context variables can leak out of asyncio.Task

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:

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

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.

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

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

The fix is merged in main branch, I'll prefer to wait for some time before backporting this to older branch just to makes sure that if any bug pops up it only affects main branch.

Which older branch?

3.14 and 3.13 branches

Makes sense. Thanks for the PR. :)

Merge remote-tracking branch 'upstream/main' into fix-pythongh-138577

Add `--reset-contextvars` flag to isolate ASGI request context

pythongh-140947: fix contextvars handling for server tasks in asyncio (…

After much thought I think backporting this is too risky as it a large behavior change of old code. Keeping this for 3.15+.

Resume reads in the connection's context

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context variables can leak out of asyncio.Task #140947

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context variables can leak out of asyncio.Task #140947

Description

Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

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kumaraditya303 commented on Dec 14, 2025

pmeier commented on Dec 14, 2025

kumaraditya303 commented on Mar 21, 2026

Kludex commented on Mar 21, 2026

kumaraditya303 commented on Mar 21, 2026

Kludex commented on Mar 21, 2026

kumaraditya303 commented on Jul 23, 2026

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