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 5ff0d0d3861a31c60532f733a76016d704734229..7481d2255ab8756cbb55b1c9624d49ca21739646 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_linkPreloadCounter(0)
     , m_archive(context().isMainFrame() ? nullptr : context().archive())
     , m_resourceTimingReportTimer(this, &ResourceFetcher::resourceTimingReportTimerFired)
     , m_autoLoadImages(true)
@@ -474,6 +475,10 @@ Resource* ResourceFetcher::requestResource(FetchRequest& request, const Resource
         initializeRevalidation(request.mutableResourceRequest(), resource);
         break;
     case Use:
+        if (resource->isLinkPreload() && !request.isLinkPreload()) {
+            resource->setLinkPreload(false);
+            --m_linkPreloadCounter;
+        }
         memoryCache()->updateForAccess(resource);
         break;
     }
@@ -869,6 +874,8 @@ void ResourceFetcher::preloadStarted(Resource* 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>>;
@@ -878,7 +885,7 @@ void ResourceFetcher::preloadStarted(Resource* 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;
         }
@@ -894,7 +901,7 @@ 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)
@@ -906,6 +913,17 @@ 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 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.
@@ -1180,7 +1198,7 @@ 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: