Author njs
Recipients YoSTEALTH, benjamin.peterson, giampaolo.rodola, martin.panter, njs, pitrou, stutzbach, vstinner, xgdomingo
Date 2019-10-10.22:16:10
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Message-id <1570745770.56.0.312606268417.issue32561@roundup.psfhosted.org>
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> If you wanted to keep async disk access separate from the io module, then what we'd have to do is to create a fork of all the code in the io module, and add this feature to it.

Thinking about this again today, I realized there *might* be another option.

The tricky thing about supporting async file I/O is that users want the whole io module interface, and we don't want to have to reimplement all the functionality in TextIOWrapper, BufferedReader, BufferedWriter, etc. And we still need the blocking functionality too, for when we fall back to threads.

But, here's a possible hack. We could implement our own version of 'FileIO' that wraps around a real FileIO. Every operation just delegates to the underlying FileIO – but with a twist. Something like:

def wrapped_op(self, *args):
    if self._cached_op.key == (op, args):
        return self._cached_op.result
    if MAGIC_THREAD_LOCAL.io_is_forbidden:
        def cache_filler():
            MAGIC_THREAD_LOCAL.io_is_forbidden = False
            self._cached_op = self._real_file.op(*args)
        raise IOForbiddenError(cache_filler)
    return self._real_file.op(*args)

And then in order to implement an async operation, we do something like:

async def op(self, *args):
    while True:
        try:
            # First try fulfilling the operation from cache
            MAGIC_THREAD_LOCAL.io_is_forbidden = True
            return self._io_obj.op(*args)
        except IOForbiddenError as exc:
            # We have to actually hit the disk
            # Run the real IO operation in a thread, then try again
            await in_thread(cache_filler)
        finally:
            del MAGIC_THREAD_LOCAL.io_is_forbidden

This is pretty convoluted: we keep trying the operation on the outer "buffered" object, seeing which low-level I/O operation it gets stuck on, doing that I/O operation, and trying again. There's all kinds of tricky non-local state here; like for example, there isn't any formal guarantee that the next time we try the "outer" I/O operation it will end up making exactly the same request to the "inner" RawIO object. If you try performing I/O operations on the same file from multiple tasks concurrently then you'll get all kinds of havoc. But if it works, then it does have two advantages:

First, it doesn't require changes to the io module, which is at least nice for experimentation.

And second, it's potentially compatible with the io_uring style of async disk I/O API. I don't actually know if this matters; if you look at the io_uring docs, the only reason they say they're more efficient than a thread pool is that they can do the equivalent of preadv(RWF_NOWAIT), and it's a lot easier to add preadv(RWF_NOWAIT) to a thread pool than it is to implement io_uring. But still, this approach is potentially more flexible than my original idea.

We'd still have to reimplement open() in order to set up our weird custom IO stacks, but hopefully that's not *too* bad, since it's mostly just a bunch of if statements to decide which wrappers to stick around the raw IO object.

My big concern is, that I'm not actually sure if this works :-).

The thing is, for this to work, we need TextIOWrapper/BufferedReader/BufferedWriter to be very well-behaved when the underlying operation raises an exception. In particular, if they're doing a complex operation that requires multiple calls to the underlying object, and the second call raises an exception, they need to keep the first call's results in their buffer so that next time they can pick up where they left off. And I have no idea if that's true.

I guess if you squint this is kind of like the non-blocking support in the io module – IOForbiddenError is like NonBlockingError. The big difference is that here, we don't have any "partial success" state at the low-level; either we do the operation immediately, or we punt and do the operation in a thread. Either way it completes as a single indivisible unit. So that might simplify things? From a quick skim of issue13322 it sounds like a lot of the problems with the current "non-blocking" mode come from these partial success states, but I haven't read it in detail.
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Date User Action Args
2019-10-10 22:16:10njssetrecipients: + njs, pitrou, vstinner, giampaolo.rodola, benjamin.peterson, stutzbach, martin.panter, YoSTEALTH, xgdomingo
2019-10-10 22:16:10njssetmessageid: <1570745770.56.0.312606268417.issue32561@roundup.psfhosted.org>
2019-10-10 22:16:10njslinkissue32561 messages
2019-10-10 22:16:10njscreate