Bug 1668134 - part 2: Make nsPIDOMWindowInner have an API to know whether a node is (was) in the window has been observed by web apps with a mutation observer r=smaug

There is similar API in `Document`, but they indicate whether a node has been
observed by any mutation receiver, not only by `MutationObserver` of JS.
However, I'd like to know the percentage of web apps which use
`MutationObserver`, but not use `beforeinput` events.  Therefore, this patch
adds similar API into `nsPIDOMWindowInner` as same as `beforeinput` and
ignores `MutationObserver`s which are created by chrome script and addons.

Differential Revision: https://phabricator.services.mozilla.com/D92547

dom/base/nsDOMMutationObserver.cpp

@@ -593,7 +593,7 @@ void nsDOMMutationObserver::RescheduleForRun() {
 
 void nsDOMMutationObserver::Observe(
     nsINode& aTarget, const mozilla::dom::MutationObserverInit& aOptions,
-    mozilla::ErrorResult& aRv) {
+    nsIPrincipal& aSubjectPrincipal, mozilla::ErrorResult& aRv) {
   bool childList = aOptions.mChildList;
   bool attributes =
       aOptions.mAttributes.WasPassed() && aOptions.mAttributes.Value();
@@ -673,6 +673,13 @@ void nsDOMMutationObserver::Observe(
   r->SetAnimations(animations);
   r->RemoveClones();
 
+  if (!aSubjectPrincipal.IsSystemPrincipal() &&
+      !aSubjectPrincipal.GetIsAddonOrExpandedAddonPrincipal()) {
+    if (nsPIDOMWindowInner* window = aTarget.OwnerDoc()->GetInnerWindow()) {
+      window->SetMutationObserverHasObservedNodeForTelemetry();
+    }
+  }
+
 #ifdef DEBUG
   for (int32_t i = 0; i < mReceivers.Count(); ++i) {
     NS_WARNING_ASSERTION(mReceivers[i]->Target(),

dom/base/nsDOMMutationObserver.h

@@ -29,6 +29,8 @@
 #include "nsTArray.h"
 #include "nsWrapperCache.h"
 
+class nsIPrincipal;
+
 class nsDOMMutationObserver;
 using mozilla::dom::MutationObservingInfo;
 
@@ -461,7 +463,7 @@ class nsDOMMutationObserver final : public nsISupports, public nsWrapperCache {
 
   void Observe(nsINode& aTarget,
                const mozilla::dom::MutationObserverInit& aOptions,
-               mozilla::ErrorResult& aRv);
+               nsIPrincipal& aSubjectPrincipal, mozilla::ErrorResult& aRv);
 
   void Disconnect();
 

dom/base/nsGlobalWindowInner.cpp

@@ -7480,6 +7480,7 @@ nsPIDOMWindowInner::nsPIDOMWindowInner(nsPIDOMWindowOuter* aOuterWindow,
       mMayHaveMouseEnterLeaveEventListener(false),
       mMayHavePointerEnterLeaveEventListener(false),
       mMayHaveBeforeInputEventListenerForTelemetry(false),
+      mMutationObserverHasObservedNodeForTelemetry(false),
       mOuterWindow(aOuterWindow),
       mWindowID(0),
       mHasNotifiedGlobalCreated(false),

dom/base/nsPIDOMWindow.h

@@ -242,6 +242,26 @@ class nsPIDOMWindowInner : public mozIDOMWindow {
     mMayHaveBeforeInputEventListenerForTelemetry = true;
   }
 
+  /**
+   * Call this to check whether some node (The document, or content in the
+   * document) has been observed by web apps with a mutation observer.
+   * (i.e., `MutationObserver.observe()` called by chrome script and addon's
+   * script does not make this returns true).
+   * Returing false may be wrong if some nodes have come from another document
+   * with `Document.adoptNode`.
+   */
+  bool MutationObserverHasObservedNodeForTelemetry() const {
+    return mMutationObserverHasObservedNodeForTelemetry;
+  }
+
+  /**
+   * Call this to indicate that some node (The document, or content in the
+   * document) is observed by web apps with a mutation observer.
+   */
+  void SetMutationObserverHasObservedNodeForTelemetry() {
+    mMutationObserverHasObservedNodeForTelemetry = true;
+  }
+
   // Sets the event for window.event. Does NOT take ownership, so
   // the caller is responsible for clearing the event before the
   // event gets deallocated. Pass nullptr to set window.event to
@@ -609,6 +629,7 @@ class nsPIDOMWindowInner : public mozIDOMWindow {
   // Only used for telemetry probes.  This may be wrong if some nodes have
   // come from another document with `Document.adoptNode`.
   bool mMayHaveBeforeInputEventListenerForTelemetry;
+  bool mMutationObserverHasObservedNodeForTelemetry;
 
   // Our inner window's outer window.
   nsCOMPtr<nsPIDOMWindowOuter> mOuterWindow;

dom/webidl/MutationObserver.webidl

@@ -43,7 +43,7 @@ interface MutationObserver {
   [Throws]
   constructor(MutationCallback mutationCallback);
 
-  [Throws]
+  [Throws, NeedsSubjectPrincipal]
   void observe(Node target, optional MutationObserverInit options = {});
   void disconnect();
   sequence<MutationRecord> takeRecords();

editor/libeditor/EditorBase.h

@@ -246,6 +246,23 @@ class EditorBase : public nsIEditor,
     return false;
   }
 
+  /**
+   * MutationObserverHasObservedNodeForTelemetry() returns true when a node in
+   * the window may have been observed by the web apps with a mutation observer
+   * (i.e., `MutationObserver.observe()` called by chrome script and addon's
+   * script does not make this returns true).
+   * Note that this may return false even if there is a node observed by
+   * a MutationObserver.  See
+   * nsPIDOMWindowInner::MutationObserverHasObservedNodeForTelemetry()'s comment
+   * for the detail.
+   */
+  bool MutationObserverHasObservedNodeForTelemetry() const {
+    if (const nsPIDOMWindowInner* window = GetInnerWindow()) {
+      return window->MutationObserverHasObservedNodeForTelemetry();
+    }
+    return false;
+  }
+
   PresShell* GetPresShell() const {
     return mDocument ? mDocument->GetPresShell() : nullptr;
   }