Clarify that "this" is not available while in parallel

Fixes #11110, by clarifying that Web IDL's this value is not available from steps running in parallel, even if those steps were kicked off my a non-parallel algorithm that _can_ access "this".

Clarify this in the parallelism example, and in prose about how to operate from while parallel, and fix all cases of "this's relevant _foo_" being used improperly from parallel steps, which is a good start to setting the right precedent.

Author: domfarolino

source

@@ -1637,23 +1637,25 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
     <li><p>Let <var>p</var> be a new promise created in <span>this</span>'s <span
     data-x="concept-relevant-realm">relevant realm</span>.</p></li>
 
+    <li><p>Let <var>global</var> be <span>this</span>'s <span>relevant global
+    object</span>.</p></li>
+
     <li>
      <p>Run the following steps <span>in parallel</span>:</p>
 
      <ol>
       <li><p>If <var>nameList</var> <span data-x="list contains">contains</span> <var>name</var>,
       then <span>queue a global task</span> on the <span>DOM manipulation task source</span> given
-      <span>this</span>'s <span>relevant global object</span> to reject <var>p</var> with a
-      <code>TypeError</code>, and abort these steps.</p></li>
+      <var>global</var> to reject <var>p</var> with a <code>TypeError</code>, and abort these
+      steps.</p></li>
 
       <li><p>Do some potentially lengthy work.</p></li>
 
       <li><p><span data-x="list append">Append</span> <var>name</var> to
       <var>nameList</var>.</p></li>
 
-      <li><p><span>Queue a global task</span> on the <span>DOM manipulation task source</span>
-      given <span>this</span>'s <span>relevant global object</span> to resolve <var>p</var> with
-      undefined.</p></li>
+      <li><p><span>Queue a global task</span> on the <span>DOM manipulation task source</span> given
+      <var>global</var> to resolve <var>p</var> with undefined.</p></li>
      </ol>
     </li>
 
@@ -1671,23 +1673,25 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
     <li><p>Let <var>p</var> be a new promise created in <span>this</span>'s <span
     data-x="concept-relevant-realm">relevant realm</span>.</p></li>
 
+    <li><p>Let <var>global</var> be <span>this</span>'s <span>relevant global
+    object</span>.</p></li>
+
     <li>
      <p><mark><span>Enqueue the following steps</span> to <var>nameListQueue</var>:</mark></p>
 
      <ol>
       <li><p>If <var>nameList</var> <span data-x="list contains">contains</span> <var>name</var>,
       then <span>queue a global task</span> on the <span>DOM manipulation task source</span> given
-      <span>this</span>'s <span>relevant global object</span> to reject <var>p</var> with a
-      <code>TypeError</code>, and abort these steps.</p></li>
+      <var>global</var> to reject <var>p</var> with a <code>TypeError</code>, and abort these
+      steps.</p></li>
 
       <li><p>Do some potentially lengthy work.</p></li>
 
       <li><p><span data-x="list append">Append</span> <var>name</var> to
       <var>nameList</var>.</p></li>
 
-      <li><p><span>Queue a global task</span> on the <span>DOM manipulation task source</span>
-      given <span>this</span>'s <span>relevant global object</span> to resolve <var>p</var> with
-      undefined.</p></li>
+      <li><p><span>Queue a global task</span> on the <span>DOM manipulation task source</span> given
+      <var>global</var> to resolve <var>p</var> with undefined.</p></li>
      </ol>
     </li>
 
@@ -72160,6 +72164,8 @@ interface <dfn interface>OffscreenCanvas</dfn> : <span>EventTarget</span> {
 
    <li><p>Let <var>result</var> be a new promise object.</p></li>
 
+   <li><p>Let <var>global</var> be <span>this</span>'s <span>relevant global object</span>.</p></li>
+
    <li>
     <p>Run these steps <span>in parallel</span>:</p>
 
@@ -72171,14 +72177,14 @@ interface <dfn interface>OffscreenCanvas</dfn> : <span>EventTarget</span> {
 
      <li>
       <p><span>Queue a global task</span> on the <span>canvas blob serialization task source</span>
-      given <span>this</span>'s <span>relevant global object</span> to run these steps:</p>
+      given <var>global</var> to run these steps:</p>
 
       <ol>
        <li><p>If <var>file</var> is null, then reject <var>result</var> with an
        <span>"<code>EncodingError</code>"</span> <code>DOMException</code>.</p></li>
 
        <li><p>Otherwise, resolve <var>result</var> with a new <code>Blob</code> object, created in
-       <span>this</span>'s <span data-x="concept-relevant-realm">relevant realm</span>, representing
+       <var>global</var>'s <span data-x="concept-relevant-realm">relevant realm</span>, representing
        <var>file</var>. <ref>FILEAPI</ref></p></li>
       </ol>
      </li>
@@ -114024,6 +114030,10 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
   Doing so would create data races observable to JavaScript code, since after all, your algorithm
   steps are running <em><span>in parallel</span></em> to the JavaScript code.</p>
 
+  <p>By extension, you cannot access Web IDL's <span>this</span> value from steps running <span>in
+  parallel</span>, even if those steps were activated by an algorithm that <em>does</em> have access
+  to the <span>this</span> value.</p>
+
   <p>You can, however, manipulate specification-level data structures and values from
   <cite>Infra</cite>, as those are realm-agnostic. They are never directly exposed to JavaScript without
   a specific conversion taking place (often <span data-x="concept-idl-convert">via Web IDL</span>).
@@ -123951,48 +123961,50 @@ dictionary <dfn dictionary>WorkletOptions</dfn> {
 
    <li><p>Let <var>promise</var> be a new promise.</p></li>
 
+   <li><p>Let <var>workletInstance</var> be <span>this</span>.</p></li>
+
    <li>
     <p>Run the following steps <span>in parallel</span>:</p>
 
     <ol>
      <li>
-      <p>If <span>this</span>'s <span data-x="concept-Worklet-global-scopes">global scopes</span>
-      <span data-x="list is empty">is empty</span>, then:</p>
+      <p>If <var>workletInstance</var>'s <span data-x="concept-Worklet-global-scopes">global
+      scopes</span> <span data-x="list is empty">is empty</span>, then:</p>
 
       <ol>
-       <li><p><span>Create a worklet global scope</span> given <span>this</span>.</p></li>
+       <li><p><span>Create a worklet global scope</span> given <var>workletInstance</var>.</p></li>
 
        <li><p>Optionally, <span data-x="create a worklet global scope">create</span> additional
-       global scope instances given <span>this</span>, depending on the specific worklet in question
-       and its specification.</p></li>
+       global scope instances given <var>workletInstance</var>, depending on the specific worklet in
+       question and its specification.</p></li>
 
        <li><p>Wait for all steps of the <span data-x="create a worklet global scope">creation</span>
        process(es) — including those taking place within the <span data-x="worklet agent">worklet
        agents</span> — to complete, before moving on.</p></li>
       </ol>
      </li>
 
-     <li><p>Let <var>pendingTasks</var> be <span>this</span>'s <span
+     <li><p>Let <var>pendingTasks</var> be <var>workletInstance</var>'s <span
      data-x="concept-Worklet-global-scopes">global scopes</span>'s <span data-x="list
      size">size</span>.</p></li>
 
      <li><p>Let <var>addedSuccessfully</var> be false.</p></li>
 
      <li>
       <p><span data-x="list iterate">For each</span> <var>workletGlobalScope</var> of
-      <span>this</span>'s <span data-x="concept-Worklet-global-scopes">global scopes</span>,
-      <span>queue a global task</span> on the <span>networking task source</span> given
-      <var>workletGlobalScope</var> to <span>fetch a worklet script graph</span> given
-      <var>moduleURLRecord</var>, <var>outsideSettings</var>, <span>this</span>'s <span>worklet
-      destination type</span>, <var>options</var>["<code
+      <var>workletInstance</var>'s <span data-x="concept-Worklet-global-scopes">global
+      scopes</span>, <span>queue a global task</span> on the <span>networking task source</span>
+      given <var>workletGlobalScope</var> to <span>fetch a worklet script graph</span> given
+      <var>moduleURLRecord</var>, <var>outsideSettings</var>, <var>workletInstance</var>'s
+      <span>worklet destination type</span>, <var>options</var>["<code
       data-x="dom-WorkletOptions-credentials">credentials</code>"], <var>workletGlobalScope</var>'s
-      <span>relevant settings object</span>, <span>this</span>'s <span
+      <span>relevant settings object</span>, <var>workletInstance</var>'s <span
       data-x="concept-Worklet-module-responses-map">module responses map</span>, and the following
       steps given <var>script</var>:</p>
 
       <p class="note">Only the first of these fetches will actually perform a network request; the
       ones for other <code>WorkletGlobalScope</code>s will reuse <span
-      data-x="concept-response">responses</span> from <span>this</span>'s <span
+      data-x="concept-response">responses</span> from <var>workletInstance</var>'s <span
       data-x="concept-Worklet-module-responses-map">module responses map</span>.</p>
 
       <ol>
@@ -124002,7 +124014,7 @@ dictionary <dfn dictionary>WorkletOptions</dfn> {
         <ol>
          <li>
           <p><span>Queue a global task</span> on the <span>networking task source</span> given
-          <span>this</span>'s <span>relevant global object</span> to perform the following
+          <var>workletInstance</var>'s <span>relevant global object</span> to perform the following
           steps:</p>
 
           <ol>
@@ -124030,7 +124042,7 @@ dictionary <dfn dictionary>WorkletOptions</dfn> {
         <ol>
          <li>
           <p><span>Queue a global task</span> on the <span>networking task source</span> given
-          <span>this</span>'s <span>relevant global object</span> to perform the following
+          <var>workletInstance</var>'s <span>relevant global object</span> to perform the following
           steps:</p>
 
           <ol>
@@ -124056,8 +124068,8 @@ dictionary <dfn dictionary>WorkletOptions</dfn> {
 
         <ol>
          <li><p><span data-x="list append">Append</span> <var>moduleURLRecord</var> to
-         <span>this</span>'s <span data-x="concept-Worklet-added-modules-list">added modules
-         list</span>.</p></li>
+         <var>workletInstance</var>'s <span data-x="concept-Worklet-added-modules-list">added
+         modules list</span>.</p></li>
 
          <li><p>Set <var>addedSuccessfully</var> to true.</p></li>
         </ol>
@@ -124066,9 +124078,9 @@ dictionary <dfn dictionary>WorkletOptions</dfn> {
        <li><p><span>Run a module script</span> given <var>script</var>.</p></li>
 
        <li>
-        <p><span>Queue a global task</span> on the
-        <span>networking task source</span> given <span>this</span>'s <span>relevant global
-        object</span> to perform the following steps:</p>
+        <p><span>Queue a global task</span> on the <span>networking task source</span> given
+        <var>workletInstance</var>'s <span>relevant global object</span> to perform the following
+        steps:</p>
 
         <ol>
          <li>