Add a warning whenever link preloads are not used

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

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller (mueller@kde.org)
     Copyright (C) 2002 Waldo Bastian (bastian@kde.org)
     Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
     Copyright (C) 2009 Torch Mobile Inc. http://www.torchmobile.com/
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Library General Public
     License as published by the Free Software Foundation; either
@@ skipping 193 matching lines @@
         return WebURLRequest::RequestContextVideo;
     case Resource::Manifest:
         return WebURLRequest::RequestContextManifest;
     }
     ASSERT_NOT_REACHED();
     return WebURLRequest::RequestContextSubresource;
 }
 
 ResourceFetcher::ResourceFetcher(FetchContext* newContext)
     : m_context(newContext)
+    , m_linkPreloadCounter(0)
     , m_archive(context().isMainFrame() ? nullptr : context().archive())
     , m_resourceTimingReportTimer(this, &ResourceFetcher::resourceTimingReportTimerFired)
     , m_autoLoadImages(true)
     , m_imagesEnabled(true)
     , m_allowStaleResources(false)
     , m_imageFetched(false)
 {
     ThreadState::current()->registerPreFinalizer(this);
 }
 
@@ skipping 243 matching lines @@
     case Reload:
         memoryCache()->remove(resource);
         // Fall through
     case Load:
         resource = createResourceForLoading(request, request.charset(), factory);
         break;
     case Revalidate:
         initializeRevalidation(request.mutableResourceRequest(), resource);
         break;
     case Use:
+        if (resource->isLinkPreload() && !request.isLinkPreload()) {
+            resource->setLinkPreload(false);
+            --m_linkPreloadCounter;
+        }
         memoryCache()->updateForAccess(resource);
         break;
     }
 
     if (!resource)
         return nullptr;
     if (resource->getType() != factory.type()) {
         ASSERT(request.forPreload());
         return nullptr;
     }
@@ skipping 375 matching lines @@
 {
     return m_loaders.size() > 0 || m_nonBlockingLoaders.size() > 0;
 }
 
 void ResourceFetcher::preloadStarted(Resource* resource)
 {
     if (m_preloads && m_preloads->contains(resource))
         return;
     TRACE_EVENT_ASYNC_STEP_INTO0("blink.net", "Resource", resource->identifier(), "Preload");
     resource->increasePreloadCount();
+    if (resource->isLinkPreload())
+        ++m_linkPreloadCounter;
 
     if (!m_preloads)
         m_preloads = new HeapListHashSet<Member<Resource>>;
     m_preloads->add(resource);
 }
 
 bool ResourceFetcher::isPreloaded(const KURL& url) const
 {
     if (m_preloads) {
-        for (auto resource : *m_preloads) {
+        for (const auto& resource : *m_preloads) {
             if (resource->url() == url)
                 return true;
         }
     }
 
     return false;
 }
 
 void ResourceFetcher::clearPreloads(ClearPreloadsPolicy policy)
 {
     if (!m_preloads)
         return;
 
     logPreloadStats();
 
-    for (auto resource : *m_preloads) {
+    for (const auto& resource : *m_preloads) {
         if (policy == ClearAllPreloads || !resource->isLinkPreload()) {
             resource->decreasePreloadCount();
             if (resource->getPreloadResult() == Resource::PreloadNotReferenced)
                 memoryCache()->remove(resource.get());
             m_preloads->remove(resource);
         }
     }
     if (!m_preloads->size())
         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 rel=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);

[Charlie Harrison, 2016/09/14 21:07:59]

Hm... but what about Link preload headers?

+        }
+    }
+}
+
 ArchiveResource* ResourceFetcher::createArchive(Resource* resource)
 {
     // Only the top-frame can load MHTML.
     if (!context().isMainFrame())
         return nullptr;
     m_archive = MHTMLArchive::create(resource->url(), resource->resourceBuffer());
     return m_archive ? m_archive->mainResource() : nullptr;
 }
 
 void ResourceFetcher::didFinishLoading(Resource* resource, double finishTime, int64_t encodedDataLength, DidFinishLoadingReason finishReason)
@@ skipping 254 matching lines @@
     unsigned fonts = 0;
     unsigned fontMisses = 0;
     unsigned medias = 0;
     unsigned mediaMisses = 0;
     unsigned textTracks = 0;
     unsigned textTrackMisses = 0;
     unsigned imports = 0;
     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:
             images++;
             imageMisses += missCount;
             break;
         case Resource::Script:
             scripts++;
             scriptMisses += missCount;
             break;
@@ skipping 129 matching lines @@
     visitor->trace(m_context);
     visitor->trace(m_archive);
     visitor->trace(m_loaders);
     visitor->trace(m_nonBlockingLoaders);
     visitor->trace(m_documentResources);
     visitor->trace(m_preloads);
     visitor->trace(m_resourceTimingInfoMap);
 }
 
 } // namespace blink