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

From: Simon Pieters <simonp@opera.com>
Date: Mon, 2 Jan 2017 14:22:24 +0100
Message-ID: <CAFUichF4RZNex_Fbpsbe7cHsixzpnsbL3Z62nJUhkSnR-yQOOA@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>
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
>
> 😸
>
>
Received on Monday, 2 January 2017 13:22:57 UTC

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