[swift-evolution] [Concurrency] Fixing race conditions in async/await example

Howard Lovatt howard.lovatt at gmail.com
Tue Aug 29 20:02:39 CDT 2017 

@David,

The signatures would be:


func processImage(_ image: Future<Image>) -> Future<Image>

func translate(_ text: Future<String>) -> Future<Image>


Inside `processImage` and `translate` you would `get` the values at the
point were needed so that downloadImage and downloadText run in parallel
(which is highly desirable).


  -- Howard.

On 30 August 2017 at 07:21, David Hart <david at hartbit.com> wrote:

> I don’t think the examples are 100% equivalent. In your version with the
> Future library, *preprocessImage* and *translate* need to accept futures
> as argument, correct? That’s more restrictive than in my example code where
> async/await specifically provide sugar over *then*. Plus I don’t
> understand why you mention that the Future version handles errors when
> async/await also plays very nicely with errors.
>
> On 29 Aug 2017, at 10:22, Howard Lovatt <howard.lovatt at gmail.com> wrote:
>
> @David,
>
> Using the `Future` library based on GCD that I have previously posted your
> example would be:
>
> let image = preprocessImage(downloadImage()) // These first two lines run in parallellet text = translate(downloadText())render(image: image.get ?? defaultImage, text: text.get ?? defaultText)
>
>
> The main difference, and I would argue an improvement, is that the
> `Future` version handles errors.
>
> So what advantage does async/await have over a `Future` library we can
> write today?
>
>
>   -- Howard.
>
> On 29 August 2017 at 15:28, David Hart via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>>
>> On 29 Aug 2017, at 02:22, Xiaodi Wu via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>> On Mon, Aug 28, 2017 at 16:10 Adam Kemp via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>> I know what the proposal said. I’m making a case that there is value in
>>> doing it differently.
>>>
>>> The composability of futures is valuable. Mixing and matching
>>> async/await with futures is also valuable. The queue-returning behavior
>>> that you can get from futures is also valuable, and building async/await on
>>> top of futures means async/await can get that for free.
>>>
>>
>> Why couldn't you mix and match async/await and futures and get the
>> queue-return behavior of futures if futures are built on top of async/await
>> instead off the other way around?
>>
>>
>> We could, but the syntax is much worse. Contrast:
>>
>> *async/await built on top of Futures*
>>
>> let image = preprocessImage(downloadImage())let text = translate(downloadText())
>> await render(image: image, text: text)
>>
>>
>> *Futures built on top of async/await*
>>
>> let image = Future(downloadImage).then({ preprocessImage($0) })let text = Future(downloadText).then({ translate($0) })
>> await render(image: image.get(), text: text.get())
>>
>>
>> Maybe you don’t value those things, which is fine. But I do, and maybe
>>> other people do too. That’s why we’re having a discussion about it.
>>>
>>> It can also be valuable having a minimal implementation, but we have to
>>> acknowledge that it comes with a downside as well. The problem with doing a
>>> minimal implementation is that you can be stuck with the consequences for a
>>> long time. I want to make sure that we’re not stuck with the consequences
>>> of a minimal implementation that doesn’t adequately address the problems
>>> that async/await should be addressing. I’d hate for Swift to get an
>>> async/await that is so weak that it has to be augmented by tedious
>>> boilerplate code before it’s useful.
>>>
>>>
>>> On Aug 28, 2017, at 1:54 PM, Wallacy <wallacyf at gmail.com> wrote:
>>>
>>> We don't need to this now!
>>>
>>> Again: (Using proposal words)
>>>
>>> "It is important to understand that this is proposing compiler support
>>> that is completely concurrency runtime-agnostic. This proposal does not
>>> include a new runtime model (like "actors") - it works just as well with
>>> GCD as with pthreads or another API. Furthermore, unlike designs in other
>>> languages, it is independent of specific coordination mechanisms, such as
>>> futures or channels, allowing these to be built as library feature"
>>>
>>> and
>>>
>>> "This proposal does not formally propose a Future type, or any other
>>> coordination abstractions. There are many rational designs for futures, and
>>> a lot of experience working with them. On the other hand, there are also
>>> completely different coordination primitives that can be used with this
>>> coroutine design, and incorporating them into this proposal only makes it
>>> larger."
>>>
>>> and
>>>
>>> We focus on task-based concurrency abstractions commonly encountered in
>>> client and server applications, particularly those that are highly event
>>> driven (e.g. responding to UI events or requests from clients). This does
>>> not attempt to be a comprehensive survey of all possible options, nor does
>>> it attempt to solve all possible problems in the space of concurrency.
>>> Instead, it outlines a single coherent design thread that can be built over
>>> the span of years to incrementally drive Swift to further greatness.
>>>
>>> and
>>>
>>> This proposal has been kept intentionally minimal, but there are many
>>> possible ways to expand this in the future.
>>>
>>> ....
>>>
>>> The point is: No Future type is indeed proposed yet!
>>>
>>> The proposal try to include de "minimum" required to implement a basic
>>> async/await to solve the problem created by the GCD! (Pyramid of doom)
>>>
>>> The question is: How do you do the same using dispatch_async ?
>>> dispatch_async also does not return nothing to do what you are intentend do
>>> do!
>>>
>>> Algo, by Swift 5 manifesto, there's no compromise to make a "complete"
>>> concurrency model by this time!
>>>
>>> My intention is only make parity to dispatch_async, but also make the
>>> ground free to make more complex implementation like Futures in another
>>> round on top of this one.
>>>
>>> This 'async T' can be a real type in the future? Maybe will... But
>>> doesn't matter now! Now we only need to is some kind of type which need to
>>> be unwrapped using await before use. Maybe this intermediary/virtual type
>>> can be a real thing and gain some abilities at some point! Maybe a full
>>> Future type, why not?
>>>
>>> Em seg, 28 de ago de 2017 às 17:33, Adam Kemp <adam.kemp at apple.com>
>>> escreveu:
>>>
>>>> How would these anonymous types get composed? If I wanted to implement
>>>> a function that takes a collection of futures and wait on it, how would I
>>>> do that? That is, how would I implement the equivalent of C#’s Task.WhenAll
>>>> and Task.WhenAny methods?
>>>>
>>>> More generally, how do you pass one of these typeless futures to some
>>>> other function so that we can do the waiting there?
>>>>
>>>>
>>>> On Aug 28, 2017, at 1:23 PM, Wallacy <wallacyf at gmail.com> wrote:
>>>>
>>>> And that's why I (and others) are suggesting:
>>>>
>>>> func processImageData1a() async -> Image {
>>>>   let dataResource  = async loadWebResource("dataprofile.txt") // No
>>>> future type here... Just another way to call dispatch_async under the hood.
>>>>   let imageResource = async loadWebResource("imagedata.dat")
>>>>
>>>>   // ... other stuff can go here to cover load latency...
>>>>
>>>>   let imageTmp    = await decodeImage(dataResource, imageResource) //
>>>> Compiles force await call here...
>>>>   let imageResult = await dewarpAndCleanupImage(imageTmp)
>>>>   return imageResult
>>>> }
>>>>
>>>> And now we gain all advantages of async/await again without to handle
>>>> with one more type.
>>>>
>>>> Em seg, 28 de ago de 2017 às 17:07, Adam Kemp via swift-evolution <
>>>> swift-evolution at swift.org> escreveu:
>>>>
>>>>> I think the biggest tradeoff is clearer when you look at the examples
>>>>> from the proposal where futures are built on top of async/await:
>>>>>
>>>>> func processImageData1a() async -> Image {
>>>>>   let dataResource  = Future { await loadWebResource("dataprofile.txt")
>>>>> }
>>>>>   let imageResource = Future { await loadWebResource("imagedata.dat")
>>>>> }
>>>>>
>>>>>   // ... other stuff can go here to cover load latency...
>>>>>
>>>>>   let imageTmp    = await decodeImage(dataResource.get(),
>>>>> imageResource.get())
>>>>>   let imageResult = await dewarpAndCleanupImage(imageTmp)
>>>>>   return imageResult
>>>>> }
>>>>>
>>>>>
>>>>> With this approach you have to wrap each call site to create a future.
>>>>> Compare to this:
>>>>>
>>>>> func processImageData1a() -> Future<Image> {
>>>>>   let dataResourceFuture  = loadWebResource("dataprofile.txt”);
>>>>>   let imageResourceFuture = loadWebResource("imagedata.dat”);
>>>>>
>>>>>   // ... other stuff can go here to cover load latency...
>>>>>
>>>>>   let imageTmp    = await decodeImage(await dataResourceFuture, await
>>>>> imageResourceFuture)
>>>>>   let imageResult = await dewarpAndCleanupImage(imageTmp)
>>>>>   return imageResult
>>>>> }
>>>>>
>>>>>
>>>>> Here, not only are the explicit wrappers gone, but this function
>>>>> itself can be used with either await or as a future. You get both options
>>>>> with one implementation.
>>>>>
>>>>> As I’ve mentioned before, C#’s implementation is not tied to any one
>>>>> particular futures implementation. The Task type is commonly used, but
>>>>> async/await does not directly depend on Task. Instead it works with any
>>>>> return type that meets certain requirements (detailed here:
>>>>> https://blogs.msdn.microsoft.com/pfxteam/2011/01/13/await-anything/).
>>>>> Swift could do this using a protocol, which can be retroactively applied
>>>>> using an extension.
>>>>>
>>>>> Obviously for this to be useful we would need some kind of existing
>>>>> future implementation, but at least we wouldn’t be tied to any particular
>>>>> one. That would mean library maintainers who have already been using their
>>>>> own futures implementations could quickly adopt async/await in their code
>>>>> without having to rewrite their futures library or throw wrappers around
>>>>> every usage of async/await. They could just adopt a protocol (using an
>>>>> extension, even) and get async/await support for free.
>>>>>
>>>>> The downside is that this feature would be specific to the async/await
>>>>> use case rather than a generic coroutine implementation (i.e., there would
>>>>> have to be a separate compiler transform for yield return). It’s not clear
>>>>> to me why it should be a goal to have just one generic coroutine feature.
>>>>> The real-world usages of async/await and yield return are different enough
>>>>> that I’m not convinced we could have a single compiler feature that meets
>>>>> the needs of both cleanly.
>>>>>
>>>>> On Aug 27, 2017, at 7:35 PM, Florent Vilmart <florent at flovilmart.com>
>>>>> wrote:
>>>>>
>>>>> Adam, you’re completely right, languages as c# and JS have been
>>>>> through the path before, (callback, Promises , async/await) I believe
>>>>> Chris’s goal it to avoid building a promise implementation and go straight
>>>>> to a coroutines model, which is more deeply integrated with the compiler. I
>>>>> don’t see a particular trade off, pursuing that route, and the main benefit
>>>>> is that coroutines can power any asynchronous metaphor (Signals, Streams,
>>>>> Futures, Promises etc...) which is not true of Futures so i would tend to
>>>>> think that for the long run, and to maximize usability, async/await/yield
>>>>> would probably be the way to go.
>>>>>
>>>>> On Aug 27, 2017, 22:22 -0400, Adam Kemp <adam.kemp at apple.com>, 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
>>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> swift-evolution mailing list
>>>>>>> swift-evolution at swift.org
>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>>
>>>>>>> --
>>>>>> -- Howard.
>>>>>>
>>>>>> _______________________________________________
>>>>>> swift-evolution mailing list
>>>>>> swift-evolution at swift.org
>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>
>>>>>>
>>>>>
>>>>> _______________________________________________
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>>>>
>>>>
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