Upgrade: Send a 'Prefer' header for feature detection.

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 701 matching lines @@
     }
     m_validatedURLs.add(request.resourceRequest().url());
 }
 
 ResourcePtr<Resource> ResourceFetcher::requestResource(Resource::Type type, FetchRequest& request)
 {
     ASSERT(request.options().synchronousPolicy == RequestAsynchronously || type == Resource::Raw);
 
     TRACE_EVENT0("blink", "ResourceFetcher::requestResource");
 
-    maybeUpgradeInsecureRequestURL(request);
+    upgradeInsecureRequest(request);
 
     KURL url = request.resourceRequest().url();
 
     WTF_LOG(ResourceLoading, "ResourceFetcher::requestResource '%s', charset '%s', priority=%d, forPreload=%u, type=%s", url.elidedString().latin1().data(), request.charset().latin1().data(), request.priority(), request.forPreload(), ResourceTypeName(type));
 
     // If only the fragment identifiers differ, it is the same resource.
     url = MemoryCache::removeFragmentIdentifierIfNeeded(url);
 
     if (!url.isValid())
         return nullptr;
@@ skipping 161 matching lines @@
     if (request.requestContext() == WebURLRequest::RequestContextUnspecified)
         determineRequestContext(request, type);
     if (type == Resource::LinkPrefetch || type == Resource::LinkSubresource)
         request.setHTTPHeaderField("Purpose", "prefetch");
     if (frame()->document())
         request.setOriginatesFromReservedIPRange(frame()->document()->isHostedInReservedIPRange());
 
     context().addAdditionalRequestHeaders(document(), request, (type == Resource::MainResource) ? FetchMainResource : FetchSubresource);
 }
 
-void ResourceFetcher::maybeUpgradeInsecureRequestURL(FetchRequest& fetchRequest)
+void ResourceFetcher::upgradeInsecureRequest(FetchRequest& fetchRequest)
 {
-    if (!m_document)
+    if (!m_document || !RuntimeEnabledFeatures::experimentalContentSecurityPolicyFeaturesEnabled())
         return;
 
     KURL url = fetchRequest.resourceRequest().url();
+
+    // Tack a 'Prefer' header to outgoing navigational requests, as described in
+    // https://w3c.github.io/webappsec/specs/upgrade/#feature-detect
+    if (fetchRequest.resourceRequest().frameType() != WebURLRequest::FrameTypeNone && !SecurityOrigin::isSecure(url))
+        fetchRequest.mutableResourceRequest().addHTTPHeaderField("Prefer", "return=secure-representation");
+
     if (m_document->insecureContentPolicy() == SecurityContext::InsecureContentUpgrade && url.protocolIs("http")) {
         // We always upgrade subresource requests and nested frames, we always upgrade form
         // submissions, and we always upgrade requests whose host matches the host of the
         // containing document's security origin.
         //
         // FIXME: We need to check the document that set the policy, not the current document.
         const ResourceRequest& request = fetchRequest.resourceRequest();
         if (request.frameType() == WebURLRequest::FrameTypeNone
             || request.frameType() == WebURLRequest::FrameTypeNested
             || request.requestContext() == WebURLRequest::RequestContextForm
@@ skipping 726 matching lines @@
     ResourceLoaderHost::trace(visitor);
 }
 
 ResourceFetcher* ResourceFetcher::toResourceFetcher(ResourceLoaderHost* host)
 {
     ASSERT(host->objectType() == ResourceLoaderHost::ResourceFetcherType);
     return static_cast<ResourceFetcher*>(host);
 }
 
 }