Bug report

Bug description:

Summary

Cancelling loop.sendfile() while its native implementation is waiting for the transport's write buffer to become empty leaves the transport in a partially modified state. The transport remains paused and retains its internal empty-buffer waiter. The selector implementation additionally leaves the transport absent from the event loop's transport registry.

The cancellation window exists in both BaseSelectorEventLoop._sendfile_native() and BaseProactorEventLoop._sendfile_native().

Reproduction Code

import asyncio


loop = asyncio.ProactorEventLoop()
asyncio.set_event_loop(loop)


class Transport:
    _sendfile_compatible = asyncio.constants._SendfileMode.TRY_NATIVE
    _sock = None

    def __init__(self):
        self.paused = False
        self.resumed = False
        self.empty_waiter = None

    def is_closing(self):
        return False

    def is_reading(self):
        return True

    def pause_reading(self):
        self.paused = True

    def resume_reading(self):
        self.resumed = True

    def _make_empty_waiter(self):
        self.empty_waiter = loop.create_future()
        return self.empty_waiter

    def _reset_empty_waiter(self):
        self.empty_waiter = None


async def main():
    transport = Transport()
    task = asyncio.create_task(loop.sendfile(transport, None))
    await asyncio.sleep(0)
    task.cancel()
    await asyncio.gather(task, return_exceptions=True)
    print(
        transport.paused,
        transport.resumed,
        transport.empty_waiter is not None,
    )


loop.run_until_complete(main())
loop.close()

Actual Behavior

The controlled transport is paused but never resumed, and its waiter is not reset:

True False True

On the selector implementation, the same cancellation point also leaves transport._sock_fd absent from loop._transports.

Expected Behavior

Cancellation should propagate without leaving the transport in a partially modified state. Reading should be restored when it was active before sendfile(), the empty-buffer waiter should be reset, and the selector implementation should restore the transport registry entry.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-157025: Fix transport cleanup when cancelling asyncio native sendfile #157027