Add a warning whenever link preloads are not used

third_party/WebKit/Source/core/fetch/ResourceFetcher.cpp

Index: third_party/WebKit/Source/core/fetch/ResourceFetcher.cpp
diff --git a/third_party/WebKit/Source/core/fetch/ResourceFetcher.cpp b/third_party/WebKit/Source/core/fetch/ResourceFetcher.cpp
index a61d582a7a65b49fe938cba6f06b2a72098347c0..0773379f3e943fe3dd158946ed8328bbc28464cc 100644
--- a/third_party/WebKit/Source/core/fetch/ResourceFetcher.cpp
+++ b/third_party/WebKit/Source/core/fetch/ResourceFetcher.cpp
@@ -211,6 +211,7 @@ static WebURLRequest::RequestContext requestContextFromType(bool isMainFrame, Re
 
 ResourceFetcher::ResourceFetcher(FetchContext* newContext)
     : m_context(newContext)
+    , m_unusedLinkPreloadCounter(0)
     , m_archive(context().isMainFrame() ? nullptr : context().archive())
     , m_resourceTimingReportTimer(this, &ResourceFetcher::resourceTimingReportTimerFired)
     , m_autoLoadImages(true)
@@ -474,8 +475,11 @@ Resource* ResourceFetcher::requestResource(FetchRequest& request, const Resource
         initializeRevalidation(request.mutableResourceRequest(), resource);
         break;
     case Use:
-        if (resource->isLinkPreload() && !request.isLinkPreload())
+        if (resource->isLinkPreload() && !request.isLinkPreload()) {
             resource->setLinkPreload(false);
+            DCHECK_GT(m_unusedLinkPreloadCounter, 0u);
+            --m_unusedLinkPreloadCounter;
+        }
         memoryCache()->updateForAccess(resource);
         break;
     }
@@ -871,6 +875,8 @@ void ResourceFetcher::preloadStarted(Resource* resource)
         return;
     TRACE_EVENT_ASYNC_STEP_INTO0("blink.net", "Resource", resource->identifier(), "Preload");
     resource->increasePreloadCount();
+    if (resource->isLinkPreload())
+        ++m_unusedLinkPreloadCounter;
 
     if (!m_preloads)
         m_preloads = new HeapListHashSet<Member<Resource>>;
@@ -905,7 +911,7 @@ void ResourceFetcher::clearPreloads(ClearPreloadsPolicy policy)
 
     logPreloadStats();
 
-    for (auto resource : *m_preloads) {
+    for (const auto& resource : *m_preloads) {
         if (policy == ClearAllPreloads || !resource->isLinkPreload()) {
             resource->decreasePreloadCount();
             if (resource->getPreloadResult() == Resource::PreloadNotReferenced)
@@ -917,6 +923,17 @@ void ResourceFetcher::clearPreloads(ClearPreloadsPolicy policy)
         m_preloads.clear();
 }
 
+void ResourceFetcher::warnUnusedPreloads()
+{
+    if (!m_preloads)
+        return;
+    for (const auto& resource : *m_preloads) {
+        if (resource && resource->isLinkPreload() && resource->getPreloadResult() == Resource::PreloadNotReferenced) {
+            context().addConsoleMessage("The resource " + resource->url().getString() + " was preloaded using link preload but not used within a few seconds from the window's load event. Please make sure it wasn't preloaded for nothing.", FetchContext::LogWarningMessage);
+        }
+    }
+}
+
 ArchiveResource* ResourceFetcher::createArchive(Resource* resource)
 {
     // Only the top-frame can load MHTML.
@@ -1192,7 +1209,7 @@ void ResourceFetcher::logPreloadStats()
     unsigned importMisses = 0;
     unsigned raws = 0;
     unsigned rawMisses = 0;
-    for (auto resource : *m_preloads) {
+    for (const auto& resource : *m_preloads) {
         int missCount = resource->getPreloadResult() == Resource::PreloadNotReferenced ? 1 : 0;
         switch (resource->getType()) {
         case Resource::Image: