context variables can leak out of asyncio.Task

[pmeier]

Bug report

Bug description:

This is a minimal reproducer for Kludex/uvicorn#2167. TL;DR when using asyncio as event loop, users of uvicorn were seeing polluted contexts after sending in large payloads. For those uvicorn paused reading on the main thread and resumed it in a task.

import asyncio
import contextvars
import sys

cvar1 = contextvars.ContextVar("cvar1")
cvar2 = contextvars.ContextVar("cvar2")
cvar3 = contextvars.ContextVar("cvar3")


def print_diagnostics(label):
 task = t.get_name() if (t := asyncio.current_task()) else None
 context = {c.name: v for c, v in contextvars.copy_context().items()}
 print(f"{label}: {task=}, {context=}\n{'-' * 80}")


class DemoProtocol(asyncio.Protocol):
 def __init__(self, on_conn_lost):
 self.transport = None
 self.on_conn_lost = on_conn_lost
 self.tasks = set()

 def connection_made(self, transport):
 print_diagnostics("connection_made")

 self.transport = transport

 def data_received(self, data):
 print_diagnostics("data_received")

 task = asyncio.create_task(self.asgi())
 self.tasks.add(task)
 task.add_done_callback(self.tasks.discard)

 self.transport.pause_reading()

 def connection_lost(self, exc):
 print_diagnostics("connection_lost")
 if not self.on_conn_lost.done():
 self.on_conn_lost.set_result(True)

 async def asgi(self):
 print_diagnostics("asgi start")

 cvar1.set(True)

 # make sure that we only resume after the pause
 # otherwise the resume does nothing
 while not self.transport._paused:
 await asyncio.sleep(0.1)

 cvar2.set(True)

 self.transport.resume_reading()

 cvar3.set(True)

 print_diagnostics("asgi end")


async def main():
 print(f"Python: {sys.version}\n{'-' * 80}")

 loop = asyncio.get_running_loop()
 on_conn_lost = loop.create_future()

 host, port = "127.0.0.1", 8888

 async with await loop.create_server(lambda: DemoProtocol(on_conn_lost), host, port):
 reader, writer = await asyncio.open_connection(host, port)
 writer.write(b"anything")
 await writer.drain()
 writer.close()
 await writer.wait_closed()
 await on_conn_lost


if __name__ == "__main__":
 asyncio.run(main())

Python: 3.14.0 (main, Oct 14 2025, 21:27:55) [Clang 20.1.4 ]
--------------------------------------------------------------------------------
connection_made: task=None, context={}
--------------------------------------------------------------------------------
data_received: task=None, context={}
--------------------------------------------------------------------------------
asgi start: task='Task-4', context={}
--------------------------------------------------------------------------------
asgi end: task='Task-4', context={'cvar2': True, 'cvar3': True, 'cvar1': True}
--------------------------------------------------------------------------------
connection_lost: task=None, context={'cvar2': True, 'cvar1': True}
--------------------------------------------------------------------------------

The asgi task sets three context variables:

1. cvar1 is set at the beginning of the function, which may or may not be before the reading is paused.
2. cvar2 is set after the reading is paused.
3. cvar3 is set after the reading is resumed.

The context variables that have been set in the task before the reading is resumed (cvar1 and cvar2) leak out of the task into the main thread.

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-140947: fix contextvars handling for server tasks in asyncio  #141158

[kumaraditya303]

Output on Windows:

❯ ./python.bat main.py 
Running Debug|x64 interpreter...
Python: 3.15.0a2+ (heads/main:af7cca3c39d, Dec 14 2025, 16:45:59) [MSC v.1944 64 bit (AMD64)]
--------------------------------------------------------------------------------
connection_made: task=None, context={}
--------------------------------------------------------------------------------
data_received: task=None, context={}
--------------------------------------------------------------------------------
asgi start: task='Task-5', context={}
--------------------------------------------------------------------------------
asgi end: task='Task-5', context={'cvar3': True, 'cvar1': True, 'cvar2': True}
--------------------------------------------------------------------------------
connection_lost: task=None, context={}
---------------------------------------

The context of connection_lost is empty which suggests that this bug is not present in windows implementation, still I'll double check it once I add tests on the PR.

[pmeier]

Both the bug report as well as the regression tests in uvicorn also suggest that Windows is not affected.