Introduce SameOrigin and SameAgentCluster for [Serializable]

Useful for WebAssembly.Module and RTCCertificate. SharedArrayBuffer can reuse the underlying infrastructure.

Fixes #4939. Helps with #4920.

source

@@ -8101,12 +8101,17 @@ interface <dfn>DOMStringList</dfn> {
   and deserialized by these steps. Typically the steps are very symmetric.</p>
 
   <p>The <code data-x="Serializable">[Serializable]</code> extended attribute must take no
-  arguments, and must not appear on anything other than an interface. It must appear only once on an
-  interface. It must not be used on a callback interface. If it appears on a partial interface or an
-  interface that is really a mixin, then it must also appear on the original or mixed-in-to
-  interface, and any supplied <span>serialization steps</span> and <span>deserialization
-  steps</span> for the partial interface or mixin should be understood as being appended to those of
-  the original or mixed-in-to interface.</p>
+  arguments, an identifier, or an identifier list, and must not appear on anything other than an
+  interface. It must appear only once on an interface. It must not be used on a callback interface.
+  If it appears on a partial interface or an interface that is really a mixin, then it must also
+  appear on the original or mixed-in-to interface, and any supplied <span>serialization steps</span>
+  and <span>deserialization steps</span> for the partial interface or mixin should be understood as
+  being appended to those of the original or mixed-in-to interface.</p>
+
+  <p>The identifiers for the <code data-x="Serializable">[Serializable]</code> extended
+  attribute are <dfn data-x="Serializable SameOrigin"><code>SameOrigin</code></dfn> and <dfn
+  data-x="Serializable SameAgentCluster"><code>SameAgentCluster</code></dfn>. These can be used to
+  restrict where a <span>serializable object</span> can be deserialized.</p>
 
   <div class="example">
    <p>Let's say we were defining a platform object <code data-x="">Person</code>, which had
@@ -8296,8 +8301,7 @@ interface <dfn>DOMStringList</dfn> {
 
        <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>
+       [[EnforceAgentCluster]]: null }.</p></li>
       </ol>
      </li>
 
@@ -8460,6 +8464,15 @@ interface <dfn>DOMStringList</dfn> {
 
      <li><p>Set <var>serialized</var> to { [[Type]]: <var>typeString</var> }.</p></li>
 
+     <li><p>If <var>value</var>'s interface <code data-x="Serializable">[Serializable]</code>
+     extended attribute contains the <code data-x="Serializable SameOrigin">SameOrigin</code>
+     identifier, then set <var>serialized</var>.[[EnforceOrigin]] to null.</p></li>
+
+     <li><p>If <var>value</var>'s interface <code data-x="Serializable">[Serializable]</code>
+     extended attribute contains the <code data-x="Serializable
+     SameAgentCluster">SameAgentCluster</code> identifier, then set
+     <var>serialized</var>.[[EnforceAgentCluster]] to null.</p></li>
+
      <li><p>Set <var>deep</var> to true.</p></li>
     </ol>
    </li>
@@ -8642,6 +8655,14 @@ interface <dfn>DOMStringList</dfn> {
     </ol>
    </li>
 
+   <li><p>If <var>serialized</var> has an [[EnforceOrigin]] internal slot, then set
+   <var>serialized</var>.[[EnforceOrigin]] to the <span>current settings object</span>'s <span
+   data-x="concept-settings-object-origin">origin</span>.</p></li>
+
+   <li><p>If <var>serialized</var> has an [[EnforceAgentCluster]] internal slot, then set
+   <var>serialized</var>.[[EnforceAgentCluster]] to the <span>surrounding agent</span>'s <span>agent
+   cluster</span>.</p></li>
+
    <li><p>Return <var>serialized</var>.</p></li>
   </ol>
 
@@ -8712,6 +8733,16 @@ o.myself = o;</code></pre>
 
    <li><p>Let <var>value</var> be an uninitialized value.</p></li>
 
+   <li><p>If <var>serialized</var> has an [[EnforceOrigin]] internal slot and its value is not
+   <span>same origin</span> with <var>targetRealm</var>'s <span
+   data-x="concept-realm-settings-object">settings object</span>'s <span
+   data-x="concept-settings-object-origin">origin</span>, then throw a
+   <span>"<code>DataCloneError</code>"</span> <code>DOMException</code>.</p></li>
+
+   <li><p>If <var>serialized</var> has an [[EnforceAgentCluster]] internal slot and its value is not
+   <var>targetRealm</var>'s corresponding <span>agent cluster</span>, then throw a
+   <span>"<code>DataCloneError</code>"</span> <code>DOMException</code>.</p></li>
+
    <li><p>If <var>serialized</var>.[[Type]] is "primitive", then set <var>value</var> to
    <var>serialized</var>.[[Value]].</p>
 
@@ -8741,20 +8772,11 @@ o.myself = o;</code></pre>
    <var>serialized</var>.[[OriginalSource]], and whose [[OriginalFlags]] internal slot value is
    <var>serialized</var>.[[OriginalFlags]].</p></li>
 
-   <li>
-    <p>Otherwise, if <var>serialized</var>.[[Type]] is "SharedArrayBuffer", then:</p>
-
-    <ol>
-     <li><p>If <var>targetRealm</var>'s corresponding <span>agent cluster</span> is not
-     <var>serialized</var>.[[AgentCluster]], then then throw a
-     <span>"<code>DataCloneError</code>"</span> <code>DOMException</code>.</p></li>
-
-     <li><p>Otherwise, set <var>value</var> to a new SharedArrayBuffer object in
-     <var>targetRealm</var> whose [[ArrayBufferData]] internal slot value is
-     <var>serialized</var>.[[ArrayBufferData]] and whose [[ArrayBufferByteLength]] internal slot
-     value is <var>serialized</var>.[[ArrayBufferByteLength]].</p></li>
-    </ol>
-   </li>
+   <li><p>Otherwise, if <var>serialized</var>.[[Type]] is "SharedArrayBuffer", then set
+   <var>value</var> to a new SharedArrayBuffer object in <var>targetRealm</var> whose
+   [[ArrayBufferData]] internal slot value is <var>serialized</var>.[[ArrayBufferData]], and whose
+   [[ArrayBufferByteLength]] internal slot value is
+   <var>serialized</var>.[[ArrayBufferByteLength]].</p></li>
 
    <li>
     <p>Otherwise, if <var>serialized</var>.[[Type]] is "ArrayBuffer", then set <var>value</var> to a