Define agents and agent clusters

Define the infrastructure for SharedArrayBuffer. This also clarifies
along which boundaries a browser implementation can use processes and
threads.

Tests: web-platform-tests/wpt#5569.

Follow-up to define similar-origin window agents upon a less shaky
foundation is #2528. Because of that, similar-origin window agents
are the best place to store state that would formerly go on unit of
related similar-origin browsing contexts.

tc39/ecma262#882 is follow-up to define
agents in more detail; in particular make their implicit realms slot
explicit. w3c/css-houdini-drafts#224 is
follow-up to define worklet ownership better which is needed to
define how they relate to agent (sub)clusters.

Fixes part of #2260. Fixes #851. Fixes
w3c/ServiceWorker#1115. Fixes most of
w3c/css-houdini-drafts#380 (no tests and no
nice grouping of multiple realms in a single agent as that is not
needed).

source

@@ -2855,6 +2855,8 @@ 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>
@@ -2936,6 +2938,7 @@ 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>
@@ -3780,6 +3783,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://w3c.github.io/ServiceWorker/#dfn-serviceworker-link">serviceworker link</dfn></li>
      <li><dfn data-x-href="https://w3c.github.io/ServiceWorker/#serviceworker"><code>ServiceWorker</code></dfn> interface</li>
      <li><dfn data-x-href="https://w3c.github.io/ServiceWorker/#serviceworkercontainer"><code>ServiceWorkerContainer</code></dfn> interface</li>
+     <li><dfn data-x-href="https://w3c.github.io/ServiceWorker/#serviceworkerglobalscope"><code>ServiceWorkerGlobalScope</code></dfn> interface</li>
      <li><dfn data-x-href="https://w3c.github.io/ServiceWorker/#dfn-use-cache">use cache</dfn></li>
     </ul>
 
@@ -3905,6 +3909,17 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
     </ul>
    </dd>
 
+   <dt>Worklets</dt>
+
+   <dd>
+    <p>The following feature is defined in the <cite>Worklets</cite> specification: <ref
+    spec=WORKLETS></p>
+
+    <ul class="brief">
+     <li><dfn data-x-href="https://drafts.css-houdini.org/worklets/#workletglobalscope"><code>WorkletGlobalScope</code></dfn></li>
+    </ul>
+   </dd>
+
   </dl>
 
   <hr>
@@ -88451,6 +88466,121 @@ 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>. Until such a time that this standard
+  has a better handle on lifetimes, we define five types of <span data-x="agent">agents</span> that
+  user agents must allocate at the appropriate time.</p>
+
+  <p class="note">In the future, when this specification has a better handle on lifetimes, we hope
+  to define exactly when <span data-x="agent">agents</span> and <span data-x="agent cluster">agent
+  clusters</span> are created.</p>
+
+  <p class="&#x0058;&#x0058;&#x0058;">JavaScript is expected to define <span
+  data-x="agent">agents</span> in more detail; in particular that they hold a set of <span
+  data-x="concept-global-object-realm">realms</span>: <a
+  href="https://github.com/tc39/ecma262/issues/882">tc39/ecma262 issue #882</a>.</p>
+
+  <dl>
+   <dt><dfn data-export="">Similar-origin window agent</dfn></dt>
+   <dd>
+    <p>An <span>agent</span> whose [[CanBlock]] is false and whose set of <span
+    data-x="JavaScript realm">realms</span> consists of all <span
+    data-x="concept-global-object-realm">realms</span> of <code>Window</code> objects whose
+    <span>relevant settings object</span>'s <span>responsible browsing context</span> is in the same
+    <span>unit of related similar-origin browsing contexts</span>.</p>
+
+    <p class="note">Two <code>Window</code> objects that are <span>same origin</span> can be in
+    different <span data-x="similar-origin window agent">similar-origin window agents</span>, for
+    instance if they are each in their own <span>unit of related similar-origin browsing
+    contexts</span>.</p>
+   </dd>
+
+   <dt><dfn data-export="">Dedicated worker agent</dfn></dt>
+   <dd><p>An <span>agent</span> whose [[CanBlock]] is true and whose set of <span data-x="JavaScript
+   realm">realms</span> consists of a single <code>DedicatedWorkerGlobalScope</code> object's <span
+   data-x="concept-global-object-realm">Realm</span>.</p></dd>
+
+   <dt><dfn data-export="">Shared worker agent</dfn></dt>
+   <dd><p>An <span>agent</span> whose [[CanBlock]] is true and whose set of <span data-x="JavaScript
+   realm">realms</span> consists a single <code>SharedWorkerGlobalScope</code> object's <span
+   data-x="concept-global-object-realm">Realm</span>.</p></dd>
+
+   <dt><dfn data-export="">Service worker agent</dfn></dt>
+   <dd><p>An <span>agent</span> whose [[CanBlock]] is false and whose set of <span data-x="JavaScript
+   realm">realms</span> consists of a single <code>ServiceWorkerGlobalScope</code> object's <span
+   data-x="concept-global-object-realm">Realm</span>.</p></dd>
+
+   <dt><dfn data-export="">Worklet agent</dfn></dt>
+   <dd>
+    <p>An <span>agent</span> whose [[CanBlock]] is false and whose set of <span
+    data-x="JavaScript realm">realms</span> consists of a single <code>WorkletGlobalScope</code>
+    object's <span data-x="concept-global-object-realm">Realm</span>.</p>
+
+    <p class="note">While conceptually it might be cleaner for worklets that end up with multiple
+    realms to put all those in the same agent, it is not observable in practice.</p>
+   </dd>
+  </dl>
+
+  <p>To determine the manifestation of JavaScript's <span>agent cluster</span> concept in user
+  agents, we define which <span data-x="agent">agents</span> <dfn>can share memory with</dfn>
+  another. Then all the <span data-x="agent">agents</span> that can <span>can share memory
+  with</span> each other form an <span>agent cluster</span>.</p>
+
+  <p class="note">The <span>can share memory with</span> relation is symmetric.</p>
+
+  <p>A <span>similar-origin window agent</span>, <span>dedicated worker agent</span>, <span>shared
+  worker agent</span>, or <span>service worker agent</span>, <var>agent</var>, <span>can share
+  memory with</span> any <span>dedicated worker agent</span> whose single <span data-x="JavaScript
+  realm">realm</span>'s <span data-x="concept-realm-global">global object</span>'s <span>owner
+  set</span> contains an item whose <span data-x="concept-relevant-realm">relevant Realm</span>
+  belongs to <var>agent</var>.</p>
+
+  <p class="note">We use item above as an <span>owner set</span> can contain <code>Document</code>
+  objects.</p>
+
+  <p class="&#x0058;&#x0058;&#x0058;">A <span>worklet agent</span> &hellip; currently worklets have
+  no clearly defined owner, see: <a
+  href="https://github.com/w3c/css-houdini-drafts/issues/224">w3c/css-houdini-drafts issue
+  #224</a>.</p>
+
+  <p>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>agent cluster</span>, and
+   thus can use <code>SharedArrayBuffer</code> instances to share memory with each other:</p>
+
+   <ul class="brief">
+    <li>A <code>Window</code> object and a dedicated worker that it created.</li>
+
+    <li>A worker (of any type) and a dedicated worker it created.</li>
+
+    <li>A <code>Window</code> object <var>A</var> and the <code>Window</code> object of an
+    <code>iframe</code> element that <var>A</var> created that could be <span>same
+    origin-domain</span> with <var>A</var>.</li>
+
+    <li>A <code>Window</code> object and a <span>same origin-domain</span> <code>Window</code>
+    object that opened it.</li>
+   </ul>
+
+   <p>The following pairs of global objects are <em>not</em> within the same <span>agent
+   cluster</span>, and thus cannot share memory:</p>
+
+   <ul class="brief">
+    <li>A <code>Window</code> object and a shared worker it created.</li>
+
+    <li>A worker (of any type) and a shared worker it created.</li>
+
+    <li>A <code>Window</code> object and a service worker it created.</li>
+
+    <li>A <code>Window</code> object <var></var> and the <code>Window</code> object of an
+    <code>iframe</code> element that <var>A</var> created that cannot be <span>same
+    origin-domain</span> with <var>A</var>.</li>
+   </ul>
+  </div>
+
   </div>
 
 
@@ -119795,6 +119925,9 @@ INSERT INTERFACES HERE
    <dt id="refsWHATWGWIKI">[WHATWGWIKI]</dt>
    <dd><cite><a href="https://wiki.whatwg.org/">The WHATWG Wiki</a></cite>. WHATWG.</dd>
 
+   <dt id="refsWORKLETS">[WORKLETS]</dt>
+   <dd><cite><a href="https://drafts.css-houdini.org/worklets/">Worklets</a></cite>. I. Kilpatrick. W3C.</dd>
+
    <dt id="refsWSP">[WSP]</dt>
    <dd><cite><a href="https://tools.ietf.org/html/rfc6455">The WebSocket protocol</a></cite>, I. Fette, A. Melnikov. IETF.</dd>