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Re: Bug in Acid3 test #72 (race condition when images decode asynchronously)

From: Simon Pieters <simonp@opera.com>
Date: Tue, 3 Jan 2017 00:11:11 +0100
Message-ID: <CAFUichFPz9R1wKgTRtu+GmNKR-qCDhCMNA3LoqG3QtBvYAhjsA@mail.gmail.com>
To: Ian Hickson <ian@hixie.ch>
Cc: Daniel Holbert <dholbert@mozilla.com>, "www-archive@w3.org" <www-archive@w3.org>, Yoav Weiss <yoav@yoav.ws>, Anne van Kesteren <annevk@annevk.nl>
Ah, OK. Yes, I agree it should be available "quickly". But it should also
*not* be available sync in the same script that sets .src, since it needs a
task.

Just trying to make sure browsers aren't going in different directions
here. :-)

On Mon, Jan 2, 2017 at 9:00 PM, Ian Hickson <ian@hixie.ch> wrote:

> I spoke a bit loosely. Here's a more accurate description:
>
> Everything up to step 13 of "update the image data" happens synchronously
> from the parser, then the fetching happens in parallel (main fetch step
> 10), which posts a task straight away (because the data: URL's data is
> available without any delay, see basic fetch, noting that nothing there
> needs time to resolve), and that triggers step 14 of "update the image
> data" which requires the image to be decoded and the size determined
> synchronously in a single task.
>
> The point is that even if it happens in parallel, there's nothing to
> fetching the data: URL that will take any time, since the data is right
> there. If it takes ten milliseconds to base64-decode a short data: URL then
> the browser is buggy regardless of what the spec says, and the acid3 test
> is really not relevant to the question at that point. The whole point of
> data: URLs is to inline the data so that it can be rendered quickly.
>
> On Mon, Jan 2, 2017 at 5:22 AM Simon Pieters <simonp@opera.com> wrote:
>
>> Where is it defined that data: is fetched sync? I thought it was defined
>> to be fetched async.
>>
>> Step 6 awaits a stable state before entering the synchronous section, so
>> that is a microtask before the fetch can start. Step 13 starts the fetch
>> but the Fetch spec doesn't special case data: URLs to be sync, as far as I
>> can tell?
>>
>> I believe Chromium is trying to make <img src="data:..."> load async. See
>> https://bugs.chromium.org/p/chromium/issues/detail?id=514206
>>
>> On Sat, Dec 31, 2016 at 4:46 AM, Ian Hickson <ian@hixie.ch> wrote:
>>
>> Glad to hear it. :-)
>>
>> Cheers,
>> --
>> Ian Hickson
>>
>> On Fri, Dec 30, 2016 at 4:12 PM Daniel Holbert <dholbert@mozilla.com>
>> wrote:
>>
>> Thanks for clarifying that -- I wasn't familiar with that "update the
>> image data" spec section.
>>
>> Based on that section, I think you're correct, and Acid3 test #72 is
>> indeed OK after all.  I've got a local patch to fix Firefox to perform
>> data-URI dimension-decodes synchronously, so that we'll go back to
>> satisfying this chunk of spec & passing that subtest.
>>
>> ~Daniel
>>
>> On 12/23/2016 02:13 AM, Ian Hickson wrote:
>> > I believe this is non-conforming. data: URLs are defined to be fetched
>> > synchronously and image decoding to obtain dimensions is defined to
>> > happen instantaneously (for non-data: URLs, this is black-box
>> > indistinguishable from it being rolled into the network delay, of
>> course).
>> >
>> > Specifically, see step 14 of "update the image data", noting that
>> > everything up to step 13 happens synchronously from the parser, then the
>> > fetching happens as per the fetch spec in one task (because it's a data:
>> > URL, it's not asynchronous), and that triggers step 14 which requires
>> > the image to be decoded and the size determined synchronously in a
>> > single task.
>> >
>> > --
>> > Ian Hickson
>> >
>> > On Fri, Dec 23, 2016 at 2:03 AM Daniel Holbert <dholbert@mozilla.com
>> > <mailto:dholbert@mozilla.com>> wrote:
>> >
>> >     On 12/23/2016 12:06 AM, Ian Hickson wrote:
>> >     > The image in question is a data: URL. There's a ten millisecond
>> gap
>> >     > between the last test and this test. That guarantees that there's
>> at
>> >     > least one opportunity to handle the microtask in which the data:
>> URL
>> >     > gets loaded. As far as I can tell this means that by the time the
>> >     height
>> >     > property accessor is read, it's guaranteed that the image is
>> loaded.
>> >
>> >     Nah, it's actually not guaranteed to be loaded -- hence the bug
>> that was
>> >     filed for Firefox failing this subtest. :)
>> >
>> >     In current Firefox versions, image-decoding actually happens on a
>> >     *helper-thread* -- not in a main-thread microtask. So, depending on
>> >     thread-switching / signalling overhead, the image very well might
>> not
>> >     have been decoded (& had its results reported back) when the 10
>> >     millisecond timeout expires. Hence, it may very well be in the
>> >     spec-defined "unavailable" state at that point, when the test reads
>> the
>> >     <img> size.
>> >
>> >     You might think that this is silly and we should redesign this to
>> decode
>> >     image sizes faster, and you'd perhaps be right.
>> >
>> >     But whether we redesign it or not, our current behavior *is*
>> compliant
>> >     with the HTML spec.  And I don't see why the Acid3 test should
>> >     arbitrarily mandate a decode-in-10ms behavior.
>> >
>> >     I linked to a patch in my initial post, which simply makes the test
>> >     check whether the image is ready before reading the size (& retry if
>> >     not).  Given the above explanation (and the spec discussion in my
>> >     initial post), does my suggested change seem reasonable?
>> >
>> >     Thanks,
>> >     ~Daniel
>> >
>> > --
>> >
>> > --
>> > Ian Hickson
>> >
>> > 😸
>> >
>>
>> --
>>
>> --
>> Ian Hickson
>>
>> 😸
>>
>>
>> --
>
> --
> Ian Hickson
>
> 😸
>
>
Received on Monday, 2 January 2017 23:11:45 UTC

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