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Fix request lookup when frame locals mutate #120

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Fix request lookup when frame locals mutate #120

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11 changes: 7 additions & 4 deletions json_logging/util.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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()):

@qodo-code-review qodo-code-review Bot Aug 12, 2026

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Remediation recommended

1. Extra locals snapshot allocations 🐞 Bug ➹ Performance

get_request_from_call_stack() now allocates tuple(f_locals.items()) for every inspected frame,
creating an O(n) tuple of 2-tuples per frame walked. This increases CPU/memory overhead for
correlation-id lookup during log formatting in FastAPI, where the library falls back to stack
scanning (no global request object).
Agent Prompt 
## Issue description
`RequestUtil.get_request_from_call_stack()` snapshots `tuple(f_locals.items())`, which allocates a tuple plus a 2-tuple per local entry for every frame visited.

## Issue Context
This codepath is exercised from `JSONLogWebFormatter` via `request_util.get_correlation_id(within_formatter=True)`. For FastAPI, `support_global_request_object()` is `False`, so correlation-id lookup may frequently fall back to scanning the call stack.

## Fix Focus Areas
- json_logging/util.py[180-200]

### Suggested approach
Preserve mutation-safety but reduce allocations by snapshotting only values:
- Keep the existing fast-path checks for `request` and `req`.
- Replace `for key, value in tuple(f_locals.items()): ...` with `for value in tuple(f_locals.values()): ...` (and drop the key filter), since you only need to detect any local whose value is an instance of `class_type`.
- Ensure the regression test still passes.

ⓘ Copy this prompt and use it to remediate the issue with your preferred AI generation tools

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:
Expand All @@ -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))
return all(map(lambda pattern: re.search(pattern, path) is None, patterns))
32 changes: 32 additions & 0 deletions tests/test_fastapi.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@
import logging
import pathlib
import re
import sys

import fastapi
import fastapi.testclient
Expand Down Expand Up @@ -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
Expand Down