[swift-evolution] [Concurrency] Fixing race conditions in async/await example
Howard Lovatt
howard.lovatt at gmail.com
Sun Aug 27 23:20:12 CDT 2017
Yes I do think the async keyword to start execution without waiting is an
improvement, but would still like to have cancel, thread control, timeout,
and improved debugging.
-- Howard.
On 28 August 2017 at 13:21, Jonathan Hull <jhull at gbis.com> wrote:
> What do you think of the idea that several of us have been proposing on
> another thread? In addition to being able to place ‘await' before an
> asynchronous function, you could instead write ‘async’ which would simply
> allow you to defer calling await (presumably until you need to await
> multiple items at once). You would be required by the compiler to call
> ‘await’ before being able to access the resulting value.
>
> let a = async longCalculationA()
> let b = async longCalculationB() //b doesn’t wait for a to complete before
> starting
> let c = async longCalculationC() //c doesn’t wait for a or b
> let result = await combineCalculations(a: a, b: b, c: c) //waits until a,
> b, and c are all available
>
> I think this would give us 80% of the value of futures, but with added
> benefits:
>
> • *It* *allows* *optimizations which futures do not* - One of swift’s
> core goals is to have the resulting program be as fast/performant as
> possible. The compiler has a lot of freedom here to optimize as it sees
> fit, because it doesn’t have to create a programmer inspectable object.
> The above could be implemented by the compiler behind the scenes using some
> sort of future, but it could also just combine the continuation blocks in a
> different way. It can provide different optimizations in different
> situations.
>
> • *It is forward compatible* - Even if we just implement this by creating
> internal futures, because we don’t expose those details, that
> implementation can be changed at any time without involving evolution.
>
> • *It works with any async function* - Futures have a different type, and
> so any function returning a future (especially in a chain of returns) must
> explicitly return Future<Type>. The proposal above returns the normal type
> (with a compiler annotation forcing await to be called before it is used).
> As a result ‘async’ can be used anywhere ‘await’ can be used, even in a
> chain of ‘async’ returns. The guarantee we are making is that the result
> will not be used without calling ‘await’.
>
> • *It may make actual futures more efficient* - None of the points above
> are arguing against having futures in Swift. I want them in the standard
> library! With the above as a base, it should be fairly simple to build
> futures as a framework with all of introspection and cancellation ability
> that implies. Those futures should also gain some of the optimizations of
> that base as a result (especially when there are intermediate stages to a
> long calculation, etc…).
>
> Thanks,
> Jon
>
> On Aug 27, 2017, at 7:22 PM, Adam Kemp via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> As has been explained, futures can be built on top of async/await (or the
> other way around). You can have the best of both worlds. We are not losing
> anything by having this feature. It would be a huge improvement to have
> this as an option.
>
> However, using futures correctly requires more nested closures than you
> have shown in your examples to avoid blocking any threads. That's why
> you're not seeing the advantage to async/await. You're comparing examples
> that have very different behaviors.
>
> That said, I have also expressed my opinion that it is better to build
> async/await on top of futures rather than the other way around. I believe
> it is more powerful and cleaner to make async/await work with any arbitrary
> future type (via a protocol). The alternative (building futures on top of
> async/await) requires more code when the two are mixed. I very much prefer
> how it's done in C#, where you can freely mix the two models without having
> to resort to ad-hoc wrappers, and you can use async/await with any futures
> implementation you might already be using.
>
> I really think we should be having more discussion about the tradeoffs
> between those two approaches, and I'm concerned that some of the opinions
> about how C# does it are not based on a clear and accurate understanding of
> how it actually works in that language.
>
> --
> Adam Kemp
>
> On Aug 27, 2017, at 6:02 PM, Howard Lovatt <howard.lovatt at gmail.com>
> wrote:
>
> The async/await is very similar to the proposed Future (as I posed
> earlier) with regard to completion-handler code, they both re-write the
> imported completion-handler function using a closure, the relevant sentence
> from the Async Proposal is:
>
> "Under the hood, the compiler rewrites this code using nested closures ..."
>
>
> Unlike the proposed future code the async code is not naturally parallel,
> in the running example the following lines from the async code are run in
> series, i.e. await blocks:
>
> let dataResource = await loadWebResource("dataprofile.txt")
> let imageResource = await loadWebResource("imagedata.dat")
>
> The equivalent lines using the proposed Future:
>
> let dataResource = loadWebResource("dataprofile.txt")
> let imageResource = loadWebResource("imagedata.dat")
>
> Run in parallel and therefore are potentially faster assuming that
> resources, like cores and IO, are available.
>
> Therefore you would be better using a Future than an async, so why provide
> an async unless you can make a convincing argument that it allows you to
> write a better future?
>
> -- Howard.
>
> On 28 August 2017 at 09:59, Adam Kemp <adam.kemp at apple.com> wrote:
>
>> This example still has nested closures (to create a Future), and still
>> relies on a synchronous get method that will block a thread. Async/await
>> does not require blocking any threads.
>>
>> I’m definitely a fan of futures, but this example isn’t even a good
>> example of using futures. If you’re using a synchronous get method then
>> you’re not using futures properly. They’re supposed to make it easy to
>> avoid writing blocking code. This example just does the blocking call on
>> some other thread.
>>
>> Doing it properly would show the benefits of async/await because it would
>> require more nesting and more complex error handling. By simplifying the
>> code you’ve made a comparison between proper asynchronous code (with
>> async/await) and improper asynchronous code (your example).
>>
>> That tendency to want to just block a thread to make it easier is exactly
>> why async/await is so valuable. You get simple code while still doing it
>> correctly.
>>
>> --
>> Adam Kemp
>>
>> On Aug 27, 2017, at 4:00 PM, Howard Lovatt via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>> The running example used in the white paper coded using a Future is:
>>
>> func processImageData1() -> Future<Image> {
>> return AsynchronousFuture { _ -> Image in
>> let dataResource = loadWebResource("dataprofile.txt") //
>> dataResource and imageResource run in parallel.
>> let imageResource = loadWebResource("imagedata.dat")
>> let imageTmp = decodeImage(dataResource.get ??
>> Resource(path: "Default data resource or prompt user"), imageResource.get
>> ?? Resource(path: "Default image resource or prompt user"))
>> let imageResult = dewarpAndCleanupImage(imageTmp.get ??
>> Image(dataPath: "Default image or prompt user", imagePath: "Default image
>> or prompt user"))
>> return imageResult.get ?? Image(dataPath: "Default image or
>> prompt user", imagePath: "Default image or prompt user")
>> }
>> }
>>
>> This also avoids the pyramid of doom; the pyramid is avoided by
>> converting continuation-handlers into either a sync or future, i.e. it is
>> the importer that eliminates the nesting by translating the code
>> automatically.
>>
>> This example using Future also demonstrates three advantages of Future:
>> they are naturally parallel (dataResource and imageResource lines run in
>> parallel), they timeout automatically (get returns nil if the Future has
>> taken too long), and if there is a failure (for any reason including
>> timeout) it provides a method of either detecting the failure or providing
>> a default (get returns nil on failure).
>>
>> There are a three of other advantages a Future has that this example
>> doesn’t show: control over which thread the Future runs on, Futures can be
>> cancelled, and debugging information is available.
>>
>> You could imagine `async` as a syntax sugar for Future, e.g. the above
>> Future example could be:
>>
>> func processImageData1() async -> Image {
>> let dataResource = loadWebResource("dataprofile.txt") //
>> dataResource and imageResource run in parallel.
>> let imageResource = loadWebResource("imagedata.dat")
>> let imageTmp = decodeImage(dataResource.get ?? Resource(path:
>> "Default data resource or prompt user"), imageResource.get ??
>> Resource(path: "Default image resource or prompt user"))
>> let imageResult = dewarpAndCleanupImage(imageTmp.get ??
>> Image(dataPath: "Default image or prompt user", imagePath: "Default image
>> or prompt user"))
>> return imageResult.get ?? Image(dataPath: "Default image or prompt
>> user", imagePath: "Default image or prompt user")
>> }
>>
>> Since an async is sugar for Future the async runs as soon as it is
>> created (as soon as the underlying Future is created) and get returns an
>> optional (also cancel and status would be still be present). Then if you
>> want control over threads and timeout they could be arguments to async:
>>
>> func processImageData1() async(queue: DispatchQueue.main, timeout:
>> .seconds(5)) -> Image { ... }
>>
>> On Sat, 26 Aug 2017 at 11:00 pm, Florent Vilmart <florent at flovilmart.com>
>> wrote:
>>
>>> Howard, with async / await, the code is flat and you don’t have to
>>> unowned/weak self to prevent hideous cycles in the callbacks.
>>> Futures can’t do that
>>>
>>> On Aug 26, 2017, 04:37 -0400, Goffredo Marocchi via swift-evolution <
>>> swift-evolution at swift.org>, wrote:
>>>
>>> With both he now built in promises in Node8 as well as libraries like
>>> Bluebird there was ample time to evaluate them and convert/auto convert at
>>> times libraries that loved callback pyramids of doom when the flow grows
>>> complex into promise based chains. Converting to Promises seems magical for
>>> the simple case, but can quickly descend in hard to follow flows and hard
>>> to debug errors when you move to non trivial multi path scenarios. JS is
>>> now solving it with their implementation of async/await, but the point is
>>> that without the full picture any single solution would break horribly in
>>> real life scenarios.
>>>
>>> Sent from my iPhone
>>>
>>> On 26 Aug 2017, at 06:27, Howard Lovatt via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>
>>> My argument goes like this:
>>>
>>> 1. You don't need async/await to write a powerful future type; you can
>>> use the underlying threads just as well, i.e. future with async/await is no
>>> better than future without.
>>>
>>> 2. Since future is more powerful, thread control, cancel, and timeout,
>>> people should be encouraged to use this; instead because async/await are
>>> language features they will be presumed, incorrectly, to be the best way,
>>> consequently people will get into trouble with deadlocks because they don't
>>> have control.
>>>
>>> 3. async/await will require some engineering work and will at best
>>> make a mild syntax improvement and at worst lead to deadlocks, therefore
>>> they just don't carry their weight in terms of useful additions to Swift.
>>>
>>> Therefore, save some engineering effort and just provide a future
>>> library.
>>>
>>> To turn the question round another way, in two forms:
>>>
>>> 1. What can async/wait do that a future can't?
>>>
>>> 2. How will future be improved if async/await is added?
>>>
>>>
>>> -- Howard.
>>>
>>> On 26 August 2017 at 02:23, Joe Groff <jgroff at apple.com> wrote:
>>>
>>>>
>>>> On Aug 25, 2017, at 12:34 AM, Howard Lovatt <howard.lovatt at gmail.com>
>>>> wrote:
>>>>
>>>> In particular a future that is cancellable is more powerful that the
>>>> proposed async/await.
>>>>
>>>>
>>>> It's not more powerful; the features are to some degree disjoint. You
>>>> can build a Future abstraction and then use async/await to sugar code that
>>>> threads computation through futures. Getting back to Jakob's example,
>>>> someone (maybe the Clang importer, maybe Apple's framework developers in an
>>>> overlay) will still need to build infrastructure on top of IBActions and
>>>> other currently ad-hoc signalling mechanisms to integrate them into a more
>>>> expressive coordination framework.
>>>>
>>>> -Joe
>>>>
>>>
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>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>> --
>> -- Howard.
>>
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