Don't start a second pre-render if one is in progress.

components/offline_pages/background/request_coordinator.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_OFFLINE_PAGES_BACKGROUND_REQUEST_COORDINATOR_H_
 #define COMPONENTS_OFFLINE_PAGES_BACKGROUND_REQUEST_COORDINATOR_H_
 
 #include <memory>
 
 #include "base/callback.h"
@@ skipping 55 matching lines @@
   RequestQueue* queue() { return queue_.get(); }
 
   // Return an unowned pointer to the Scheduler.
   Scheduler* scheduler() { return scheduler_.get(); }
 
   // Returns the status of the most recent offlining.
   Offliner::RequestStatus last_offlining_status() {
     return last_offlining_status_;
   }
 
+  bool is_busy() {
+    return is_busy_;
+  }
+
  private:
   void AddRequestResultCallback(RequestQueue::AddRequestResult result,
                                 const SavePageRequest& request);
 
   void UpdateRequestCallback(RequestQueue::UpdateRequestResult result);
 
   // Callback from the request picker when it has chosen our next request.
   void RequestPicked(const SavePageRequest& request);
 
   // Callback from the request picker when no more requests are in the queue.
   void RequestQueueEmpty();
 
   void SendRequestToOffliner(const SavePageRequest& request);
 
   // Called by the offliner when an offlining request is completed. (and by
   // tests).
   void OfflinerDoneCallback(const SavePageRequest& request,
                             Offliner::RequestStatus status);
 
   void TryNextRequest();
 
   friend class RequestCoordinatorTest;
 
+  // The offliner can only handle one request at a time - if the offliner is
+  // busy, prevent other requests.  This flag marks whether the offliner is in
+  // use.
+  bool is_busy_;
   // RequestCoordinator takes over ownership of the policy
   std::unique_ptr<OfflinerPolicy> policy_;
   // OfflinerFactory.  Used to create offline pages. Owned.
   std::unique_ptr<OfflinerFactory> factory_;
   // RequestQueue.  Used to store incoming requests. Owned.
   std::unique_ptr<RequestQueue> queue_;
   // Scheduler. Used to request a callback when network is available.  Owned.
   std::unique_ptr<Scheduler> scheduler_;
   // Status of the most recent offlining.
   Offliner::RequestStatus last_offlining_status_;
   // Class to choose which request to schedule next
   std::unique_ptr<RequestPicker> picker_;
   // Calling this returns to the scheduler across the JNI bridge.
   base::Callback<void(bool)> scheduler_callback_;
   // Allows us to pass a weak pointer to callbacks.
   base::WeakPtrFactory<RequestCoordinator> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(RequestCoordinator);
 };
 
 }  // namespace offline_pages
 
 #endif  // COMPONENTS_OFFLINE_PAGES_BACKGROUND_REQUEST_COORDINATOR_H_