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Re: New proposal for fixing race conditions

From: Marcus Geelnard <mage@opera.com>
Date: Tue, 23 Jul 2013 20:00:35 +0200
Message-ID: <CAL8YEv5qy4zt69PdnhUaza+GNgHYzxKa2izGXKK_4E+RBSXeqg@mail.gmail.com>
To: Chris Wilson <cwilso@google.com>
Cc: Ehsan Akhgari <ehsan.akhgari@gmail.com>, "Robert O'Callahan" <robert@ocallahan.org>, Jer Noble <jer.noble@apple.com>, Russell McClellan <russell@motu.com>, WG <public-audio@w3.org>
Hi Chris,

On Tuesday, July 23, 2013, Chris Wilson <cwilso@google.com> wrote:
> I've been hanging back from this discussion a bit, but I feel the need to
express my own take (since I come at the API from a very different
perspective than Chris).
> I understand (and support) Robert's initial introduction of this issue
(first line of
http://lists.w3.org/Archives/Public/public-audio/2013AprJun/0644.html) - we
should avoid having internal implementation details affect observable
output.  However, that's not the same thing as "we must prevent any
possible race conditions" - in this case, the race condition is between the
Web Audio "thread" and the main execution thread.  This is not so much
about internal implementation details as it is about the fact that Web
Audio developers need to have their expectations set around interactions
with the audio "thread".
> AFAICT, all the proposals made so far - Jer's included - put quite a
heavy weight on interacting with audio buffer data. For the purposes of
synthesizing my own audio data, this will require a memcpy.

Just so that I'll understand, could you please give an example of such real
time synthesis that you can't do with a ScriptProcessorNode.

If you're talking about pre-rendering sound into an AudioBuffer (in a way
that can't be done using an OfflineAudioContext), I doubt that memcpy will
do much harm. Again (if this is the case), could you please provide an
exanple?

/Marcus

> In mobile scenarios, and desktop scenarios with large buffers of data
(i.e. a DAW), this will put a significantly destructive additional burden
on the environment required to play (and likely record/process) audio.
 This seems like an awfully big deal to me, so I have to question - what's
the benefit?  It is not to my knowledge required to avoid crashes or other
potential security issues; the only downside is if an author modifies a
playing audio buffer, they could get differing playback results depending
on precise timing.  That doesn't seem any different, to me, than what
happens with small timing differences in event delivery today, or setting
audio times that are too close to "now" - if you want the full power of the
audio system, you have to learn how to work closely with the system and
adapt to environments.  As Chris pointed out, there is some experience
working with the API as it is today, and I haven't heard of (or personally
experienced) any problems traced to this issue.
> Also, correct me if I'm mistaken, but I don't believe this is equal to
"browser x will operate differently than browser y" - timing is everything
in this scenario anyway, and actually even Jer's proposal could enable
different behavior across browsers/environments, it would just be replacing
the entire buffer instead of a portion.
> I feel designing the API around prevent race conditions everywhere is 1)
ultimately not going to be successful anyway, and 2) is like wrapping
everything with bubble wrap.  It will prevent some minor bruises, but it
will also make it quite a bit more costly (in memory and time) to get the
tasks needed done.
> Olivier, to answer your question, I believe this would currently be an
Objection.
> -Chris
>
> On Tue, Jul 23, 2013 at 8:07 AM, Ehsan Akhgari <ehsan.akhgari@gmail.com>
wrote:
>>
>> On Mon, Jul 22, 2013 at 6:44 PM, Robert O'Callahan <robert@ocallahan.org>
wrote:
>>>
>>> On Tue, Jul 23, 2013 at 5:44 AM, Marcus Geelnard <mage@opera.com> wrote:
>>>>
>>>> My guess is that this is very similar to the current solution in gecko
(Ehsan?).
>>>
>>>
>>> It's close to what we do. We neuter and recycle the output buffers, but
currently we don't neuter and recycle the input buffers. I think that's a
good idea though.
>>
>> We also lazily create the input buffers when JS first accesses the
inputBuffer property, to optimize the case where the ScriptProcessorNode is
only used for synthesis, not as a filter.
>>
>> Cheers,
>> Ehsan
>
>
Received on Tuesday, 23 July 2013 18:01:04 UTC

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