Raise the bar for SharedArrayBuffer via postMessage()

This depends on the work to add Cross-Origin-Opener-Policy and Cross-Origin-Embedder-Policy. It specifies how postMessage() is affected when both those headers are set for the agent clusters they impact.

Additionally, it exposes this state through self.crossOriginIsolated.

Tests: see links in #4732.

Closes #4732.

source

@@ -8302,13 +8302,26 @@ interface <dfn>DOMStringList</dfn> {
       <p>If ! <span>IsSharedArrayBuffer</span>(<var>value</var>) is true, then:
 
       <ol>
+       <li><p>Let <var>agentCluster</var> be <span>surrounding agent</span>'s
+       <span>agent cluster</span>.</p></li>
+
+       <li>
+        <p>If <var>agentCluster</var>'s <dfn>cross-origin isolated</dfn> is false, then throw a
+        <!-- TODO: this is not the place to <dfn> this, but doing this for now so the build does not
+             fail -->
+        <span>"<code>DataCloneError</code>"</span> <code>DOMException</code>.</p>
+
+        <p class="note">This check is only needed when serializing (and not when deserializing) as
+        <span>cross-origin isolated</span> cannot change over time and a
+        <code>SharedArrayBuffer</code> cannot leave an <span>agent cluster</span>.</p>
+       </li>
+
        <li><p>If <var>forStorage</var> is true, then throw a
        <span>"<code>DataCloneError</code>"</span> <code>DOMException</code>.</p></li>
 
        <li><p>Set <var>serialized</var> to { [[Type]]: "SharedArrayBuffer", [[ArrayBufferData]]:
        <var>value</var>.[[ArrayBufferData]], [[ArrayBufferByteLength]]: <var>size</var>,
-       [[AgentCluster]]: the <span>current Realm Record</span>'s corresponding <span>agent
-       cluster</span> }.</p></li>
+       [[AgentCluster]]: <var>agentCluster</var> }.</p></li>
       </ol>
      </li>
 
@@ -94167,6 +94180,7 @@ interface mixin <dfn>DocumentAndElementEventHandlers</dfn> {
 
 interface mixin <dfn>WindowOrWorkerGlobalScope</dfn> {
   [Replaceable] readonly attribute USVString <span data-x="dom-origin">origin</span>;
+  readonly attribute boolean <span data-x="dom-crossOriginIsolated">crossOriginIsolated</span>;
 
   // base64 utility methods
   DOMString <span data-x="dom-btoa">btoa</span>(DOMString data);
@@ -94190,8 +94204,17 @@ interface mixin <dfn>WindowOrWorkerGlobalScope</dfn> {
 
   <dl class="domintro">
    <dt><var>origin</var> = self . <code subdfn data-x="dom-origin">origin</code></dt>
-
    <dd><p>Returns the global object's <span>origin</span>, serialized as string.</p></dd>
+
+   <dt><var>origin</var> = self . <code subdfn data-x="dom-crossOriginIsolated">crossOriginIsolated</code></dt>
+   <dd>
+    <p>Returns the <span>surrounding agent</span>'s <span>agent cluster</span>'s
+    <span>cross-origin isolated</span>.</p>
+
+    <p>(This value depends on the <code data-x="">Cross-Origin-Opener-Policy</code> and <code
+    data-x="">Cross-Origin-Embedder-Policy</code> HTTP response headers and determines whether
+    <code>SharedArrayBuffer</code> can be used with <code data-x="">postMessage()</code> APIs.)</p>
+   </dd>
   </dl>
 
   <div class="example">
@@ -94216,6 +94239,10 @@ document.body.appendChild(frame)</code></pre>
   data-x="concept-settings-object-origin">origin</span>, <span data-x="serialization of an
   origin">serialized</span>.</p>
 
+  <p>The <dfn data-x="dom-crossOriginIsolated"><code>crossOriginIsolated</code></dfn> attribute's
+  getter must return the <span>surrounding agent</span>'s <span>agent cluster</span>'s
+  <span>cross-origin isolated</span>.</p>
+
 
   <h3 id="atob">Base64 utility methods</h3>