Add support for fallback content for the frame. This includes main and subframes.

content/browser/loader/navigation_url_loader_network_service.cc

Index: content/browser/loader/navigation_url_loader_network_service.cc
diff --git a/content/browser/loader/navigation_url_loader_network_service.cc b/content/browser/loader/navigation_url_loader_network_service.cc
index d55894595e17accb1220a5630c4ad2b6f119843a..3cd6a4da42751bcabd188988012fbb2277d76ab0 100644
--- a/content/browser/loader/navigation_url_loader_network_service.cc
+++ b/content/browser/loader/navigation_url_loader_network_service.cc
@@ -105,7 +105,8 @@ class NavigationURLLoaderNetworkService::URLLoaderRequestController
       : resource_request_(std::move(resource_request)),
         resource_context_(resource_context),
         default_url_loader_factory_getter_(default_url_loader_factory_getter),
-        owner_(owner) {}
+        owner_(owner),
+        fallback_response_binding_(this) {}
 
   ~URLLoaderRequestController() override {
     DCHECK_CURRENTLY_ON(BrowserThread::IO);
@@ -214,6 +215,7 @@ class NavigationURLLoaderNetworkService::URLLoaderRequestController
       factory = default_url_loader_factory_getter_->GetBlobFactory()->get();
     } else {
       factory = default_url_loader_factory_getter_->GetNetworkFactory()->get();
+      default_loader_used_ = true;
     }
     url_loader_ = ThrottlingURLLoader::CreateLoaderAndStart(
         factory,
@@ -243,6 +245,12 @@ class NavigationURLLoaderNetworkService::URLLoaderRequestController
       const ResourceResponseHead& head,
       const base::Optional<net::SSLInfo>& ssl_info,
       mojom::DownloadedTempFilePtr downloaded_file) override {
+    response_ = head;
+    // If the default loader (network) was used to handle the URL load request
+    // we need to see if the handlers want to load a fallback for the response.
+    if (MaybeLoadFallbackForResponse(head))
+      return;
+
     BrowserThread::PostTask(
         BrowserThread::UI, FROM_HERE,
         base::Bind(&NavigationURLLoaderNetworkService::OnReceiveResponse,
@@ -251,6 +259,13 @@ class NavigationURLLoaderNetworkService::URLLoaderRequestController
 
   void OnReceiveRedirect(const net::RedirectInfo& redirect_info,
                          const ResourceResponseHead& head) override {
+    response_ = head;
+
+    // If the default loader (network) was used to handle the URL load request
+    // we need to see if the handlers want to load a fallback for the response.
+    if (MaybeLoadFallbackForRedirect(head, redirect_info))
+      return;
+
     BrowserThread::PostTask(
         BrowserThread::UI, FROM_HERE,
         base::Bind(&NavigationURLLoaderNetworkService::OnReceiveRedirect,
@@ -278,12 +293,74 @@ class NavigationURLLoaderNetworkService::URLLoaderRequestController
 
   void OnComplete(
       const ResourceRequestCompletionStatus& completion_status) override {
+    // If the default loader (network) was used to handle the URL load request
+    // we need to see if the handlers want to load a fallback for the response.
+    if (MaybeLoadFallbackForResponse(response_))
+      return;
+
     BrowserThread::PostTask(
         BrowserThread::UI, FROM_HERE,
         base::Bind(&NavigationURLLoaderNetworkService::OnComplete, owner_,
                    completion_status));
   }
 
+  // Returns true if a handler returns a fallback response.
+  bool MaybeLoadFallbackForResponse(const ResourceResponseHead& response) {
+    mojom::URLLoaderClientPtr fallback_client;
+    mojom::URLLoaderRequest fallback_request;
+    if (!CreateFallbackClientAndRequest(&fallback_client, &fallback_request))
+      return false;
+
+    for (size_t index = 0; index < handlers_.size(); ++index) {
+      if (handlers_[index]->MaybeGetFallbackForResponse(
+              response, &fallback_client, &fallback_request)) {
+        OnFallbackHandlerFound();
+        return true;
+      }
+    }
+    return false;
+  }
+
+  // Returns true if a handler returns a fallback response for the redirect.
+  bool MaybeLoadFallbackForRedirect(const ResourceResponseHead& response,
+                                    const net::RedirectInfo& redirect_info) {
+    mojom::URLLoaderClientPtr fallback_client;
+    mojom::URLLoaderRequest fallback_request;
+    if (!CreateFallbackClientAndRequest(&fallback_client, &fallback_request))
+      return false;
+
+    for (size_t index = 0; index < handlers_.size(); ++index) {
+      if (handlers_[index]->MaybeGetFallbackForRedirect(
+              response, redirect_info, &fallback_client, &fallback_request)) {
+        OnFallbackHandlerFound();
+        return true;
+      }
+    }
+    return false;
+  }
+
+  // Creates an instance of the URLLoaderClient interface which can be bound
+  // to a handler for receiving fallback responses.
+  // Returns true on success.
+  bool CreateFallbackClientAndRequest(mojom::URLLoaderClientPtr* client,
+                                      mojom::URLLoaderRequest* request) {
+    // Ignore requests which were not sent to the network loader.
+    if (!default_loader_used_)
+      return false;
+    fallback_response_binding_.Bind(mojo::MakeRequest(client));
+    *request = mojo::MakeRequest(&fallback_url_loader_);
+    return true;
+  }
+
+  // Called if we find a handler who delivers fallback content for the URL.
+  void OnFallbackHandlerFound() {
+    // We reset this flag as we expect a response from the fallback handler.
+    default_loader_used_ = false;
+    // Disconnect from the network loader to stop receiving further data
+    // or notifications for the URL.
+    url_loader_->DisconnectClient();
+  }
+
   std::vector<std::unique_ptr<URLLoaderRequestHandler>> handlers_;
   size_t handler_index_ = 0;
 
@@ -306,6 +383,19 @@ class NavigationURLLoaderNetworkService::URLLoaderRequestController
   // This is referenced only on the UI thread.
   base::WeakPtr<NavigationURLLoaderNetworkService> owner_;
 
+  // Set to true if the default URLLoader (network service) was used for the
+  // current navigation.
+  bool default_loader_used_ = false;
+
+  // Contains a copy of the response.
+  ResourceResponseHead response_;
+
+  // URLLoaderClient binding for handling fallback responses.
+  mojo::Binding<mojom::URLLoaderClient> fallback_response_binding_;
+
+  // URLLoader instance for fallback responses.
+  mojom::URLLoaderPtr fallback_url_loader_;
+
   DISALLOW_COPY_AND_ASSIGN(URLLoaderRequestController);
 };