Need a way to protect a region from cancellation (like "masking interrupts") #96

Here's an example of a simple bidirectional proxy using curio: https://gist.github.com/njsmith/4b4e75a9ab9e975705412f8f8e866948

The core idea is that we spawn two tasks to handle the copying in each direction:

async def copy_all_bidir(sock1, sock2):
    await all_or_none([
        await curio.spawn(copy_all(sock1, sock2)),
        await curio.spawn(copy_all(sock2, sock1)),
])

The tricky part is managing these tasks. Basically with regards to exceptions/timeouts/cancellation/etc., we want it to act like the two copy_all tasks are both "part of" the copy_all_bidir task. So:

• If one of the tasks raises an exception, the other task should immediately be canceled, and the exception should propagate out of copy_all_bidir
• If both of the tasks complete, then copy_all_bidir should complete successfully
• If copy_all_bidir is cancelled, then both tasks should be canceled as part of copy_all_bidir's cleanup

The all_or_none helper there tries to do this, but AFAICT it can't quite achieve the last requirement given curio's current APIs. The problem sequence is:

1. One of the tasks raises an exception
2. all_or_none catches this exception, and starts to cancel the other task
3. all_or_none itself gets canceled, thus canceling the cancellation and making everyone's head explode (and also leaking the other task)

In general this is a problem for cleanup code -- once you're executing the cancellation cleanup path you really don't want to get canceled again!

My suggestion for what it should look like is:

async def all_or_none(tasks):
    try:
        async for task in curio.wait(tasks):
            await task.join()
    except:
        async with curio.defer_cancellation:
            for task in tasks:
                await task.cancel()
        raise

except that curio.defer_cancellation isn't a thing that exists right now.

So this issue is to propose that curio.defer_cancellation be a thing that exists.

(Guess how I'm distracting myself from election news!)