Teach MessageEvent to work with origins

Currently, MessageEvent's origin holds a serialized origin. In order for Origin.from(...) to function as intended for sources with opaque origins, we need to hold an origin on MessageEvent instead.

This patch adds an "origin" concept to `MessageEvent`, and shifts call sites to initialize that value with an origin whenever possible. It also adjusts the "extract an origin" steps to return the stored origin when present, and to return null in cases where the object was initialized with an untrusted origin representation (e.g., when its constructor is called with a MessageEventInit dictionary).

Closes #11993.

Author: mikewest

source

@@ -127963,6 +127963,10 @@ dictionary <dfn dictionary>MessageEventInit</dfn> : <span>EventInit</span> {
 
 typedef (<span>WindowProxy</span> or <span>MessagePort</span> or <span>ServiceWorker</span>) <dfn typedef>MessageEventSource</dfn>;</code></pre>
 
+  <p>Each <code>MessageEvent</code> has an <dfn data-x="concept-MessageEvent-origin"
+  for="MessageEvent" export>origin</dfn> (an <span>origin</span>, a string, or null), initially
+  null.</p>
+
   <dl class="domintro">
    <dt><code data-x=""><var>event</var>.<span subdfn data-x="dom-MessageEvent-data">data</span></code></dt>
 
@@ -128010,17 +128014,37 @@ typedef (<span>WindowProxy</span> or <span>MessagePort</span> or <span>ServiceWo
 
   <div algorithm>
   <p>The <dfn attribute for="MessageEvent"><code
-  data-x="dom-MessageEvent-origin">origin</code></dfn> attribute must return the value it was
-  initialized to. It represents, in <span>server-sent events</span> and <span>cross-document
-  messaging</span>, the <span data-x="concept-document-origin">origin</span> of the document that
-  sent the message (typically the scheme, hostname, and port of the document, but not its path or
-  <span data-x="concept-url-fragment">fragment</span>).</p>
+  data-x="dom-MessageEvent-origin">origin</code></dfn> attribute represents, in <span>server-sent
+  events</span> and <span>cross-document messaging</span>, the <span
+  data-x="concept-document-origin">origin</span> of the document that sent the message (typically
+  the scheme, hostname, and port of the document, but not its path or <span
+  data-x="concept-url-fragment">fragment</span>).</p>
+
+  <p>The <code data-x="dom-MessageEvent-origin">origin</code> getter steps are:</p>
+
+  <ol>
+   <li><p>If <span>this</span>'s <span data-x="concept-MessageEvent-origin">origin</span> is
+   an <span>origin</span>, then return the <span
+   data-x="serialization of an origin">serialization</span> of <span>this</span>'s <span
+   data-x="concept-MessageEvent-origin">origin</span>.</p></li>
+
+   <li><p>If <span>this</span>'s <span data-x="concept-MessageEvent-origin">origin</span> is
+   null, then return the empty string.</p></li>
+
+   <li><p>Return <span>this</span>'s <span
+   data-x="concept-MessageEvent-origin">origin</span>.</p></li>
+  </ol>
+
+  <p>When the <code data-x="dom-MessageEvent-origin">origin</code> attribute is "initialized"
+  (during <code>MessageEvent</code>'s <span data-x="dom-Event-constructor">constructor</span>, for
+  example), the initialization value is placed into the object's <span
+  data-x="concept-MessageEvent-origin">origin</span>.</p>
   </div>
 
   <div algorithm>
   <p>Objects implementing the <code>MessageEvent</code> interface's <span>extract an origin</span>
-  steps are to return <span>this</span>'s <span>relevant settings object</span>'s
-  <span data-x="concept-settings-object-origin">origin</span>.</p>
+  steps are to return <span>this</span>'s <span data-x="concept-MessageEvent-origin">origin</span>
+  if it is an <span>origin</span>; otherwise null.</p>
   </div>
 
   <div algorithm>
@@ -128687,10 +128711,10 @@ any-char      = %x0000-0009 / %x000B-000C / %x000E-10FFFF
 
    <li><p>Initialize <var>event</var>'s <code data-x="dom-Event-type">type</code> attribute to
    "<code data-x="event-message">message</code>", its <code
-   data-x="dom-MessageEvent-data">data</code> attribute to <var>data</var>, its <code
-   data-x="dom-MessageEvent-origin">origin</code> attribute to the <span data-x="serialization of
-   an origin">serialization</span> of the <span data-x="concept-url-origin">origin</span> of the
-   event stream's final URL (i.e., the URL after redirects), and its <code
+   data-x="dom-MessageEvent-data">data</code> attribute to <var>data</var>, its <span
+   data-x="concept-MessageEvent-origin">origin</span> to the <span
+   data-x="concept-url-origin">origin</span> of the event stream's final URL (i.e., the URL after
+   redirects), and its <code
    data-x="dom-MessageEvent-lastEventId">lastEventId</code> attribute to the <span
    data-x="concept-event-stream-last-event-id">last event ID string</span> of the event
    source.</p></li>
@@ -129158,8 +129182,7 @@ function receiver(e) {
      <code>Document</code></span>'s <span data-x="concept-document-origin">origin</span> is not
      <span>same origin</span> with <var>targetOrigin</var>, then return.</p></li>
 
-     <li><p>Let <var>origin</var> be the <span data-x="serialization of an
-     origin">serialization</span> of <var>incumbentSettings</var>'s <span
+     <li><p>Let <var>origin</var> be <var>incumbentSettings</var>'s <span
      data-x="concept-settings-object-origin">origin</span>.</p></li>
 
      <li><p>Let <var>source</var> be the <code>WindowProxy</code> object corresponding to
@@ -129173,8 +129196,8 @@ function receiver(e) {
 
       <p>If this throws an exception, catch it, <span data-x="concept-event-fire">fire an
       event</span> named <code data-x="event-messageerror">messageerror</code> at
-      <var>targetWindow</var>, using <code>MessageEvent</code>, with the <code
-      data-x="dom-MessageEvent-origin">origin</code> attribute initialized to <var>origin</var> and
+      <var>targetWindow</var>, using <code>MessageEvent</code>, with its <span
+      data-x="concept-MessageEvent-origin">origin</span> initialized to <var>origin</var> and
       the <code data-x="dom-MessageEvent-source">source</code> attribute initialized to
       <var>source</var>, and then return.</p>
      </li>
@@ -129187,8 +129210,8 @@ function receiver(e) {
 
      <li><p><span data-x="concept-event-fire">Fire an event</span> named <code
      data-x="event-message">message</code> at <var>targetWindow</var>, using
-     <code>MessageEvent</code>, with the <code data-x="dom-MessageEvent-origin">origin</code>
-     attribute initialized to <var>origin</var>, the <code
+     <code>MessageEvent</code>, with its <span
+     data-x="concept-MessageEvent-origin">origin</span> initialized to <var>origin</var>, the <code
      data-x="dom-MessageEvent-source">source</code> attribute initialized to <var>source</var>, the
      <code data-x="dom-MessageEvent-data">data</code> attribute initialized to
      <var>messageClone</var>, and the <code data-x="dom-MessageEvent-ports">ports</code> attribute
@@ -130057,17 +130080,16 @@ interface <dfn interface>BroadcastChannel</dfn> : <span>EventTarget</span> {
 
       <p>If this throws an exception, catch it, <span data-x="concept-event-fire">fire an
       event</span> named <code data-x="event-messageerror">messageerror</code> at
-      <var>destination</var>, using <code>MessageEvent</code>, with the <code
-      data-x="dom-MessageEvent-origin">origin</code> attribute initialized to the <span
-      data-x="serialization of an origin">serialization</span> of <var>sourceOrigin</var>, and then
-      abort these steps.</p>
+      <var>destination</var>, using <code>MessageEvent</code>, with its <span
+      data-x="concept-MessageEvent-origin">origin</span> initialized to <var>sourceOrigin</var>, and
+      then abort these steps.</p>
      </li>
 
      <li><p><span data-x="concept-event-fire">Fire an event</span> named <code
      data-x="event-message">message</code> at <var>destination</var>, using
      <code>MessageEvent</code>, with the <code data-x="dom-MessageEvent-data">data</code> attribute
-     initialized to <var>data</var> and the <code data-x="dom-MessageEvent-origin">origin</code>
-     attribute initialized to the <span data-x="serialization of an origin">serialization</span> of
+     initialized to <var>data</var> and its <span
+     data-x="concept-MessageEvent-origin">origin</span> initialized to
      <var>sourceOrigin</var>.</p></li>
     </ol>
    </li>