Add a feature flag and about_flags entry to disable Mutation Events

Mutation events are being deprecated, and this feature allows testing with the events completely disabled. Bug: 1446498 Change-Id: Ia5584320f82b7eb1c7643b36a5edd47d4419b56e Cq-Do-Not-Cancel-Tryjobs: true Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4547496 Reviewed-by: David Baron <dbaron@chromium.org> Auto-Submit: Mason Freed <masonf@chromium.org> Commit-Queue: Mason Freed <masonf@chromium.org> Commit-Queue: David Baron <dbaron@chromium.org> Cr-Commit-Position: refs/heads/main@{#1147523}
chromium · May 22, 2023 · e8de880 · e8de880
1 parent 62e4362
commit e8de880
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Showing 17 changed files with 238 additions and 49 deletions.
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
@@ -5431,6 +5431,9 @@ const FeatureEntry kFeatureEntries[] = {
      flag_descriptions::kSecurePaymentConfirmationDebugName,
      flag_descriptions::kSecurePaymentConfirmationDebugDescription, kOsAll,
      FEATURE_VALUE_TYPE(features::kSecurePaymentConfirmationDebug)},
+    {"mutation-events", flag_descriptions::kMutationEventsName,
+     flag_descriptions::kMutationEventsDescription, kOsAll,
+     FEATURE_VALUE_TYPE(blink::features::kMutationEvents)},
     {"fill-on-account-select", flag_descriptions::kFillOnAccountSelectName,
      flag_descriptions::kFillOnAccountSelectDescription, kOsAll,
      FEATURE_VALUE_TYPE(password_manager::features::kFillOnAccountSelect)},

diff --git a/chrome/browser/flag-metadata.json b/chrome/browser/flag-metadata.json
@@ -5159,6 +5159,11 @@
     "owners": [ "christianxu", "stkhapugin", "bling-flags@google.com"],
     "expiry_milestone": 120
   },
+  {
+    "name": "mutation-events",
+    "owners": [ "masonf" ],
+    "expiry_milestone": 127
+  },
   {
     "name": "mute-notification-snooze-action",
     "owners": [ "knollr@chromium.org", "peter@chromium.org" ],

diff --git a/chrome/browser/flag_descriptions.cc b/chrome/browser/flag_descriptions.cc
@@ -1572,6 +1572,14 @@ const char kFillingAcrossGroupedSitesDescription[] =
     "This flag enables password filling across grouped websites. Information "
     "about website groups is provided by the affiliation service.";
 
+const char kMutationEventsName[] =
+    "Enable (deprecated) synchronous mutation events";
+const char kMutationEventsDescription[] =
+    "Mutation Events are a deprecated set of events which cause performance "
+    "issues. Disabling this feature turns off Mutation Events. NOTE: Disabling "
+    "these events can cause breakage on some sites that are still reliant on "
+    "these deprecated features.";
+
 const char kFillOnAccountSelectName[] = "Fill passwords on account selection";
 const char kFillOnAccountSelectDescription[] =
     "Filling of passwords when an account is explicitly selected by the user "

diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
@@ -877,6 +877,9 @@ extern const char kFillingAcrossAffiliatedWebsitesDescription[];
 extern const char kFillingAcrossGroupedSitesName[];
 extern const char kFillingAcrossGroupedSitesDescription[];
 
+extern const char kMutationEventsName[];
+extern const char kMutationEventsDescription[];
+
 extern const char kFillOnAccountSelectName[];
 extern const char kFillOnAccountSelectDescription[];
 

diff --git a/third_party/blink/renderer/core/dom/document.cc b/third_party/blink/renderer/core/dom/document.cc
@@ -5728,8 +5728,13 @@ Event* Document::createEvent(ScriptState* script_state,
 
 void Document::AddMutationEventListenerTypeIfEnabled(
     ListenerType listener_type) {
-  if (ContextFeatures::MutationEventsEnabled(this))
-    AddListenerType(listener_type);
+  // Mutation events can be disabled by the embedder via a ContextFeatures
+  // switch, or via the runtime enabled feature.
+  if (!ContextFeatures::MutationEventsEnabled(this) ||
+      !RuntimeEnabledFeatures::MutationEventsEnabled()) {
+    return;
+  }
+  AddListenerType(listener_type);
 }
 
 void Document::AddListenerTypeIfNeeded(const AtomicString& event_type,
@@ -5738,7 +5743,6 @@ void Document::AddListenerTypeIfNeeded(const AtomicString& event_type,
   ListenerType listener_type;
   if (event_util::IsDOMMutationEventType(event_type, mutation_event_feature,
                                          listener_type)) {
-    UseCounter::Count(*this, mutation_event_feature);
     AddMutationEventListenerTypeIfEnabled(listener_type);
   } else if (event_type == event_type_names::kWebkitAnimationStart ||
              event_type == event_type_names::kAnimationstart) {

diff --git a/third_party/blink/renderer/core/dom/document.h b/third_party/blink/renderer/core/dom/document.h
@@ -1125,9 +1125,16 @@ class CORE_EXPORT Document : public ContainerNode,
     kScrollListener = 1 << 14,
     kLoadListenerAtCapturePhaseOrAtStyleElement = 1 << 15,
     // 0 bits remaining
+    kDOMMutationEventListener =
+        kDOMSubtreeModifiedListener | kDOMNodeInsertedListener |
+        kDOMNodeRemovedListener | kDOMNodeRemovedFromDocumentListener |
+        kDOMNodeInsertedIntoDocumentListener |
+        kDOMCharacterDataModifiedListener,
   };
 
   bool HasListenerType(ListenerType listener_type) const {
+    DCHECK(RuntimeEnabledFeatures::MutationEventsEnabled() ||
+           !(listener_types_ & kDOMMutationEventListener));
     return (listener_types_ & listener_type);
   }
   void AddListenerTypeIfNeeded(const AtomicString& event_type, EventTarget&);

diff --git a/third_party/blink/renderer/core/dom/events/event_dispatcher.cc b/third_party/blink/renderer/core/dom/events/event_dispatcher.cc
@@ -32,6 +32,7 @@
 #include "third_party/blink/public/common/input/web_keyboard_event.h"
 #include "third_party/blink/public/web/web_local_frame_client.h"
 #include "third_party/blink/renderer/core/accessibility/ax_object_cache.h"
+#include "third_party/blink/renderer/core/dom/context_features.h"
 #include "third_party/blink/renderer/core/dom/element.h"
 #include "third_party/blink/renderer/core/dom/events/event_dispatch_forbidden_scope.h"
 #include "third_party/blink/renderer/core/dom/events/event_dispatch_result.h"
@@ -41,6 +42,7 @@
 #include "third_party/blink/renderer/core/dom/events/window_event_context.h"
 #include "third_party/blink/renderer/core/dom/node.h"
 #include "third_party/blink/renderer/core/editing/editor.h"
+#include "third_party/blink/renderer/core/event_type_names.h"
 #include "third_party/blink/renderer/core/events/keyboard_event.h"
 #include "third_party/blink/renderer/core/events/mouse_event.h"
 #include "third_party/blink/renderer/core/events/simulated_event_util.h"
@@ -61,7 +63,6 @@
 #include "third_party/blink/renderer/platform/instrumentation/use_counter.h"
 #include "third_party/blink/renderer/platform/keyboard_codes.h"
 #include "ui/events/keycodes/dom/keycode_converter.h"
-
 namespace blink {
 
 DispatchEventResult EventDispatcher::DispatchEvent(Node& node, Event& event) {
@@ -187,7 +188,8 @@ DispatchEventResult EventDispatcher::Dispatch() {
     return DispatchEventResult::kNotCanceled;
   }
   std::unique_ptr<EventTiming> eventTiming;
-  LocalFrame* frame = node_->GetDocument().GetFrame();
+  auto& document = node_->GetDocument();
+  LocalFrame* frame = document.GetFrame();
   LocalDOMWindow* window = nullptr;
   if (frame) {
     window = frame->DomWindow();
@@ -218,10 +220,24 @@ DispatchEventResult EventDispatcher::Dispatch() {
     DCHECK(!frame->GetAdTracker() || !frame->GetAdTracker()->IsAdScriptInStack(
                                          AdTracker::StackType::kBottomAndTop));
     if (frame->IsAdFrame()) {
-      UseCounter::Count(node_->GetDocument(), WebFeature::kAdClick);
+      UseCounter::Count(document, WebFeature::kAdClick);
     }
   }
 
+#if DCHECK_IS_ON()
+  // If Mutation Events are disabled, we should never dispatch trusted ones.
+  if (event_->isTrusted() &&
+      (event_->type() == event_type_names::kDOMCharacterDataModified ||
+       event_->type() == event_type_names::kDOMSubtreeModified ||
+       event_->type() == event_type_names::kDOMNodeInserted ||
+       event_->type() == event_type_names::kDOMNodeInsertedIntoDocument ||
+       event_->type() == event_type_names::kDOMNodeRemoved ||
+       event_->type() == event_type_names::kDOMNodeRemovedFromDocument)) {
+    DCHECK(RuntimeEnabledFeatures::MutationEventsEnabled());
+    DCHECK(ContextFeatures::MutationEventsEnabled(&document));
+  }
+#endif
+
   // 6. Let isActivationEvent be true, if event is a MouseEvent object and
   // event's type attribute is "click", and false otherwise.
   //

diff --git a/third_party/blink/renderer/core/dom/events/event_target.cc b/third_party/blink/renderer/core/dom/events/event_target.cc
@@ -42,6 +42,7 @@
 #include "third_party/blink/renderer/bindings/core/v8/v8_union_boolean_eventlisteneroptions.h"
 #include "third_party/blink/renderer/core/dom/abort_signal.h"
 #include "third_party/blink/renderer/core/dom/abort_signal_registry.h"
+#include "third_party/blink/renderer/core/dom/context_features.h"
 #include "third_party/blink/renderer/core/dom/events/add_event_listener_options_resolved.h"
 #include "third_party/blink/renderer/core/dom/events/event.h"
 #include "third_party/blink/renderer/core/dom/events/event_dispatch_forbidden_scope.h"
@@ -63,6 +64,7 @@
 #include "third_party/blink/renderer/platform/bindings/v8_dom_activity_logger.h"
 #include "third_party/blink/renderer/platform/instrumentation/histogram.h"
 #include "third_party/blink/renderer/platform/instrumentation/use_counter.h"
+#include "third_party/blink/renderer/platform/runtime_enabled_features.h"
 #include "third_party/blink/renderer/platform/wtf/assertions.h"
 #include "third_party/blink/renderer/platform/wtf/functional.h"
 #include "third_party/blink/renderer/platform/wtf/std_lib_extras.h"
@@ -513,50 +515,54 @@ bool EventTarget::AddEventListenerInternal(
 void EventTarget::AddedEventListener(
     const AtomicString& event_type,
     RegisteredEventListener& registered_listener) {
-  if (const LocalDOMWindow* executing_window = ExecutingWindow()) {
-    if (Document* document = executing_window->document()) {
-      if (event_type == event_type_names::kAuxclick) {
-        UseCounter::Count(*document, WebFeature::kAuxclickAddListenerCount);
-      } else if (event_type == event_type_names::kAppinstalled) {
-        UseCounter::Count(*document, WebFeature::kAppInstalledEventAddListener);
-      } else if (event_util::IsPointerEventType(event_type)) {
-        UseCounter::Count(*document, WebFeature::kPointerEventAddListenerCount);
-      } else if (event_type == event_type_names::kSlotchange) {
-        UseCounter::Count(*document, WebFeature::kSlotChangeEventAddListener);
-      } else if (event_type == event_type_names::kBeforematch) {
-        UseCounter::Count(*document, WebFeature::kBeforematchHandlerRegistered);
-      } else if (event_type ==
-                 event_type_names::kContentvisibilityautostatechange) {
-        UseCounter::Count(
-            *document,
-            WebFeature::kContentVisibilityAutoStateChangeHandlerRegistered);
-      } else if (event_type == event_type_names::kScrollend) {
-        UseCounter::Count(*document, WebFeature::kScrollend);
-      }
+  const LocalDOMWindow* executing_window = ExecutingWindow();
+  Document* document =
+      executing_window ? executing_window->document() : nullptr;
+  if (document) {
+    if (event_type == event_type_names::kAuxclick) {
+      UseCounter::Count(*document, WebFeature::kAuxclickAddListenerCount);
+    } else if (event_type == event_type_names::kAppinstalled) {
+      UseCounter::Count(*document, WebFeature::kAppInstalledEventAddListener);
+    } else if (event_util::IsPointerEventType(event_type)) {
+      UseCounter::Count(*document, WebFeature::kPointerEventAddListenerCount);
+    } else if (event_type == event_type_names::kSlotchange) {
+      UseCounter::Count(*document, WebFeature::kSlotChangeEventAddListener);
+    } else if (event_type == event_type_names::kBeforematch) {
+      UseCounter::Count(*document, WebFeature::kBeforematchHandlerRegistered);
+    } else if (event_type ==
+               event_type_names::kContentvisibilityautostatechange) {
+      UseCounter::Count(
+          *document,
+          WebFeature::kContentVisibilityAutoStateChangeHandlerRegistered);
+    } else if (event_type == event_type_names::kScrollend) {
+      UseCounter::Count(*document, WebFeature::kScrollend);
     }
   }
 
-  WebFeature mutation_event_feature;
-  Document::ListenerType listener_type;
-  if (event_util::IsDOMMutationEventType(event_type, mutation_event_feature,
-                                         listener_type)) {
-    if (ExecutionContext* context = GetExecutionContext()) {
-      String message_text = String::Format(
-          "Listener added for a synchronous '%s' DOM Mutation Event. "
-          "This event type is deprecated "
-          "(https://w3c.github.io/uievents/#legacy-event-types) "
-          "and work is underway to remove it from this browser. Usage of this "
-          "event listener will cause performance issues today, and represents "
-          "a risk of future incompatibility. Consider using MutationObserver "
-          "instead.",
-          event_type.GetString().Utf8().c_str());
-      PerformanceMonitor::ReportGenericViolation(
-          context, PerformanceMonitor::kDiscouragedAPIUse, message_text,
-          base::TimeDelta(), nullptr);
-      context->AddConsoleMessage(MakeGarbageCollected<ConsoleMessage>(
-          mojom::blink::ConsoleMessageSource::kDeprecation,
-          mojom::blink::ConsoleMessageLevel::kWarning, message_text));
-      Deprecation::CountDeprecation(context, mutation_event_feature);
+  if (RuntimeEnabledFeatures::MutationEventsEnabled() &&
+      (!document || ContextFeatures::MutationEventsEnabled(document))) {
+    WebFeature mutation_event_feature;
+    Document::ListenerType listener_type;
+    if (event_util::IsDOMMutationEventType(event_type, mutation_event_feature,
+                                           listener_type)) {
+      if (ExecutionContext* context = GetExecutionContext()) {
+        String message_text = String::Format(
+            "Listener added for a synchronous '%s' DOM Mutation Event. "
+            "This event type is deprecated "
+            "(https://w3c.github.io/uievents/#legacy-event-types) "
+            "and work is underway to remove it from this browser. Usage of "
+            "this event listener will cause performance issues today, and "
+            "represents a risk of future incompatibility. Consider using "
+            "MutationObserver instead.",
+            event_type.GetString().Utf8().c_str());
+        PerformanceMonitor::ReportGenericViolation(
+            context, PerformanceMonitor::kDiscouragedAPIUse, message_text,
+            base::TimeDelta(), nullptr);
+        context->AddConsoleMessage(MakeGarbageCollected<ConsoleMessage>(
+            mojom::blink::ConsoleMessageSource::kDeprecation,
+            mojom::blink::ConsoleMessageLevel::kWarning, message_text));
+        Deprecation::CountDeprecation(context, mutation_event_feature);
+      }
     }
   }
 }

diff --git a/third_party/blink/renderer/core/events/event_util.cc b/third_party/blink/renderer/core/events/event_util.cc
@@ -39,6 +39,11 @@ bool IsPointerEventType(const AtomicString& event_type) {
          event_type == event_type_names::kPointerup;
 }
 
+bool IsDOMMutationEventType(const AtomicString& event_type) {
+  WebFeature web_feature;
+  Document::ListenerType listener_type;
+  return IsDOMMutationEventType(event_type, web_feature, listener_type);
+}
 bool IsDOMMutationEventType(const AtomicString& event_type,
                             WebFeature& web_feature,
                             Document::ListenerType& listener_type) {

diff --git a/third_party/blink/renderer/core/events/event_util.h b/third_party/blink/renderer/core/events/event_util.h
@@ -27,6 +27,7 @@ CORE_EXPORT bool IsPointerEventType(const AtomicString& event_type);
 bool IsDOMMutationEventType(const AtomicString& event_type,
                             WebFeature& web_feature,
                             Document::ListenerType& listener_type);
+bool IsDOMMutationEventType(const AtomicString& event_type);
 
 }  // namespace event_util
 

diff --git a/third_party/blink/renderer/core/events/mutation_event.idl b/third_party/blink/renderer/core/events/mutation_event.idl
@@ -20,7 +20,8 @@
 // https://w3c.github.io/uievents/#interface-MutationEvent
 
 [
-    Exposed=Window
+    Exposed=Window,
+    RuntimeEnabled=MutationEvents
 ] interface MutationEvent : Event {
     // attrChangeType
     const unsigned short MODIFICATION = 1;

diff --git a/third_party/blink/renderer/platform/runtime_enabled_features.json5 b/third_party/blink/renderer/platform/runtime_enabled_features.json5
@@ -2321,7 +2321,6 @@
       base_feature: "none",
     },
     {
-
       name: "MobileLayoutTheme",
       base_feature: "none",
     },
@@ -2337,6 +2336,12 @@
       status: "test",
       base_feature: "none",
     },
+    // crbug.com/1446498: This feature is being used for the deprecation of
+    // Mutation Events.
+    {
+      name: "MutationEvents",
+      status: "stable",
+    },
     {
       name: "NavigateEventCancelableTraversals",
       status: "stable",

diff --git a/third_party/blink/web_tests/VirtualTestSuites b/third_party/blink/web_tests/VirtualTestSuites
@@ -1300,6 +1300,13 @@
              "--enable-blink-features=SerializeViewTransitionStateInSPA"],
     "expires": "Feb 1, 2024"
   },
+  {
+    "prefix": "mutation-events-disabled",
+    "platforms": ["Linux"],
+    "bases": [],
+    "args": ["--disable-blink-features=MutationEvents"],
+    "expires": "Sep 1, 2024"
+  },
   {
     "prefix": "popover-hint-disabled",
     "platforms": ["Linux", "Mac", "Win"],

diff --git a/third_party/blink/web_tests/fast/dom/mutation-events-usecounter.html b/third_party/blink/web_tests/fast/dom/mutation-events-usecounter.html
@@ -0,0 +1,53 @@
+<!DOCTYPE html>
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+
+<div id=target>foo</div>
+
+<script>
+test(() => {
+  const target = document.getElementById('target');
+
+  // From web_feature.mojom
+  const kDOMSubtreeModifiedEvent = 143;
+  const kDOMNodeInsertedEvent = 144;
+  const kDOMNodeRemovedEvent = 145;
+  const kDOMNodeRemovedFromDocumentEvent = 146;
+  const kDOMNodeInsertedIntoDocumentEvent = 147;
+  const kDOMCharacterDataModifiedEvent = 148;
+
+  const events = [
+    {counter: kDOMSubtreeModifiedEvent, event: 'DOMSubtreeModified'},
+    {counter: kDOMNodeInsertedEvent, event: 'DOMNodeInserted'},
+    {counter: kDOMNodeRemovedEvent, event: 'DOMNodeRemoved'},
+    {counter: kDOMNodeRemovedFromDocumentEvent, event: 'DOMNodeRemovedFromDocument'},
+    {counter: kDOMNodeInsertedIntoDocumentEvent, event: 'DOMNodeInsertedIntoDocument'},
+    {counter: kDOMCharacterDataModifiedEvent, event: 'DOMCharacterDataModified'},
+  ];
+
+  const expectedCounters = new Set();
+  function assertUseCounted(msg) {
+    events.forEach(evt => {
+      assert_equals(internals.isUseCounted(document, evt.counter), expectedCounters.has(evt.counter), evt.event + ', ' + msg);
+    });
+  }
+  function triggerThemAll() {
+    target.remove();
+    document.body.appendChild(target);
+    target.setAttribute('foo','bar');
+    target.removeAttribute('foo');
+    target.firstChild.textContent = "bar";
+  }
+
+  assertUseCounted('initial');
+  triggerThemAll();
+  assertUseCounted('before event listeners attached');
+  events.forEach(evt => {
+    target.addEventListener(evt.event,() => {});
+    expectedCounters.add(evt.counter);
+    assertUseCounted('after adding ' + evt.event + ' listener, before firing');
+    triggerThemAll();
+    assertUseCounted('after adding ' + evt.event + ' listener, after firing (no difference)');
+  });
+}, 'Use of Mutation Events is use counted.');
+</script>
diff --git a/third_party/blink/web_tests/virtual/mutation-events-disabled/README.md b/third_party/blink/web_tests/virtual/mutation-events-disabled/README.md
@@ -0,0 +1,4 @@
+# Overview
+
+This suite tests the case where Mutation Events are disabled via
+`--disable-blink-features=MutationEvents`.