Bug 1755226 - Only notify 'http-on-image-cache-response' when there are registered observers r=tnikkel

This is an optimization to avoid doing redundant things when there are no
registered observers.

Differential Revision: https://phabricator.services.mozilla.com/D138853

image/imgLoader.cpp

@@ -1905,15 +1905,24 @@ void imgLoader::NotifyObserversForCachedImage(
   if (aEntry->HasNotified()) {
     return;
   }
+
+  nsCOMPtr<nsIObserverService> obsService = services::GetObserverService();
+
+  bool found;
+  nsresult rv = obsService->HasObservers("http-on-image-cache-response", found);
+  MOZ_ASSERT(NS_SUCCEEDED(rv));
+  if (!found) {
+    return;
+  }
+
   aEntry->SetHasNotified();
 
   nsCOMPtr<nsIChannel> newChannel;
   bool forcePrincipalCheck;
-  nsresult rv =
-      NewImageChannel(getter_AddRefs(newChannel), &forcePrincipalCheck, aURI,
-                      nullptr, aCORSMode, aReferrerInfo, nullptr, 0,
-                      nsIContentPolicy::TYPE_INTERNAL_IMAGE,
-                      aTriggeringPrincipal, aLoadingDocument, mRespectPrivacy);
+  rv = NewImageChannel(getter_AddRefs(newChannel), &forcePrincipalCheck, aURI,
+                       nullptr, aCORSMode, aReferrerInfo, nullptr, 0,
+                       nsIContentPolicy::TYPE_INTERNAL_IMAGE,
+                       aTriggeringPrincipal, aLoadingDocument, mRespectPrivacy);
   if (NS_FAILED(rv)) {
     return;
   }
@@ -1926,7 +1935,6 @@ void imgLoader::NotifyObserversForCachedImage(
     if (image) {
       newChannel->SetContentLength(aEntry->GetDataSize());
     }
-    nsCOMPtr<nsIObserverService> obsService = services::GetObserverService();
     obsService->NotifyObservers(newChannel, "http-on-image-cache-response",
                                 nullptr);
   }

xpcom/ds/nsIObserverService.idl

@@ -81,6 +81,20 @@ interface nsIObserverService : nsISupports
     nsresult NotifyWhenScriptSafe(nsISupports* aSubject,
                                   const char* aTopic,
                                   const char16_t* aData = nullptr);
+
+    /**
+     * hasObservers
+     *
+     * Checks to see if there are registered listeners for the given topic.
+     *
+     * Implemented in "nsObserverService.cpp".
+     *
+     * @param aTopic   : The notification topic or subject.
+     * @param aFound : An out parameter; True if there are registered observers,
+     * False otherwise.
+     */
+    virtual nsresult HasObservers(const char* aTopic,
+                                  bool& aFound);
     %}
 
     /**

xpcom/ds/nsObserverService.cpp

@@ -320,6 +320,12 @@ nsObserverService::UnmarkGrayStrongObservers() {
   return NS_OK;
 }
 
+NS_IMETHODIMP nsObserverService::HasObservers(const char* aTopic,
+                                              bool& aFound) {
+  aFound = mObserverTopicTable.Contains(aTopic);
+  return NS_OK;
+}
+
 namespace {
 
 class NotifyWhenScriptSafeRunnable : public mozilla::Runnable {
@@ -362,3 +368,8 @@ nsresult nsIObserverService::NotifyWhenScriptSafe(nsISupports* aSubject,
       this, aSubject, aTopic, aData));
   return NS_OK;
 }
+
+NS_IMETHODIMP
+nsIObserverService::HasObservers(const char* aTopic, bool& aFound) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}

xpcom/ds/nsObserverService.h

@@ -37,6 +37,7 @@ class nsObserverService final : public nsIObserverService,
   [[nodiscard]] static nsresult Create(nsISupports* aOuter, const nsIID& aIID,
                                        void** aInstancePtr);
 
+  NS_IMETHODIMP HasObservers(const char* aTopic, bool& aFound) override;
   // Unmark any strongly held observers implemented in JS so the cycle
   // collector will not traverse them.
   NS_IMETHOD UnmarkGrayStrongObservers();