DOM: Add iframe insertion & removal steps WPTs

To help resolve whatwg/dom#808, we need WPTs
asserting exactly when (DOM-observing) script can and cannot be invoked
during the insertion and removing steps for iframes and script elements.

The tests in this CL assert the current spec behavior of:

Iframe:
 - Insertion:
    - Synchronously fire the `load` event in the iframe document
 - Removal:
    - No script is run in between multiple iframe removals. Script
      cannot observe the state of the DOM in between multiple
      synchronous removals because, i.e., no `unload` events are fired
      in this case per HTML [1].
Script:
 - Insertion:
    - Synchronously execute <script> elements upon insertion, even in
      between the insertions of individual <script> elements that are
      added "atomically" by a single DocumentFragment insertion. Note
      that Chromium and Gecko fail this test. In the DocumentFragment
      case, both of these browsers insert all <scripts> into the DOM
      before executing any of them. This means that once the scripts
      start executing, they can all observe each other's participation
      in the DOM tree.

At the moment, there is ongoing discussion [2] about the possibility of
changing the DOM/HTML Standard's model to more-closely match what Gecko
and Chromium do with "atomic" DOM insertion (i.e., running script as a
side effect very explicitly after all DOM node insertion is done).

[1]: https://html.spec.whatwg.org/C#the-iframe-element:html-element-removing-steps
[2]: whatwg/dom#808 (comment)

R=masonf@chromium.org

Bug: N/A
Change-Id: Iff959bbb0d32d772ae7162d5d9e54a5817959086

dom/nodes/insertion-removal-steps-iframe.window.js

@@ -0,0 +1,100 @@
+// These tests ensure that:
+//   1. The HTML element insertion steps for iframes [1] can synchronously run
+//      script during iframe insertion, which can observe an iframe's
+//      participation in the DOM tree mid-insertion.
+//   2. The HTML element removing steps for iframes [2] *do not* synchronously
+//      run script during child navigable destruction. Therefore, script cannot
+//      observe the state of the DOM in the middle of iframe removal, even when
+//      multiple iframes are being removed in the same task. Iframe removal,
+//      from the perspective of the parent's DOM tree, is atomic.
+//
+// [1]: https://html.spec.whatwg.org/C#the-iframe-element:html-element-insertion-steps
+// [2]: https://html.spec.whatwg.org/C#the-iframe-element:html-element-removing-steps
+
+promise_test(async t => {
+  const fragment = new DocumentFragment();
+
+  const iframe1 = fragment.appendChild(document.createElement('iframe'));
+  const iframe2 = fragment.appendChild(document.createElement('iframe'));
+
+  t.add_cleanup(() => {
+    iframe1.remove();
+    iframe2.remove();
+  });
+
+  let iframe1Loaded = false, iframe2Loaded = false;
+  iframe1.onload = e => {
+    iframe1Loaded = true;
+    assert_equals(window.frames.length, 1,
+        "iframe1 load event can observe its own participation in the frame tree");
+    assert_equals(iframe1.contentWindow, window.frames[0]);
+  };
+
+  iframe2.onload = e => {
+    iframe2Loaded = true;
+    assert_equals(window.frames.length, 2,
+        "iframe2 load event can observe its own participation in the frame tree");
+    assert_equals(iframe1.contentWindow, window.frames[0]);
+    assert_equals(iframe2.contentWindow, window.frames[1]);
+  };
+
+  // Synchronously consecutively adds both `iframe1` and `iframe2` to the DOM,
+  // invoking their insertion steps (and thus firing each of their `load`
+  // events) in order. `iframe1` will be able to observe itself in the DOM but
+  // not `iframe2`, and `iframe2` will be able to observe both itself and
+  // `iframe1`.
+  document.body.append(fragment);
+  assert_true(iframe1Loaded, "iframe1 loaded");
+  assert_true(iframe2Loaded, "iframe2 loaded");
+}, "Insertion steps: load event fires synchronously during iframe insertion steps");
+
+promise_test(async t => {
+  const div = document.createElement('div');
+
+  const iframe1 = div.appendChild(document.createElement('iframe'));
+  const iframe2 = div.appendChild(document.createElement('iframe'));
+  document.body.append(div);
+
+  // Now that both iframes have been inserted into the DOM, we'll set up a
+  // MutationObserver that we'll use to ensure that multiple synchronous
+  // mutations (removals) are only observed atomically at the end. Specifically,
+  // the observer's callback is not invoked synchronously for each removal.
+  let observerCallbackInvoked = false;
+  const removalObserver = new MutationObserver(mutations => {
+    assert_false(observerCallbackInvoked,
+        "MO callback is only invoked once, not multiple times, i.e., for " +
+        "each removal");
+    observerCallbackInvoked = true;
+    assert_equals(mutations.length, 1, "Exactly one MutationRecord are recorded");
+    assert_equals(mutations[0].removedNodes.length, 2);
+    assert_equals(window.frames.length, 0,
+        "No iframe Windows exist when the MO callback is run");
+    assert_equals(document.querySelector('iframe'), null,
+        "No iframe elements are connected to the DOM when the MO callback is " +
+        "run");
+  });
+
+  removalObserver.observe(div, {childList: true});
+  t.add_cleanup(() => removalObserver.disconnect());
+
+  let iframe1UnloadFired = false, iframe2UnloadFired = false;
+  iframe1.contentWindow.addEventListener('unload', e => iframe1UnloadFired = true);
+  iframe2.contentWindow.addEventListener('unload', e => iframe2UnloadFired = true);
+
+  // replaceChildren() will trigger the synchronous removal of each of `div`'s
+  // (iframe) children. This will synchronously, consecutively invoke HTML's
+  // "destroy a child navigable" (per [1]), for each iframe.
+  //
+  // [1]: https://html.spec.whatwg.org/C#the-iframe-element:destroy-a-child-navigable
+  div.replaceChildren();
+  assert_false(iframe1UnloadFired, "iframe1 unload did not fire");
+  assert_false(iframe2UnloadFired, "iframe2 unload did not fire");
+
+  assert_false(observerCallbackInvoked,
+      "MO callback is not invoked synchronously after removals");
+
+  // Wait one microtask.
+  await Promise.resolve();
+
+  assert_true(observerCallbackInvoked, "MO callback is invoked asynchronously after removals");
+}, "Removing steps: script does not run synchronously during iframe destruction");

dom/nodes/insertion-removal-steps-script.window.js

@@ -0,0 +1,45 @@
+promise_test(async t => {
+  const fragmentWithTwoScripts = new DocumentFragment();
+  const script0 = document.createElement('script');
+  const script1 = fragmentWithTwoScripts.appendChild(document.createElement('script'));
+  const script2 = fragmentWithTwoScripts.appendChild(document.createElement('script'));
+
+  window.kBaselineNumberOfScripts = 3;
+  assert_equals(document.scripts.length, kBaselineNumberOfScripts,
+      "The WPT infra starts out with exactly 3 scripts");
+
+  window.script0Executed = false;
+  script0.innerText = `
+    script0Executed = true;
+    assert_equals(document.scripts.length, kBaselineNumberOfScripts + 1,
+        'script0 can observe itself and no other scripts');
+  `;
+
+  window.script1Executed = false;
+  script1.innerText = `
+    script1Executed = true;
+    assert_equals(document.scripts.length, kBaselineNumberOfScripts + 2,
+        "script1 executes synchronously, and thus observes only itself and " +
+        "previous scripts");
+  `;
+
+  window.script2Executed = false;
+  script2.innerText = `
+    script2Executed = true;
+    assert_equals(document.scripts.length, kBaselineNumberOfScripts + 3,
+        "script2 executes synchronously, and thus observes itself and all " +
+        "previous scripts");
+  `;
+
+  document.body.append(script0);
+  assert_true(script0Executed,
+      "Script0 executes synchronously during append()");
+
+  document.body.append(fragmentWithTwoScripts);
+  assert_true(script1Executed,
+      "Script1 executes synchronously during fragment append()");
+  assert_true(script2Executed,
+      "Script2 executes synchronously during fragment append()");
+}, "Script node insertion is not atomic with regard to execution. Each " +
+   "script is synchronously executed during the HTML element insertion " +
+   "steps hook");