ResourceFetcher: fix an assertion failure on defered resource revalidation

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 493 matching lines @@
         // reference to the image is parsed (which would be at a lower priority).
         if (priority > resource->resourceRequest().priority())
             resource->didChangePriority(priority, 0);
     }
 
     // If only the fragment identifiers differ, it is the same resource.
     ASSERT(equalIgnoringFragmentIdentifier(resource->url(), url));
     requestLoadStarted(resource, request, policy == Use ? ResourceLoadingFromCache : ResourceLoadingFromNetwork, isStaticData);
     m_documentResources.set(urlWithoutFragment, resource);
 
+    // Returns with an existing resource if the resource does not need to start
+    // loading immediately.
+    // If revalidation policy was determined as |Revalidate|, the resource was
+    // already initialized for the revalidation here, but won't start loading.
     if (!resourceNeedsLoad(resource, request, policy))
         return resource;
 
     if (!context().shouldLoadNewResource(factory.type())) {
         if (memoryCache()->contains(resource))
             memoryCache()->remove(resource);
         return nullptr;
     }
 
     resource->load(this);
@@ skipping 264 matching lines @@
     if (!existingResource->canReuseRedirectChain()) {
         WTF_LOG(ResourceLoading, "ResourceFetcher::determineRevalidationPolicy reloading due to an uncacheable redirect");
         return Reload;
     }
 
     // Check if the cache headers requires us to revalidate (cache expiration for example).
     if (cachePolicy == CachePolicyRevalidate || existingResource->mustRevalidateDueToCacheHeaders()
         || request.cacheControlContainsNoCache()) {
         // See if the resource has usable ETag or Last-modified headers.
         // If the page is controlled by the ServiceWorker, we choose the Reload policy because the revalidation headers should not be exposed to the ServiceWorker.(crbug.com/429570)
-        if (existingResource->canUseCacheValidator() && !context().isControlledByServiceWorker())
+        if (existingResource->canUseCacheValidator() && !context().isControlledByServiceWorker()) {
+            // If the resource is already a cache validator but not started yet,
+            // the |Use| policy should be applied to subsequent requests.
+            if (existingResource->isCacheValidator()) {
+                DCHECK(existingResource->stillNeedsLoad());

[Takashi Toyoshima, 2016/06/06 13:14:19]

existingResource->canUseCacheValidator() ensures the Status is not Pending, and
not Load/DecodeError. Also, we already have a check that confirms
m_revalidatingRequest.isNull() is true on changing the Status to Cached. So,
here the Status should be always NotStarted until we make another change that
breaks this assumption.

+                return Use;
+            }
             return Revalidate;
+        }
 
         // No, must reload.
         WTF_LOG(ResourceLoading, "ResourceFetcher::determineRevalidationPolicy reloading due to missing cache validators.");
         return Reload;
     }
 
     return Use;
 }
 
 void ResourceFetcher::setAutoLoadImages(bool enable)
@@ skipping 438 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