Bug 1461278 - Print a note to the web console when a web site is using shadow DOM (v1), preffed off by default, r=emilio

dom/base/Element.cpp

@@ -14,6 +14,7 @@
 
 #include "AnimationCommon.h"
 #include "mozilla/DebugOnly.h"
+#include "mozilla/StaticPrefs.h"
 #include "mozilla/dom/Animation.h"
 #include "mozilla/dom/Attr.h"
 #include "mozilla/dom/Flex.h"
@@ -1247,6 +1248,10 @@ Element::AttachShadow(const ShadowRootInit& aInit, ErrorResult& aError)
 
   shadowRoot->SetIsComposedDocParticipant(IsInComposedDoc());
 
+  if (StaticPrefs::dom_webcomponents_shadowdom_report_usage()) {
+    OwnerDoc()->ReportShadowDOMUsage();
+  }
+
   /**
    * 5. Set context object’s shadow root to shadow.
    */

dom/base/nsDocument.cpp

@@ -1456,6 +1456,7 @@ nsIDocument::nsIDocument()
     mWidthStrEmpty(false),
     mParserAborted(false),
     mReportedUseCounters(false),
+    mHasReportedShadowDOMUsage(false),
 #ifdef DEBUG
     mWillReparent(false),
 #endif
@@ -13220,3 +13221,27 @@ nsIDocument::ModuleScriptsEnabled()
 
   return nsContentUtils::IsChromeDoc(this) || sEnabledForContent;
 }
+
+void
+nsIDocument::ReportShadowDOMUsage()
+{
+  if (mHasReportedShadowDOMUsage) {
+    return;
+  }
+
+  nsIDocument* topLevel = GetTopLevelContentDocument();
+  if (topLevel && !topLevel->mHasReportedShadowDOMUsage) {
+    topLevel->mHasReportedShadowDOMUsage = true;
+    nsString uri;
+    Unused << topLevel->GetDocumentURI(uri);
+    if (!uri.IsEmpty()) {
+      nsAutoString msg = NS_LITERAL_STRING("Shadow DOM used in [") + uri +
+        NS_LITERAL_STRING("] or in some of its subdocuments.");
+      nsContentUtils::ReportToConsoleNonLocalized(msg, nsIScriptError::infoFlag,
+                                                  NS_LITERAL_CSTRING("DOM"),
+                                                  topLevel);
+    }
+  }
+
+  mHasReportedShadowDOMUsage = true;
+}

dom/base/nsIDocument.h

@@ -3536,6 +3536,8 @@ public:
    */
   nsIContent* GetContentInThisDocument(nsIFrame* aFrame) const;
 
+  void ReportShadowDOMUsage();
+
 protected:
   already_AddRefed<nsIPrincipal> MaybeDowngradePrincipal(nsIPrincipal* aPrincipal);
 
@@ -4052,6 +4054,8 @@ protected:
   // that we only report them once for the document.
   bool mReportedUseCounters: 1;
 
+  bool mHasReportedShadowDOMUsage: 1;
+
 #ifdef DEBUG
 public:
   bool mWillReparent: 1;

modules/libpref/init/StaticPrefList.h

@@ -87,6 +87,16 @@ VARCACHE_PREF(
   bool, false
 )
 
+//---------------------------------------------------------------------------
+// DOM prefs
+//---------------------------------------------------------------------------
+
+VARCACHE_PREF(
+  "dom.webcomponents.shadowdom.report_usage",
+   dom_webcomponents_shadowdom_report_usage,
+  bool, false
+)
+
 //---------------------------------------------------------------------------
 // Full-screen prefs
 //---------------------------------------------------------------------------