(stack_trace[-1][0], __file__)
+
+    @override_settings(DEBUG_TOOLBAR_CONFIG={"ENABLE_STACKTRACES_LOCALS": True})
+    def test_locals(self):
+        # This wrapper class is necessary to mask the repr() of the list
+        # returned by get_stack_trace(); otherwise the 'test_locals_value_1'
+        # string will also be present in rendered_stack_2.
+        class HideRepr:
+            def __init__(self, value):
+                self.value = value
+
+        x = "test_locals_value_1"
+        stack_1_wrapper = HideRepr(get_stack_trace())
+
+        x = x.replace("1", "2")
+        stack_2_wrapper = HideRepr(get_stack_trace())
+
+        rendered_stack_1 = render_stacktrace(stack_1_wrapper.value)
+        self.assertIn("test_locals_value_1", rendered_stack_1)
+        self.assertNotIn("test_locals_value_2", rendered_stack_1)
+
+        rendered_stack_2 = render_stacktrace(stack_2_wrapper.value)
+        self.assertNotIn("test_locals_value_1", rendered_stack_2)
+        self.assertIn("test_locals_value_2", rendered_stack_2)