Handle module resolution when there is no active script

Closes #3295. This also fixes a related inaccurate assert in EnqueueJob,
putting in guards for a null active script there as well and adding
examples explaining the various scenarios.

Author: domenic

source

@@ -86477,6 +86477,10 @@ interface <dfn>ApplicationCache</dfn> : <span>EventTarget</span> {
    <li><p>Return <var>record</var>.[[HostDefined]].</p></li>
   </ol>
 
+  <p class="note">The <span>active script</span> concept is so far only used by the
+  <code>import()</code> feature, to determine the <span data-x="concept-script-base-url">base
+  URL</span> to use for resolving relative module specifiers.</p>
+
   <hr>
 
   <p>An <dfn data-export="">environment</dfn> is an object that identifies the settings of a
@@ -88278,14 +88282,54 @@ document.querySelector("button").addEventListener("click", bound);
 
    <li><p>Let <var>active script</var> be the <span>active script</span>.</p></li>
 
-   <li><p>Assert: <var>active script</var> is not null, as jobs are only enqueued by the JavaScript
-   specification while a script is active.</p></li>
+   <li><p>Let <var>script execution context</var> be null.</p></li>
 
-   <li><p>Let <var>script execution context</var> be a new <span>JavaScript execution
-   context</span>, with its Function field set to null, its Realm field set to <var>active
-   script</var>'s <span>settings object</span>'s <span data-x="environment settings object's
-   Realm">Realm</span>, and its ScriptOrModule set to <var>active script</var>'s <span
-   data-x="concept-script-record">record</span>.</p></li>
+   <li>
+    <p>If <var>active script</var> is not null, set <var>script execution context</var> to a new
+    <span>JavaScript execution context</span>, with its Function field set to null, its Realm field
+    set to <var>active script</var>'s <span>settings object</span>'s <span data-x="environment
+    settings object's Realm">Realm</span>, and its ScriptOrModule set to <var>active script</var>'s
+    <span data-x="concept-script-record">record</span>.</p>
+
+    <p class="note">As seen below, this is used in order to propagate the current <span>active
+    script</span> forward to the time when the job is executed.</p>
+
+    <div class="example">
+     <p>A case where <var>active script</var> is non-null, and saving it in this way is useful, is
+     the following:</p>
+
+     <pre><code class="js" data-x="">Promise.resolve('import(`./example.mjs`)').then(eval);</code></pre>
+
+     <p>Without this step (and the steps below that use it), there would be no <span>active
+     script</span> when the <code>import()</code> expression is evaluated, since <code>eval()</code>
+     is a built-in function that does not originate from any particular <span
+     data-x="concept-script">script</span>.</p>
+
+     <p>With this step in place, the active script is propagated from the above code into the job,
+     allowing <code>import()</code> to use the original script's <span
+     data-x="concept-script-base-url">base URL</span> appropriately.</p>
+    </div>
+
+    <div class="example">
+     <p><var>active script</var> can be null if the user clicks on the following button, before any
+     script ever accesses the button's <code data-x="">onclick</code> property:</p>
+
+     <pre><code class="html" data-x="">&lt;button onclick="Promise.resolve('import(`./example.mjs`)').then(eval)">Click me&lt;/button></code></pre>
+
+     <p>In this case, the JavaScript function for the <span data-x="event handlers">event
+     handler</span> will be created by the <span data-x="getting the current value of the event
+     handler">get the current value of the event handler</span> algorithm as a direct result of user
+     action, with no script on the stack (i.e., no <span>active script</span>). Thus, when the
+     promise machinery calls <span>EnqueueJob</span>, there will be no <span>active script</span> to
+     pass along.</p>
+
+     <p>As a consequence, this means that when the <code>import()</code> expression is evaluated,
+     there will still be no <span>active script</span>. Fortunately that is handled by our
+     implementations of <span>HostResolveImportedModule</span> and
+     <span>HostImportModuleDynamically</span>, by falling back to using the <span>current settings
+     object</span>'s <span>API base URL</span>.</p>
+    </div>
+   </li>
 
    <li>
     <p><span>Queue a microtask</span>, on <var>job settings</var>'s <span>responsible event
@@ -88310,20 +88354,20 @@ document.querySelector("button").addEventListener("click", bound);
      </li>
 
      <li>
-      <p><span data-x="stack push">Push</span> <var>script execution context</var> onto the
-      <span>JavaScript execution context stack</span>.</p>
+      <p>If <var>script execution context</var> is not null, then <span data-x="stack
+      push">push</span> <var>script execution context</var> onto the <span>JavaScript execution
+      context stack</span>.</p>
 
-      <p class="note">This affects the <span>active script</span> while the job runs, in cases like
-      <code data-x="">Promise.resolve("...").then(eval)</code> where there would otherwise be no
-      active script since <code>eval()</code> is a built-in function that does not originate from
-      any particular <span data-x="concept-script">script</span>.</p>
+      <p class="note">As explained above, this affects the <span>active script</span> while the job
+      runs.</p>
      </li>
 
      <li><p>Let <var>result</var> be the result of performing the abstract operation specified by
      <var>job</var>, using the elements of <var>arguments</var> as its arguments.</p></li>
 
-     <li><p><span data-x="stack pop">Pop</span> <var>script execution context</var> from the
-     <span>JavaScript execution context stack</span>.</p></li>
+     <li><p>If <var>script execution context</var> is not null, then <span data-x="stack
+     pop">pop</span> <var>script execution context</var> from the <span>JavaScript execution context
+     stack</span>.</p></li>
 
      <li><p><span>Clean up after running a callback</span> with <var>incumbent
      settings</var>.</p></li>
@@ -88470,20 +88514,48 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
   implementation: <ref spec=JAVASCRIPT> <ref spec=JSIMPORT></p>
 
   <ol>
-   <li><p>Let <var>referencing script</var> be
-   <var>referencingScriptOrModule</var>.[[HostDefined]].</p></li>
+   <li><p>Let <var>settings object</var> be the <span>current settings object</span>.</p></li>
 
-   <li><p>Let <var>moduleMap</var> be <var>referencing script</var>'s <span>settings object</span>'s
-   <span data-x="concept-settings-object-module-map">module map</span>.</p></li>
+   <li><p>Let <var>base URL</var> be <var>settings object</var>'s <span>API base
+   URL</span>.</p></li>
+
+   <li>
+    <p>If <var>referencingScriptOrModule</var> is not null, then:</p>
+
+    <ol>
+     <li><p>Let <var>referencing script</var> be
+     <var>referencingScriptOrModule</var>.[[HostDefined]].</p></li>
+
+     <li><p>Set <var>settings object</var> to <var>referencing script</var>'s <span>settings
+     object</span>.</p></li>
+
+     <li><p>Set <var>base URL</var> to <var>referencing script</var>'s <span
+     data-x="concept-script-base-url">base URL</span>.</p></li>
+    </ol>
+
+    <div class="example">
+     <p><var>referencingScriptOrModule</var> is not usually null. One case where it <em>can</em> be
+     null is if the user clicks the control in the following example:</p>
+
+     <pre><code class="html" data-x="">&lt;button onclick="import('./foo.mjs')">Click me&lt;/button></code></pre>
+
+     <p>In this case, at the time the <code>import()</code> expression runs,
+     <span>GetActiveScriptOrModule</span> will return null, which will be passed to this abstract
+     operation when <span data-x="js-HostResolveImportedModule">HostResolveImportedModule</span> is
+     called by <span>FinishDynamicImport</span>.</p>
+    </div>
+   </li>
+
+   <li><p>Let <var>moduleMap</var> be <var>settings object</var>'s <span
+   data-x="concept-settings-object-module-map">module map</span>.</p></li>
 
    <li><p>Let <var>url</var> be the result of <span data-x="resolve a module specifier">resolving a
-   module specifier</span> given <var>referencing script</var>'s <span
-   data-x="concept-script-base-url">base URL</span> and <var>specifier</var>.</p></li>
+   module specifier</span> given <var>base URL</var> and <var>specifier</var>.</p></li>
 
    <li><p>Assert: <var>url</var> is never failure, because <span data-x="resolve a module
-   specifier">resolving a module specifier</span> must have been <a
-   href="#validate-requested-module-specifiers">previously successful</a> with these
-   same two arguments.</p></li>
+   specifier">resolving a module specifier</span> must have been previously successful with these
+   same two arguments (either <a href="#validate-requested-module-specifiers">while creating the
+   corresponding module script</a>, or in <span>HostImportModuleDynamically</span>).</p></li>
 
    <li><p>Let <var>resolved module script</var> be <var>moduleMap</var>[<var>url</var>]. (This entry
    must <span data-x="map exists">exist</span> for us to have gotten to this point.)</p></li>
@@ -88506,12 +88578,38 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
   User agents must use the following implementation: <ref spec=JSIMPORT></p>
 
   <ol>
-   <li><p>Let <var>referencing script</var> be
-   <var>referencingScriptOrModule</var>.[[HostDefined]].</p></li>
+   <li><p>Let <var>settings object</var> be the <span>current settings object</span>.</p></li>
+
+   <li><p>Let <var>base URL</var> be <var>settings object</var>'s <span>API base
+   URL</span>.</p></li>
+
+   <li><p>Let <var>fetch options</var> be the <span>default classic script fetch
+   options</span>.</p></li>
+
+   <li>
+    <p>If <var>referencingScriptOrModule</var> is not null, then:</p>
+
+    <ol>
+     <li><p>Let <var>referencing script</var> be
+     <var>referencingScriptOrModule</var>.[[HostDefined]].</p>
+
+     <li><p>Set <var>settings object</var> to <var>referencing script</var>'s <span>settings
+     object</span>.</p></li>
+
+     <li><p>Set <var>base URL</var> to <var>referencing script</var>'s <span
+     data-x="concept-script-base-url">base URL</span>.</p></li>
+
+     <li><p>Set <var>fetch options</var> to the <span>descendant script fetch options</span> for
+     <var>referencing script</var>'s <span data-x="concept-script-script-fetch-options">fetch
+     options</span>.</p></li>
+    </ol>
+
+    <p class="note">As explained above for <span>HostResolveImportedModule</span>, in the common
+    case, <var>referencingScriptOrModule</var> is non-null.</p>
+   </li>
 
    <li><p>Let <var>url</var> be the result of <span data-x="resolve a module specifier">resolving a
-   module specifier</span> given <var>referencing script</var>'s <span
-   data-x="concept-script-base-url">base URL</span> and <var>specifier</var>.</p></li>
+   module specifier</span> given <var>base URL</var> and <var>specifier</var>.</p></li>
 
    <li>
     <p>If <var>url</var> is failure, then:</p>
@@ -88527,14 +88625,9 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
     </ol>
    </li>
 
-   <li><p>Let <var>options</var> be the <span>descendant script fetch options</span> for
-   <var>referencing script</var>'s <span data-x="concept-script-script-fetch-options">fetch
-   options</span>.</p></li>
-
-   <li><p><span>Fetch a module script graph</span> given <var>url</var>, <var>referencing
-   script</var>'s <span>settings object</span>, "<code data-x="">script</code>", and
-   <var>options</var>. Wait until the algorithm asynchronously completes with
-   <var>result</var>.</p></li>
+   <li><p><span>Fetch a module script graph</span> given <var>url</var>, <var>settings object</var>,
+   "<code data-x="">script</code>", and <var>fetch options</var>. Wait until the algorithm
+   asynchronously completes with <var>result</var>.</p></li>
 
    <li>
     <p>If <var>result</var> is null, then:</p>