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Integrate JavaScript SharedArrayBuffers and agents/agent clusters #2361

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Integrate JavaScript SharedArrayBuffers and agents/agent clusters #2361

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bb5f0a7
Agents and agent clusters
domenic Feb 15, 2017
a4946cf
Structured clone of SABs
domenic Feb 16, 2017
d0555e2
zero-length SABs for the error case
domenic Feb 16, 2017
8f2105f
"Serializability of script execution" needs updating
domenic Feb 16, 2017
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Expand Up @@ -495,6 +495,13 @@
be thought of as completely serializing the execution of all scripts in all <span data-x="browsing
context">browsing contexts</span>.</p>

<p>The exception to this general design principle is the JavaScript <code>SharedArrayBuffer</code>
class. Using <code>SharedArrayBuffer</code>s, it can in fact be observed that scripts in other
<span data-x="event loop">event loops</span> are executing simultaneously. Furthermore, due to the
JavaScript memory model, there are situations which not only are un-representable via serialized
<em>script</em> execution, but also un-representable via serialized <em>statement</em> execution
among those scripts.</p>

</div>


Expand Down Expand Up @@ -2865,6 +2872,8 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute

<ul class="brief">
<li><dfn data-x-href="https://tc39.github.io/ecma262/#active-function-object">active function object</dfn></li>
<li><dfn data-x-href="https://tc39.github.io/ecma262/#sec-agents">agent</dfn> and
<dfn data-x-href="https://tc39.github.io/ecma262/#sec-agent-clusters">agent cluster</dfn></li>
<li><dfn data-x-href="https://tc39.github.io/ecma262/#sec-automatic-semicolon-insertion">automatic semicolon insertion</dfn></li>
<li>The <dfn data-x-href="https://tc39.github.io/ecma262/#current-realm">current Realm Record</dfn></li>
<li><dfn data-x-href="https://tc39.github.io/ecma262/#early-error-rule">early error</dfn></li>
Expand All @@ -2882,8 +2891,9 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
<dfn>@@toStringTag</dfn></li>
<li><dfn data-x-href="https://tc39.github.io/ecma262/#sec-well-known-intrinsic-objects">Well-Known Intrinsic Objects</dfn>, including
<dfn data-x-href="https://tc39.github.io/ecma262/#sec-arraybuffer-constructor">%ArrayBuffer%</dfn>,
<dfn data-x-href="https://tc39.github.io/ecma262/#sec-properties-of-the-array-prototype-object">%ArrayPrototype%</dfn>, and
<dfn data-x-href="https://tc39.github.io/ecma262/#sec-object.prototype.valueof">%ObjProto_valueOf%</dfn></li>
<dfn data-x-href="https://tc39.github.io/ecma262/#sec-properties-of-the-array-prototype-object">%ArrayPrototype%</dfn>,
<dfn data-x-href="https://tc39.github.io/ecma262/#sec-object.prototype.valueof">%ObjProto_valueOf%</dfn>, and
<dfn data-x-href="https://tc39.github.io/ecma262/#sec-sharedarraybuffer-constructor">%SharedArrayBuffer%</dfn></li>

<li>The <dfn data-x="js-prod-FunctionBody" data-x-href="https://tc39.github.io/ecma262/#prod-FunctionBody"><i>FunctionBody</i></dfn> production</li>
<li>The <dfn data-x="js-prod-Module" data-x-href="https://tc39.github.io/ecma262/#prod-Module"><i>Module</i></dfn> production</li>
Expand All @@ -2898,6 +2908,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
<dfn data-x="js-ModuleEvaluation" data-x-href="https://tc39.github.io/ecma262/#sec-moduleevaluation">ModuleEvaluation</dfn> and
<dfn data-x="js-ModuleDeclarationInstantiation" data-x-href="https://tc39.github.io/ecma262/#sec-moduledeclarationinstantiation">ModuleDeclarationInstantiation</dfn> methods</li>

<li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-allocatesharedarraybuffer">AllocateSharedArrayBuffer</dfn> abstract operation</li>
<li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-arraycreate">ArrayCreate</dfn> abstract operation</li>
<li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-call">Call</dfn> abstract operation</li>
<li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-clonearraybuffer">CloneArrayBuffer</dfn> abstract operation</li>
Expand All @@ -2919,6 +2930,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
<li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-isconstructor">IsConstructor</dfn> abstract operation</li>
<li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-isdatadescriptor">IsDataDescriptor</dfn> abstract operation</li>
<li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-isdetachedbuffer">IsDetachedBuffer</dfn> abstract operation</li>
<li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-issharedarraybuffer">IsSharedArrayBuffer</dfn> abstract operation</li>
<li>The <dfn data-x="js-NewObjectEnvironment" data-x-href="https://tc39.github.io/ecma262/#sec-newobjectenvironment">NewObjectEnvironment</dfn> abstract operation</li>
<!-- the next set of Ordinary* abstract operations are in order of appearance -->
<li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-ordinarygetprototypeof">OrdinaryGetPrototypeOf</dfn> abstract operation</li>
Expand All @@ -2944,6 +2956,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
<li>The <dfn data-x="js-abstract-equality" data-x-href="https://tc39.github.io/ecma262/#sec-abstract-equality-comparison">Abstract Equality Comparison</dfn> algorithm</li>
<li>The <dfn data-x="js-strict-equality" data-x-href="https://tc39.github.io/ecma262/#sec-strict-equality-comparison">Strict Equality Comparison</dfn> algorithm</li>
<li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-date-objects"><code>Date</code></dfn> class</li>
<li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-sharedarraybuffer-objects"><code>SharedArrayBuffer</code></dfn> class</li>
<li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-typeerror"><code>TypeError</code></dfn> class</li>
<li>The <dfn data-x="js-RangeError" data-x-href="https://tc39.github.io/ecma262/#sec-native-error-types-used-in-this-standard-rangeerror"><code>RangeError</code></dfn> class</li>
<li>The <dfn data-x="js-typeof" data-x-href="https://tc39.github.io/ecma262/#sec-typeof-operator"><code>typeof</code></dfn> operator</li>
Expand Down Expand Up @@ -7791,11 +7804,54 @@ interface <dfn>DOMStringList</dfn> {
<li><p>If <span>IsDetachedBuffer</span>(<var>input</var>) is true, then throw a
<span>"<code>DataCloneError</code>"</span> <code>DOMException</code>.</p></li>

<li><p>Let <var>outputArrayBuffer</var> be the <span>%ArrayBuffer%</span> intrinsic object in
<var>targetRealm</var>.
<li>
<p>If <span>IsSharedArrayBuffer</span>(<var>input</var>) is true, then:</p>

<ol>
<li>
<p>If <var>targetRealm</var>'s <span data-x="concept-realm-settings-object">settings
object</span>'s <span>responsible event loop</span> is not in the same <span>unit of
share-within-able event loops</span> as the <span>current Realm Record</span>'s <span
data-x="concept-realm-settings-object">settings object</span>'s <span>responsible event
loop</span>, then:</p>

<ol>
<li><p>Let <var>outputConstructor</var> be the <span>%SharedArrayBuffer%</span> intrinsic
object in <var>targetRealm</var>.</p></li>

<li><p>Let <var>output</var> be !
<span>AllocateSharedArrayBuffer</span>(<var>outputConstructor</var>, 0).</p></li>
</ol>

<p class="note">The reason that this step creates a zero-length
<code>SharedArrayBuffer</code> instance, instead of throwing an error, is due to the fact
that at the time <span>StructuredCloneWithTransfer</span> is performed, the
<var>targetRealm</var> is not always known, so we cannot synchronously throw an error to the
caller. (An example is when we use <code
data-x="dom-MessagePort-postMessage">postMessage()</code> on a <code>MessagePort</code>
whose messages are initially paused.) This subtlety is unfortunately obfuscated by the
current <span>StructuredCloneWithTransfer</span> algorithm; see <a
href="https://github.com/whatwg/html/issues/935">issue #935</a>.</p>
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Is this really how we want to signal errors? Wouldn't it be much better if the message contained an indication of the error in some way? (Note that we also need to solve this for ArrayBuffer, as its allocation can fail.)

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It's also a little sad that we don't solve the underlying issue of the model being wrong first. It makes it harder to see the correct solutions.

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@domenic domenic Feb 16, 2017 •
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My understanding is that this is how it was shipped in all browsers already, based on the spec this is integrating, but maybe I'm wrong.

Last time this was discussed in #936 the idea was to not mix up error messages and data.

The advantage of this design, by the way, is that it allows you to get all the other data you cloned/transferred, instead of throwing it away (in the transferred case, forever).

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So is this how browsers deal with ArrayBuffer allocation failure?

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In that case I think they mostly just crash.

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Really? JavaScript prescribes a TypeError and I've seen that in code.

</li>

<li><p>Otherwise, let <var>output</var> be a new SharedArrayBuffer object in
<var>targetRealm</var> whose [[ArrayBufferData]] internal slot value is
<var>input</var>.[[ArrayBufferData]] and whose [[ArrayBufferByteLength]] internal slot value
is <var>input</var>.[[ArrayBufferByteLength]].</p></li>
</ol>
</li>

<li>
<p>Otherwise:</p>

<ol>
<li><p>Let <var>outputConstructor</var> be the <span>%ArrayBuffer%</span> intrinsic object in
<var>targetRealm</var>.</p></li>

<li><p>Let <var>output</var> be ? <span>CloneArrayBuffer</span>(<var>input</var>, 0,
<var>outputArrayBuffer</var>).</p></li>
<li><p>Let <var>output</var> be ? <span>CloneArrayBuffer</span>(<var>input</var>, 0,
<var>outputConstructor</var>).</p></li>
</ol>
</li>
</ol>
</li>

Expand Down Expand Up @@ -8064,6 +8120,8 @@ interface <dfn>DOMStringList</dfn> {
<ol>
<li><p>If <span>IsDetachedBuffer</span>(<var>O</var>) is true, then return false.</p></li>

<li><p>If <span>IsSharedArrayBuffer</span>(<var>O</var>) is true, then return false.</p></li>

<li><p>Return true.</p></li>
</ol>
</li>
Expand All @@ -8086,6 +8144,8 @@ interface <dfn>DOMStringList</dfn> {
data-x="transfer-abstract-op">Transfer</dfn> ( <var>input</var>, <var>targetRealm</var> )</h4>

<ol>
<li><p>Assert: <span>IsTransferable</span>(<var>input</var>) is true.</p></li>

<li>
<p>If <var>input</var> has an [[ArrayBufferData]] internal slot, then:</p>

Expand Down Expand Up @@ -87910,6 +87970,72 @@ import "https://example.com/foo/../module2.js";</pre>
data-x="concept-module-script-module-record">module record</span>.</p></li>
</ol>

<h5>Integration with the JavaScript agent formalism</h5>

<p>JavaScript defines the concept of an <span>agent</span>. This corresponds, in a 1-to-1 manner,
with this specification's concept of an <span>event loop</span>.</p>

<p>To determine the manifestation of JavaScript's <span>agent cluster</span> concept in user
agents, we first define the set of <dfn>directly share-to-able event loops</dfn> for a given
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share-to-able read a little awkward.

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Yep, any suggestions?

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Maybe "connected event loops" and "transitive-connected event loops"?

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I like including something about "sharing" in the name since this is largely about shared memory but that is indeed less awkward so I'm happy to switch to "connected".

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"Directly-shared event loops" and "shared event loops" seems okay too. I like it a little better as it has a shorter term for the thing that maps to JavaScript.

<span>event loop</span> <var>loop</var>:</p>

<dl class="switch">
<dt>For a <span>browsing context</span> <span>event loop</span></dt>
<dd>The set containing <var>loop</var>, plus the <a href="#worker-event-loop">worker event
loops</a> of any worker whose <span>environment settings object</span>'s <span>responsible
browsing context</span> is one of <var>loop</var>'s <span data-x="browsing context">browsing
contexts</span></dd>

<dt>For a <a href="#worker-event-loop">worker event loop</a> of a dedicated worker</dt>
<dd>The set containing <var>loop</var>, plus the worker's <span>environment settings
object</span>'s <span>responsible browsing context</span>'s <span>event loop</span>, plus the
<a href="#worker-event-loop">worker event loops</a> of each of <span>the worker's
workers</span></dd>
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What if this worker was inside a service worker or a shared worker?

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My understanding is that all workers spawned from a given shared/service worker are share-to-able.

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Sure, but what about its responsible browsing context then? That's either null or it should not share with that.

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I see, yeah. Hopefully it is null, in which case this is a wording tweak, but I'll need to check.

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It's never null.


<dt>For a <a href="#worker-event-loop">worker event loop</a> of any other type of worker</dt>
<dd>The set containing <var>loop</var>, plus the <a href="#worker-event-loop">worker event
loops</a> of each of <span>the worker's workers</span></dd>
</dl>

<p>Then, the transitive closure of all <span data-x="event loop">event loops</span> that are
<span>directly share-to-able event loops</span> forms a <dfn>unit of share-within-able event
loops</dfn>, and a JavaScript <span>agent cluster</span> consists of all the corresponding <span
data-x="agent">agents</span>.</p>

<p class="note">The <span>agent cluster</span> concept is crucial for defining the JavaScript
memory model, and in particular among which <span data-x="agent">agents</span> the backing data of
<code>SharedArrayBuffer</code> objects can be shared.</p>

<div class="example">
<p>The following pairs of global objects are each within the same <span>unit of share-within-able
event loops</span>, and thus can use <code>SharedArrayBuffer</code> instances to share memory
with each other:</p>

<ul class="brief">
<li>A <code>Window</code> and a dedicated worker that it created</li>

<li>A <code>Window</code> and the <code>Window</code> of a <span>same origin-domain</span>
<code>iframe</code></li>

<li>A <code>Window</code> and the <code>Window</code> that opened it</li>

<li>A worker (of any type) and any nested workers it creates</li>
</ul>

<p>The following pairs of global objects are <em>not</em> within the same <span>unit of
share-within-able event loops</span>, and thus cannot share memory:</p>

<ul class="brief">
<li>A <code>Window</code> and a shared worker it creates</li>

<li>A <code>Window</code> and a service worker it creates</li>

<li>A <code>Window</code> and the <code>Window</code> of a non-<span>same origin-domain</span>
<code>iframe</code></li>
</ul>

</div>

</div>


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