Editorial: clarify worker event retargeting

Previously, the standard used the ill-defined phrase "All messages received by that port must immediately be retargeted at the Worker object." This was not a sufficiently clear processing model. Instead, introduce the "message event target" concept to fire the events on the appropriate EventTarget.

This also adds linkable definitions for "inside port" and "outside port", and references them appropriately.

Author: nicolo-ribaudo

source

@@ -112761,7 +112761,7 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
   </ol>
 
   <p class="note">Certain <span>event handler IDL attributes</span> have additional requirements, in
-  particular the <code data-x="handler-MessagePort-onmessage">onmessage</code> attribute of
+  particular the <code data-x="handler-MessageEventTarget-onmessage">onmessage</code> attribute of
   <code>MessagePort</code> objects.</p>
 
   </div>
@@ -118994,13 +118994,13 @@ function handleMessage(event) {
    <p>The key difference is that when using <code
    data-x="dom-EventTarget-addEventListener">addEventListener()</code>, the <code
    data-x="dom-MessagePort-start">start()</code> method must also be invoked. When using <code
-   data-x="handler-MessagePort-onmessage">onmessage</code>, the call to <code
+   data-x="handler-MessageEventTarget-onmessage">onmessage</code>, the call to <code
    data-x="dom-MessagePort-start">start()</code> is implied.</p>
 
    <p>The <code data-x="dom-MessagePort-start">start()</code> method, whether called explicitly or
-   implicitly (by setting <code data-x="handler-MessagePort-onmessage">onmessage</code>), starts the
-   flow of messages: messages posted on message ports are initially paused, so that they don't get
-   dropped on the floor before the script has had a chance to set up its handlers.</p>
+   implicitly (by setting <code data-x="handler-MessageEventTarget-onmessage">onmessage</code>),
+   starts the flow of messages: messages posted on message ports are initially paused, so that they
+   don't get dropped on the floor before the script has had a chance to set up its handlers.</p>
 
   </div>
 
@@ -119142,7 +119142,29 @@ interface <dfn interface>MessageChannel</dfn> {
 
   </div>
 
+  <h4 w-nodev><span id="the-messageeventtarget-abstract-interface"></span>The
+  <code>MessageEventTarget</code> mixin</h4>
+  <h4 w-dev>Properties present on <code>MessagePort</code>, <code>Worker</code>, and
+  <code>DedicatedWorkerGlobalScope</code></h4>
 
+  <pre><code class="idl">interface mixin <dfn interface>MessageEventTarget</dfn> {
+  attribute <span>EventHandler</span> <span data-x="handler-MessageEventTarget-onmessage">onmessage</span>;
+  attribute <span>EventHandler</span> <span data-x="handler-MessageEventTarget-onmessageerror">onmessageerror</span>;
+};</code></pre>
+
+   <p>The following are the <span>event handlers</span> (and their corresponding <span data-x="event
+   handler event type">event handler event types</span>) <span w-nodev>that must be</span> supported,
+   as <span>event handler IDL attributes</span>, by <span w-nodev>objects implementing the
+   <code>MessageEventTarget</code> interface</span><span w-dev><code>MessagePort</code>,
+   <code>Worker</code>, and <code>DedicatedWorkerGlobalScope</code> objects</span>:</p>
+
+   <table>
+    <thead>
+     <tr><th><span data-x="event handlers">Event handler</span> <th><span>Event handler event type</span>
+    <tbody>
+     <tr><td><span id="handler-messageport-onmessage"></span><span id="handler-worker-onmessage"></span><span id="handler-dedicatedworkerglobalscope-onmessage"></span><dfn attribute for="MessageEventTarget"><code data-x="handler-MessageEventTarget-onmessage">onmessage</code></dfn> <td> <code data-x="event-message">message</code>
+     <tr><td><span id="handler-messageport-onmessageerror"></span><span id="handler-worker-onmessageerror"></span><span id="handler-dedicatedworkerglobalscope-onmessageerror"></span><dfn attribute for="MessageEventTarget"><code data-x="handler-MessageEventTarget-onmessageerror">onmessageerror</code></dfn> <td> <code data-x="event-messageerror">messageerror</code>
+   </table>
 
   <h4>Message ports</h4>
 
@@ -119157,11 +119179,11 @@ interface <dfn interface>MessagePort</dfn> : <span>EventTarget</span> {
   undefined <span data-x="dom-MessagePort-close">close</span>();
 
   // event handlers
-  attribute <span>EventHandler</span> <span data-x="handler-MessagePort-onmessage">onmessage</span>;
-  attribute <span>EventHandler</span> <span data-x="handler-MessagePort-onmessageerror">onmessageerror</span>;
   attribute <span>EventHandler</span> <span data-x="handler-MessagePort-onclose">onclose</span>;
 };
 
+<span>MessagePort</span> includes <span>MessageEventTarget</span>;
+
 dictionary <dfn dictionary>StructuredSerializeOptions</dfn> {
   sequence&lt;<span data-x="idl-object">object</span>&gt; <dfn dict-member for="StructuredSerializeOptions" data-x="dom-StructuredSerializeOptions-transfer">transfer</dfn> = [];
 };</code></pre>
@@ -119187,6 +119209,11 @@ dictionary <dfn dictionary>StructuredSerializeOptions</dfn> {
 
   <div w-nodev>
 
+  <p>Each <code>MessagePort</code> object has a <dfn>message event target</dfn> (a
+  <code>MessageEventTarget</code>), to which the <code data-x="event-message">message</code> and <code
+  data-x="event-messageerror">messageerror</code> events are dispatched. Unless otherwise
+  specified, it defaults to the <code>MessagePort</code> object itself.</p>
+
   <p>Each <code>MessagePort</code> object can be entangled with another (a symmetric relationship).
   Each <code>MessagePort</code> object also has a <span>task source</span> called the <dfn>port
   message queue</dfn>, initially empty. A <span>port message queue</span> can be enabled or
@@ -119377,6 +119404,9 @@ dictionary <dfn dictionary>StructuredSerializeOptions</dfn> {
       itself was transferred and thus all its tasks moved along with it.</p>
      </li>
 
+     <li><p>Let <var>messageEventTarget</var> be <var>finalTargetPort</var>'s <span>message event
+     target</span>.</p></li>
+
      <li><p>Let <var>targetRealm</var> be <var>finalTargetPort</var>'s <span
      data-x="concept-relevant-realm">relevant realm</span>.</p></li>
 
@@ -119387,7 +119417,7 @@ dictionary <dfn dictionary>StructuredSerializeOptions</dfn> {
 
       <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>finalTargetPort</var>, using <code>MessageEvent</code>, and then return.
+      <var>messageEventTarget</var>, using <code>MessageEvent</code>, and then return.
      </li>
 
      <li><p>Let <var>messageClone</var> be <var>deserializeRecord</var>.[[Deserialized]].</p></li>
@@ -119397,7 +119427,7 @@ dictionary <dfn dictionary>StructuredSerializeOptions</dfn> {
      maintaining their relative order.</p></li>
 
      <li><p><span data-x="concept-event-fire">Fire an event</span> named <code
-     data-x="event-message">message</code> at <var>finalTargetPort</var>, using
+     data-x="event-message">message</code> at <var>messageEventTarget</var>, using
      <code>MessageEvent</code>, with 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 initialized to
@@ -119462,15 +119492,13 @@ dictionary <dfn dictionary>StructuredSerializeOptions</dfn> {
    <thead>
     <tr><th><span data-x="event handlers">Event handler</span> <th><span>Event handler event type</span>
    <tbody>
-    <tr><td><dfn attribute for="MessagePort"><code data-x="handler-MessagePort-onmessage">onmessage</code></dfn> <td> <code data-x="event-message">message</code>
-    <tr><td><dfn attribute for="MessagePort"><code data-x="handler-MessagePort-onmessageerror">onmessageerror</code></dfn> <td> <code data-x="event-messageerror">messageerror</code>
     <tr><td><dfn attribute for="MessagePort"><code data-x="handler-MessagePort-onclose">onclose</code></dfn> <td> <code data-x="event-close">close</code>
   </table>
 
   <p>The first time a <code>MessagePort</code> object's <code
-  data-x="handler-MessagePort-onmessage">onmessage</code> IDL attribute is set, the port's <span>port
-  message queue</span> must be enabled, as if the <code data-x="dom-MessagePort-start">start()</code>
-  method had been called.</p>
+  data-x="handler-MessageEventTarget-onmessage">onmessage</code> IDL attribute is set, the port's
+  <span>port message queue</span> must be enabled, as if the <code
+  data-x="dom-MessagePort-start">start()</code> method had been called.</p>
 
   </div>
 
@@ -119797,8 +119825,8 @@ function showLogout() {
 
   <p>The <code data-x="dom-Worker">Worker()</code> constructor call creates a worker and returns a
   <code>Worker</code> object representing that worker, which is used to communicate with the worker.
-  That object's <code data-x="handler-Worker-onmessage">onmessage</code> event handler allows the
-  code to receive messages from the worker.</p>
+  That object's <code data-x="handler-MessageEventTarget-onmessage">onmessage</code> event handler
+  allows the code to receive messages from the worker.</p>
 
   <p>The worker itself is as follows:</p>
 
@@ -120065,8 +120093,8 @@ function showLogout() {
   other ports.</p>
 
   <p>To receive messages from a dedicated worker, use the <code
-  data-x="handler-worker-onmessage">onmessage</code> <span data-x="event handler IDL attributes">event
-  handler IDL attribute</span> on the <code>Worker</code> object:</p>
+  data-x="handler-MessageEventTarget-onmessage">onmessage</code> <span data-x="event handler IDL
+  attributes">event handler IDL attribute</span> on the <code>Worker</code> object:</p>
 
   <pre><code class="js">worker.onmessage = function (event) { ... };</code></pre>
 
@@ -120091,7 +120119,7 @@ function showLogout() {
 }, [buffer]);</code></pre>
 
   <p>To receive a message inside the worker, the <code
-  data-x="handler-DedicatedWorkerGlobalScope-onmessage">onmessage</code> <span data-x="event handler
+  data-x="handler-MessageEventTarget-onmessage">onmessage</code> <span data-x="event handler
   IDL attributes">event handler IDL attribute</span> is used.</p>
 
   <pre><code class="js">onmessage = function (event) { ... };</code></pre>
@@ -120295,18 +120323,14 @@ interface <dfn interface>DedicatedWorkerGlobalScope</dfn> : <span>WorkerGlobalSc
   undefined <span data-x="dom-DedicatedWorkerGlobalScope-postMessage-options">postMessage</span>(any message, optional <span>StructuredSerializeOptions</span> options = {});
 
   undefined <span data-x="dom-DedicatedWorkerGlobalScope-close">close</span>();
+};
 
-  attribute <span>EventHandler</span> <span data-x="handler-DedicatedWorkerGlobalScope-onmessage">onmessage</span>;
-  attribute <span>EventHandler</span> <span data-x="handler-DedicatedWorkerGlobalScope-onmessageerror">onmessageerror</span>;
-};</code></pre>
-
-  <p><code>DedicatedWorkerGlobalScope</code> objects act as if they had an implicit
-  <code>MessagePort</code> associated with them. This port is part of a channel that is set up when
-  the worker is created, but it is not exposed.<span w-nodev> This object must never be garbage
-  collected before the <code>DedicatedWorkerGlobalScope</code> object.</span></p>
+<span>DedicatedWorkerGlobalScope</span> includes <span>MessageEventTarget</span>;</code></pre>
 
-  <p w-nodev>All messages received by that port must immediately be retargeted at the
-  <code>DedicatedWorkerGlobalScope</code> object.</p>
+  <p><code>DedicatedWorkerGlobalScope</code> objects have an associated <dfn>inside port</dfn> (a
+  <code>MessagePort</code>). This port is part of a channel that is set up when the worker is
+  created, but it is not exposed.<span w-nodev> This object must never be garbage collected before
+  the <code>DedicatedWorkerGlobalScope</code> object.</span></p>
 
   <dl class="domintro">
    <dt><code data-x=""><var>dedicatedWorkerGlobal</var>.<span subdfn data-x="dom-DedicatedWorkerGlobalScope-name">name</span></code></dt>
@@ -120363,21 +120387,6 @@ interface <dfn interface>DedicatedWorkerGlobalScope</dfn> : <span>WorkerGlobalSc
 
   </div>
 
-  <p>The following are the <span>event handlers</span> (and their corresponding <span data-x="event
-  handler event type">event handler event types</span>) <span w-nodev>that must be</span> supported,
-  as <span>event handler IDL attributes</span>, by objects implementing the
-  <code>DedicatedWorkerGlobalScope</code> interface:</p>
-
-  <table>
-   <thead>
-    <tr><th><span data-x="event handlers">Event handler</span> <th><span>Event handler event type</span>
-   <tbody>
-    <tr><td><dfn attribute for="DedicatedWorkerGlobalScope"><code data-x="handler-DedicatedWorkerGlobalScope-onmessage">onmessage</code></dfn> <td> <code data-x="event-message">message</code>
-    <tr><td><dfn attribute for="DedicatedWorkerGlobalScope"><code data-x="handler-DedicatedWorkerGlobalScope-onmessageerror">onmessageerror</code></dfn> <td> <code data-x="event-messageerror">messageerror</code>
-  </table>
-
-
-
   <h5>Shared workers and the <code subdfn>SharedWorkerGlobalScope</code> interface</h5>
 
   <pre><code class="idl">[Global=(Worker,SharedWorker),Exposed=SharedWorker]
@@ -120733,7 +120742,17 @@ interface <dfn interface>SharedWorkerGlobalScope</dfn> : <span>WorkerGlobalScope
      <var>inside settings</var>'s <span data-x="environment settings object's
      realm">realm</span>.</p></li>
 
-     <li><p>Associate <var>inside port</var> with <var>worker global scope</var>.</p></li>
+     <li>
+      <p>If <var>shared</var> is false, then:</p>
+
+      <ol>
+       <li><p>Set <var>inside port</var>'s <span>message event target</span> to <var>worker global
+       scope</var>.</p></li>
+
+       <li><p>Set <var>worker global scope</var>'s <span>inside port</span> to <var>inside
+       port</var>.</p></li>
+      </ol>
+     </li>
 
      <li><p><span>Entangle</span> <var>outside port</var> and <var>inside port</var>.</p></li>
 
@@ -120987,8 +121006,6 @@ interface <dfn interface>Worker</dfn> : <span>EventTarget</span> {
 
   undefined <span data-x="dom-Worker-postMessage">postMessage</span>(any message, sequence&lt;<span data-x="idl-object">object</span>&gt; transfer);
   undefined <span data-x="dom-Worker-postMessage-options">postMessage</span>(any message, optional <span>StructuredSerializeOptions</span> options = {});
-  attribute <span>EventHandler</span> <span data-x="handler-Worker-onmessage">onmessage</span>;
-  attribute <span>EventHandler</span> <span data-x="handler-Worker-onmessageerror">onmessageerror</span>;
 };
 
 dictionary <dfn dictionary>WorkerOptions</dfn> {
@@ -120999,7 +121016,8 @@ dictionary <dfn dictionary>WorkerOptions</dfn> {
 
 enum <dfn enum>WorkerType</dfn> { "classic", "module" };
 
-<span>Worker</span> includes <span>AbstractWorker</span>;</code></pre>
+<span>Worker</span> includes <span>AbstractWorker</span>;
+<span>Worker</span> includes <span>MessageEventTarget</span>;</code></pre>
 
   <dl class="domintro">
    <dt><code data-x=""><var>worker</var> = new <span subdfn data-x="dom-Worker">Worker</span>(<var>scriptURL</var> [, <var>options</var> ])</code></dt>
@@ -121024,17 +121042,15 @@ enum <dfn enum>WorkerType</dfn> { "classic", "module" };
 
   <div w-nodev>
 
+  <p>Each <code>Worker</code> object has an associated <dfn for="Worker">outside port</dfn> (a
+  <code>MessagePort</code>). This port is part of a channel that is set up when the worker is
+  created, but it is not exposed. This object must never be garbage collected before the
+  <code>Worker</code> object.</p>
+
   <p>The <dfn method for="Worker"><code data-x="dom-Worker-terminate">terminate()</code></dfn>
   method, when invoked, must cause the <span>terminate a worker</span> algorithm to be run on the
   worker with which the object is associated.</p>
 
-  <p><code>Worker</code> objects act as if they had an implicit <code>MessagePort</code> associated
-  with them. This port is part of a channel that is set up when the worker is created, but it is not
-  exposed. This object must never be garbage collected before the <code>Worker</code> object.</p>
-
-  <p>All messages received by that port must immediately be retargeted at the <code>Worker</code>
-  object.</p>
-
   <p>The <dfn method for="Worker"><code
   data-x="dom-Worker-postMessage">postMessage(<var>message</var>, <var>transfer</var>)</code></dfn>
   and <dfn method for="Worker"><code
@@ -121043,8 +121059,8 @@ enum <dfn enum>WorkerType</dfn> { "classic", "module" };
   they immediately invoked the respective <code
   data-x="dom-MessagePort-postMessage">postMessage(<var>message</var>, <var>transfer</var>)</code>
   and <code data-x="dom-MessagePort-postMessage-options">postMessage(<var>message</var>,
-  <var>options</var>)</code> on the port, with the same arguments, and returned the same return
-  value.</p>
+  <var>options</var>)</code> on <span>this</span>'s <span>outside port</span>, with the same
+  arguments, and returned the same return value.</p>
 
   </div>
 
@@ -121057,19 +121073,6 @@ enum <dfn enum>WorkerType</dfn> { "classic", "module" };
 
   </div>
 
-  <p>The following are the <span>event handlers</span> (and their corresponding <span data-x="event
-  handler event type">event handler event types</span>) <span w-nodev>that must be</span> supported,
-  as <span>event handler IDL attributes</span>, by objects implementing the <code>Worker</code>
-  interface:</p>
-
-  <table>
-   <thead>
-    <tr><th><span data-x="event handlers">Event handler</span> <th><span>Event handler event type</span>
-   <tbody>
-    <tr><td><dfn attribute for="Worker"><code data-x="handler-Worker-onmessage">onmessage</code></dfn> <td> <code data-x="event-message">message</code>
-    <tr><td><dfn attribute for="Worker"><code data-x="handler-Worker-onmessageerror">onmessageerror</code></dfn> <td> <code data-x="event-messageerror">messageerror</code>
-  </table>
-
   <div w-nodev>
 
   <hr>
@@ -121105,7 +121108,10 @@ enum <dfn enum>WorkerType</dfn> { "classic", "module" };
    <li><p>Let <var>outside port</var> be a <span>new</span> <code>MessagePort</code> in <var>outside
    settings</var>'s <span data-x="environment settings object's realm">realm</span>.</p></li>
 
-   <li><p>Associate the <var>outside port</var> with <var>worker</var>.</p></li>
+   <li><p>Set <var>outside port</var>'s <span>message event target</span> to
+   <var>worker</var>.</p></li>
+
+   <li><p>Set <var>worker</var>'s <span>outside port</span> to <var>outside port</var>.</p></li>
 
    <li>
     <p>Run this step <span>in parallel</span>:</p>