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Re: [whatwg] An API for unhandled promise rejections

From: Domenic Denicola <d@domenic.me>
Date: Mon, 18 May 2015 22:49:10 +0000
To: WHATWG <whatwg@whatwg.org>
Message-ID: <CY1PR0501MB1369F58357058F671E3F0547DFC40@CY1PR0501MB1369.namprd05.prod.outlook.com>
Cc: Boris Zbarsky <bzbarsky@mit.edu>, Brian Terlson <Brian.Terlson@microsoft.com>
Ping. We're considering implementing this in Chrome and it would be helpful to get a sense if other vendors support this.

> -----Original Message-----
> From: whatwg [mailto:whatwg-bounces@lists.whatwg.org] On Behalf Of
> Domenic Denicola
> Sent: Monday, February 9, 2015 20:45
> Cc: Petka Antonov
> Subject: Re: [whatwg] An API for unhandled promise rejections
> An update on this: although the conversation somewhat fizzled here, in io.js
> (Node.js fork) something very similar is landing and I'm trying to guide it
> toward being reasonably compatible with browsers [1]. Additionally several
> promise libraries have implemented similar hooks (see [2] and links in the
> comments; also of interest is the analysis of how promise-using libraries use
> these hooks).
> The community largely settled on unhandledRejection/rejectionHandled,
> instead of hijacking the error event like the original post below proposes.
> Which is not to say that we have to respect that in browsers, but it's a data
> point to consider.
> One interesting question that came up is the exact timing of the
> unhandledRejection event. In my proto-spec at [3] I proposed queuing a
> separate notify-rejected task for each rejection, largely because it seemed
> the easiest thing to spec. We now have some experience from the field. The
> implementation at [1] considered a few approaches and cycled through
> implementing some subset of them to make a progressively-larger set of
> tests pass:
> - Queue a single task
> - Queue a separate task per rejection (as in proto-spec)
> - Queue a microtask that occurs after all other microtasks
> - Queue a task that occurs after all other tasks
> Hopefully Petka, CC'ed, can correct me if I misstated these and fill in any
> details I missed.
> In general I am in favor of pushing off notification as long as possible to give
> more time for the rejection to potentially become handled (and thus
> decrease false positives). From this perspective either separate task per
> rejection or after-all-others task seems good. I was hoping to get a web
> platform perspective on what sounds good and would be implementable?
> For example I think the after-all-others-task can be specced with a new task
> source that HTML mandates is drained after all others, right?
> Anyway, I mostly just wanted to give people an update and show that we're
> prototyping this in io.js. Hopefully the interchange of ideas here can help
> push the progress in browsers too.
> [1]: https://github.com/iojs/io.js/pull/758
> [2]: https://gist.github.com/benjamingr/0237932cee84712951a2
> [3]: https://gist.github.com/domenic/9b40029f59f29b822f3b#promise-error-
> handling-hooks-rough-spec-algorithm
> -----Original Message-----
> From: Domenic Denicola
> Sent: Friday, September 12, 2014 14:34
> Subject: An API for unhandled promise rejections
> ## Problem
> A common desire in web programming is to log any uncaught exceptions
> back to the server. The typical method for doing this is
>     window.onerror = (message, url, line, column, error) => {
>       // log `error` back to the server
>     };
> When programming asynchronously with promises, asynchronous
> exceptions are encapsulated as _rejected promises_. They can be caught
> and handled with `promise.catch(err => ...)`, and propagate up through an
> "asynchronous call stack" (i.e. a promise chain) in a similar manner to
> synchronous errors.
> However, for promises, there is no notion of the "top-level" of the promise
> chain at which the rejection is known to be unhandled. Promises are
> inherently temporal, and at any time code that has access to a given promise
> could handle the rejection it encapsulates. Thus, unlike with synchronous
> code, there is not an ever-growing list of unhandled exceptions: instead,
> there is a growing and shrinking list of currently-unhandled rejections.
> For developers to be able to debug promises effectively, this live list of
> currently-unhandled rejections certainly needs to be exposed via developer
> tools, similar to how devtools exposes the ever-growing list of unhandled
> exceptions (via console output). However, developer tools are not sufficient
> to satisfy the telemetry use case, i.e. the use case which is currently handled
> via `window.onerror` for synchronous code.
> ## Proposed Solution
> We propose that
> 1. `window.onerror` be extended to handle the rejected-promise use case,
> notifying about any promises that, "at the end of the task queue", contain
> rejections that are not yet handled; and 2. A new hook,
> `window.onrejectionhandled`, be added, to notify when (or if) such
> rejections eventually become handled.
> By "at the end of the task queue" I mean that upon a promise being rejected
> with no handlers, we would queue a task to fire an error event; however if a
> handler is then attached to a promise in the meantime, a flag would be set so
> that when the task executes nothing actually happens.
> ### Developer Experience
> In terms of developer experience, the result is that if a promise is rejected
> without any rejection handler present, and one is not attached by "the end
> of the event loop turn", the resulting `(message, url, line, column, error,
> promise)` tuple will hit `window.onerror`. If the developer subsequently
> attaches a rejection handler to that promise, then the `promise` object will
> be passed to any handlers for the `rejectionhandled` event.
> As usual, if one or both of these events is missing listeners, nothing will
> happen. (In this case, the developer likely does not want to do telemetry on
> errors, but instead will be availing themselves to the devtools.)
> A robust error-reporting system would use `rejectionhandled` events to
> cancel out earlier `error` events, never displaying them to the person reading
> the error report.
> ### Specification Details
> We would extend [`ErrorEvent`](http://www.whatwg.org/specs/web-

> apps/current-work/multipage/webappapis.html#the-errorevent-interface)
> and `ErrorEventInit` with a `promise` member. Similarly, we would extend
> the  [`OnErrorEventHandlerNonNull`](http://www.whatwg.org/specs/web-

> apps/current-
> work/multipage/webappapis.html#onerroreventhandlernonnull) callback
> type to take as its last argument that same promise. In both cases, the
> promise would be `undefined` for synchronous errors.
> We would add a new event to the global, named `rejectionhandled`, along
> with a `RejectionHandledEvent` class that contains only a `promise` member.
> We would need to hook into rejecting promises and `then`-ing promises, and
> track unhandled rejections:
> * When a promise is rejected, if it has no handlers, we would queue a task to
> potentially-fire-an-error.
> * When a promise is `then`'d (either by user code or by the spec's chaining
> mechanisms) but the rejection has not yet been reported, we would set a
> flag saying "don't fire that error after all."
> * When the task is executed, if that flag is still unset, we would then fire the
> appropriate `error` event.
> * If a promise is `then`-ed in such a way as to handle the rejection, but that
> promise had previously been reported as an unhandled rejection, we would
> need to fire the appropriate `rejectionhandled` event.
> I can go into details on how to modify the promises spec to have these
> hooks, if desired, as well as how HTML would exploit them to maintain the
> appropriate list and report it at the end of the task queue. I can also help with
> the spec work here, on both the ES side and the HTML side, if desired.
> ### Potential Variants
> The `error` event and its idiosyncratic handler are not the best possible
> extension points. We may be better off with a separate `unhandledrejection`
> event (or, more accurately and as [popular
> libraries](https://github.com/petkaantonov/bluebird/#error-handling) call it,
> `possiblyunhandledrejection`). We could even unify on a single event class
> used for both, e.g. `PromiseRejectionEvent` with members `promise` and
> `reason`. This improves clarity and reduces piling kludges on top of
> `window.onerror`, but requires any existing telemetry code to upgrade to
> support the new event.
> I personally think this is a better solution, both because it has less kludges
> and because I can see server telemetry tools that aren't upgraded to
> recognize the new duality becoming overwhelmed with useless `error`
> events that are later canceled by `unhandledrejection` events they are
> unaware of. That is, if you try to plug asynchronous errors into your existing
> telemetry systems, you will be pulling your hair out over spurious, and
> sometimes hard-to-reproduce, errors in your logs. But other members of the
> Chrome team feel strongly about re-using onerror and I am happy to let this
> play out in the real world.
> Note that we are proposing this for the web, and not for ES, because the
> web has `window.onerror` (not to mention an event system) already. A
> more generic unhandled-rejection-tracking mechanism for all ES
> environments might be something like an `Object.observe`able
> `Promise.unhandledRejections` array, but that discussion can be left for
> another time.
> ## Implementer Interest
> Chrome is interested in implementing this ASAP. I'm broaching the idea for
> the first time publicly to hopefully get other implementer interest or at least
> rough consensus :).
Received on Monday, 18 May 2015 22:49:40 UTC

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