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Re: Proposal: Constraints as dictionaries

From: Stefan Håkansson LK <stefan.lk.hakansson@ericsson.com>
Date: Thu, 21 Nov 2013 13:29:46 +0000
To: Jan-Ivar Bruaroey <jib@mozilla.com>, "public-media-capture@w3.org" <public-media-capture@w3.org>
Message-ID: <1447FA0C20ED5147A1AA0EF02890A64B1C3F3856@ESESSMB209.ericsson.se>
On 2013-11-20 22:31, Jan-Ivar Bruaroey wrote:
> I've been asked to make a concrete proposal to fix problems brought up
> with constraints. There are three parts (ABC).
>       A. Don't violate WebIDL
> Constraints must be WebIDL dictionaries. We cannot afford to reinvent
> dictionaries or webidl just for Constraints, at this point. The WebIDL
> WG have thought more about JS and JS processing models than I think we
> have, and for the complexity and long-tail use-cases inherent in this
> API, we need explicit definitive language that implementers can follow,
> and WebIDL is that language.
> WebIDL dictionaries ignore unknown keys. The following tweaks in our
> model let constraints be dictionaries:
>   1. Conservative programmer calls if
> (!getGumKnownConstraints().hasOwnProperty("3D")) then bail
>   2. getUserMedia({ mandatory: { 3D: true } }, succ, fail); // where 3D
> is ignored if unimplemented
>   3. (minor) Remove limit of one key-value pair in dictionaries in
> optional array (over-specified)
>   4. KeepConstrainable as a pattern, but root it in actual gUM types
> (remove abstractions, 'any' etc.)
>   5. Change abstract PropertyValueRange to UnsignedLongRange, FloatRange
> etc.
> Most controversial difference: The API for detecting browser-support for
> a constraint is now explicit, and gUM() itself only constrains by
> properties it knows.
> This avoids blanket blocking by browser for bad as well as good cameras,
> something simple apps may not have signed on for, and lets apps
> explicitly block access on those same browsers with one line of code.
> Not new: Code that plans to control the camera must already use
> getCapabilities() to learn the known things they can modify, so there is
> no added cost in those cases.
> We then describe Constraints, Capabilities and Settings in the spec
> using WebIDL instead of abstract prose. I circulated
> http://lists.w3.org/Archives/Public/public-media-capture/2013Nov/0084.htmlearlier
> which shows the proposed WebIDL at the bottom. Using WebIDL
> descriptively in this manner does not preclude IANA.

I have argued before that a (by the browser) unknown mandatory 
constraint should never give a success callback. However, if we 
introduce getGumKnownConstraints() I think it could be acceptable, and 
if it in addition allows for using WebIDL when speccing I would shift 
view. So, I think A is a good idea.

>       B. Don't leak
> Stop returning ConstraintNotSatisfiedError, because it leaks information
> about capabilities to the website without user consent, letting a
> malicious website paint a full picture after a few visits.
> Instead, always bring up the permission prompt, even when there is no
> match.The UA MUST warn the user differently of such non-matches and
> offer the Deny-equivalent choice only. The UA MUST NOT let the user
> grant access to a camera in this case (not because of a leak, but to
> give webpages an invariant for known properties).

I care a lot about my own privacy - so I like this too.

>       C. New syntax
> A new syntax for Constraints only, is aimed at moving us away from the
> mandatory/optional language to a model focused on expressiveness, and
> away from Constraints being "things" to being dictionaries used in a
> pattern, just like Settings and Capabilities are now.
> In this syntax, Constraints is an array of Settings (dictionaries) in
> decreasing order of preference, where a set is accepted only if all its
> specified keys are. The first accepted set is chosen.
> To describe your camera preferences in fine detail, you provide all the
> combinations of settings you accept, specifying only the things you care
> about, in all the combinations you accept.
> I show a width/height example here
> http://lists.w3.org/Archives/Public/public-media-capture/2013Nov/0052.htmlwhich
> in turn has a link to the original post.
> Let me elaborate on the example in the original post (sans typo):
>> Consider an alternative to:
>>           { mandatory: { a, b }, optional: [ { c }, { d } ] }
>> like this:
>>         [ { a, b, c, d }, { a, b, c }, { a, b, d }, { a, b } ]
>> Old familiars and what they look like now:
>>      { mandatory: { a, b } } ==> [ { a, b } ]
>> and separately:
>>      { optional: [{ a }, { b }] } ==> [ { a }, { b }, {} ]
> Benefits of new syntax:
> + Simpler. No keyword-dependent structure variance ([{}] vs. {}).
> + Advantage of being able to combine related things like width + height
> naturally and logically.
> + {} at the end of the array naturally makes everything optional, since
> it means "I'll accept anything".
> + More power (allows for finer, more explicit selection).
> + (AND)OR array logic is easier to read than the current optional-array
> logic (esp. width/height combos)
> + Bye bye misunderstood mandatory keyword. Hello dictionaries. <3
> Cons:
> - Can't handle large number of optionals, grows exponentially in
> edge-cases. (6 = ugh, 30 = no way)
>    I'm banking on there always being patterns in what programmers desire
> (OR-ing sets not 1by1. Try it!)
> - Pilot error: Forget ending array with {} and everything is mandatory.
> Since we no longer tailor any data-types for our special needs, and rely
> solely on well-known constructs, how things work and don't work should
> be much clearer both to developers and programmers.
>       Note
> Again, a result of this proposal is that any invariant given a webpage
> from using mandatory constraints is now only provided for known
> constraints. Some have pointed out that this is a flimsy guarantee
> anyway, since OSes may lie.

I don't have a strong opinion on C. I appreciate that it is simpler, and 
that we don't have to split up constraints into two distinct categories, 
but on the other hand we have been talking about mandatory and optional 
for a long time. I think I have a slight preference for the new proposal.

> Feel free to consider ABC individually as well as a whole.
> Thanks for reading,
> .: Jan-Ivar :.
Received on Thursday, 21 November 2013 13:30:14 UTC

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