Add Blink support for showing image placeholders using range requests.

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
@@ skipping 462 matching lines @@
   // Aims to count Resource only referenced from MemoryCache (i.e. what would be
   // dead if MemoryCache holds weak references to Resource). Currently we check
   // references to Resource from ResourceClient and |m_preloads| only, because
   // they are major sources of references.
   if (resource && !resource->isAlive() &&
       (!m_preloads || !m_preloads->contains(resource))) {
     DEFINE_RESOURCE_HISTOGRAM("Dead.");
   }
 }
 
+static void modifyRequestForPlaceholderImageRequest(FetchRequest& request,
+                                                    Resource::Type type) {
+  if (request.placeholderImageRequestType() != FetchRequest::AllowPlaceholder)
+    return;
+  DCHECK_EQ(Resource::Image, type);
+
+  if (!request.url().protocolIsInHTTPFamily() ||
+      request.resourceRequest().httpMethod() != "GET" ||
+      !request.resourceRequest().httpHeaderField("range").isNull()) {
+    request.setPlaceholderImageRequestType(FetchRequest::DisallowPlaceholder);

[Nate Chapin, 2016/10/18 18:23:17]

Everything in this function should probably all be done as part of
setPlaceholderImageRequestType()?

Also, this seems like the only place where setPlaceholderImageRequestType
changes an AllowPlaceholder to a DisallowPlaceholder. Can we change it to
setAllowImagePlaceholder() with no arguments, since the default will always be
DisallowPlaceholder?

[sclittle, 2016/10/18 23:53:03]

Done.

+    return;
+  }
+
+  // Fetch the first few bytes of the image. This number is tuned to both (a)
+  // likely capture the entire image for small images and (b) likely contain
+  // the dimensions for larger images.
+  // TODO(sclittle): Calculate the optimal value for this number.
+  request.mutableResourceRequest().setHTTPHeaderField("range", "bytes=0-2047");
+
+  // TODO(sclittle): Indicate somehow (e.g. through a new request bit) to the
+  // embedder that it should return the full resource if the entire resource is
+  // fresh in the cache.
+}
+
 Resource* ResourceFetcher::requestResource(
     FetchRequest& request,
     const ResourceFactory& factory,
     const SubstituteData& substituteData) {
   SCOPED_BLINK_UMA_HISTOGRAM_TIMER("Blink.Fetch.RequestResourceTime");
   DCHECK(request.options().synchronousPolicy == RequestAsynchronously ||
          factory.type() == Resource::Raw ||
          factory.type() == Resource::XSLStyleSheet);
 
   context().populateRequestData(request.mutableResourceRequest());
@@ skipping 16 matching lines @@
           request.getOriginRestriction())) {
     DCHECK(!substituteData.forceSynchronousLoad());
     return resourceForBlockedRequest(request, factory);
   }
 
   unsigned long identifier = createUniqueIdentifier();
   request.mutableResourceRequest().setPriority(computeLoadPriority(
       factory.type(), request, ResourcePriority::NotVisible));
   initializeResourceRequest(request.mutableResourceRequest(), factory.type(),
                             request.defer());
+
+  modifyRequestForPlaceholderImageRequest(request, factory.type());
+
   context().willStartLoadingResource(
       identifier, request.mutableResourceRequest(), factory.type());
   if (!request.url().isValid())
     return nullptr;
 
   if (!request.forPreload()) {
     V8DOMActivityLogger* activityLogger = nullptr;
     if (request.options().initiatorInfo.name ==
         FetchInitiatorTypeNames::xmlhttprequest) {
       activityLogger = V8DOMActivityLogger::currentActivityLogger();
@@ skipping 626 matching lines @@
       if (resource->options().requestInitiatorContext == DocumentContext)
         context().addResourceTiming(*info);
       resource->reportResourceTimingToClients(*info);
     }
   }
   context().dispatchDidFinishLoading(resource->identifier(), finishTime,
                                      encodedDataLength);
   if (finishReason == DidFinishLoading)
     resource->finish(finishTime);
   context().didLoadResource(resource);
+
+  if (resource->isImage() &&
+      toImageResource(resource)->shouldReloadBrokenPlaceholder()) {
+    toImageResource(resource)->reloadIfLoFiOrPlaceholder(this);

[Nate Chapin, 2016/10/18 18:23:17]

I may have asked this in the design doc, but if I did, I can't find the answer
:(

Is it important that this happens after didLoadResource()? Could we do these
calls in ImageResource::error and ImageResource::finish instead?

[sclittle, 2016/10/18 23:53:03]

I don't think the ordering here is important with respect to didLoadResource.

You did ask this in the design doc, but I resolved the comment:
https://docs.google.com/document/d/1691W7yFDI1FJv69N2MEtaSzpnqO2EqkgGD3T0O-pQ...

The issue with doing the reload in ImageResource::finish/error is that we need a
ResourceFetcher in order to start the reload. To get that ResourceFetcher, we'd
either need to pass it into finish()/error(), or ImageResource would need to
hold on to a ResourceFetcher.

Doing the reload here seems a bit nicer.

+  }
 }
 
 void ResourceFetcher::didFailLoading(Resource* resource,
                                      const ResourceError& error) {
   TRACE_EVENT_ASYNC_END0("blink.net", "Resource", resource->identifier());
   removeResourceLoader(resource->loader());
   m_resourceTimingInfoMap.take(const_cast<Resource*>(resource));
   bool isInternalRequest = resource->options().initiatorInfo.name ==
                            FetchInitiatorTypeNames::internal;
   context().dispatchDidFail(resource->identifier(), error, isInternalRequest);
   resource->error(error);
   context().didLoadResource(resource);
+
+  if (resource->isImage() &&
+      toImageResource(resource)->shouldReloadBrokenPlaceholder()) {
+    toImageResource(resource)->reloadIfLoFiOrPlaceholder(this);
+  }
 }
 
 void ResourceFetcher::didReceiveResponse(Resource* resource,
                                          const ResourceResponse& response,
                                          WebDataConsumerHandle* rawHandle) {
   // |rawHandle|'s ownership is transferred to the callee.
   std::unique_ptr<WebDataConsumerHandle> handle = wrapUnique(rawHandle);
 
   if (response.wasFetchedViaServiceWorker()) {
     if (resource->options().corsEnabled == IsCORSEnabled &&
@@ skipping 227 matching lines @@
         resource->identifier(), resourceLoadPriority,
         resourcePriority.intraPriorityValue);
   }
 }
 
 void ResourceFetcher::reloadLoFiImages() {
   for (const auto& documentResource : m_documentResources) {
     Resource* resource = documentResource.value.get();
     if (resource && resource->isImage()) {
       ImageResource* imageResource = toImageResource(resource);
-      imageResource->reloadIfLoFi(this);
+      imageResource->reloadIfLoFiOrPlaceholder(this);
     }
   }
 }
 
 void ResourceFetcher::logPreloadStats() {
   if (!m_preloads)
     return;
   unsigned scripts = 0;
   unsigned scriptMisses = 0;
   unsigned stylesheets = 0;
@@ skipping 175 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