From 71fe7d65147d2fc66e0d1dd824a291c943243b9d Mon Sep 17 00:00:00 2001
From: ducky_56789 <163338797+duclucky@users.noreply.github.com>
Date: Thu, 13 Aug 2026 06:08:19 +0700
Subject: [PATCH] Fix frame locals mutation during request lookup

---
 json_logging/util.py  | 11 +++++++----
 tests/test_fastapi.py | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 4 deletions(-)

diff --git a/json_logging/util.py b/json_logging/util.py
index c69d98a..45301ea 100644
--- a/json_logging/util.py
+++ b/json_logging/util.py
@@ -187,9 +187,12 @@ def get_request_from_call_stack(self, within_formatter=False):
                 if isinstance(f_locals['req'], class_type):
                     return f_locals['req']
 
-            for key in f_locals:
-                if key not in {'request', 'req'} and isinstance(f_locals[key], class_type):
-                    return f_locals[key]
+            for key, value in tuple(f_locals.items()):
+                if (
+                    key not in {'request', 'req'}
+                    and isinstance(value, class_type)
+                ):
+                    return value
             if f.f_back is not None:
                 f = f.f_back
             else:
@@ -207,4 +210,4 @@ def _get_correlation_id_in_request_header(request_adapter, request):
 
 
 def is_not_match_any_pattern(path, patterns):
-    return all(map(lambda pattern: re.search(pattern, path) is None, patterns))
\ No newline at end of file
+    return all(map(lambda pattern: re.search(pattern, path) is None, patterns))
diff --git a/tests/test_fastapi.py b/tests/test_fastapi.py
index 3cd3493..5dcbedd 100644
--- a/tests/test_fastapi.py
+++ b/tests/test_fastapi.py
@@ -3,6 +3,7 @@
 import logging
 import pathlib
 import re
+import sys
 
 import fastapi
 import fastapi.testclient
@@ -147,6 +148,37 @@ def test_get_correlation_id(client_and_log_handler):
     assert response.json()["correlation_id"] == "abc-def"
 
 
+def test_get_request_from_call_stack_handles_frame_locals_mutation(
+    client_and_log_handler,
+):
+    """Test stack inspection while a tracer changes the frame locals."""
+    import json_logging
+    from json_logging.util import RequestUtil
+
+    mutation_happened = False
+
+    def tracer(frame, event, arg):
+        nonlocal mutation_happened
+        if (
+            frame.f_code is RequestUtil.get_request_from_call_stack.__code__
+            and event == "line"
+            and "key" in frame.f_locals
+            and not mutation_happened
+        ):
+            frame.f_locals["added_by_tracer"] = object()
+            mutation_happened = True
+        return tracer
+
+    previous_trace = sys.gettrace()
+    sys.settrace(tracer)
+    try:
+        assert json_logging._request_util.get_request_from_call_stack() is None
+    finally:
+        sys.settrace(previous_trace)
+
+    assert mutation_happened
+
+
 def test_extra_property(client_and_log_handler):
     """Test adding an extra property to a log message"""
     api_client, handler = client_and_log_handler