[Offline Pages] Turn Offliner::Cancel into an async operation to resolve conflicting assumptions

chrome/browser/android/offline_pages/prerendering_offliner.cc

 // 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.
 
 #include "chrome/browser/android/offline_pages/prerendering_offliner.h"
 
 #include "base/bind.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/sys_info.h"
 #include "chrome/browser/android/offline_pages/offline_page_mhtml_archiver.h"
@@ skipping 192 matching lines @@
   } else {
     // Create app listener for the pending request.
     app_listener_.reset(new base::android::ApplicationStatusListener(
         base::Bind(&PrerenderingOffliner::OnApplicationStateChange,
                    weak_ptr_factory_.GetWeakPtr())));
   }
 
   return accepted;
 }
 
-void PrerenderingOffliner::Cancel() {
+void PrerenderingOffliner::Cancel(const CancelCallback& callback) {
+  int64_t request_id = 0LL;
   if (pending_request_) {
+    request_id = pending_request_->request_id();
     pending_request_.reset(nullptr);
     app_listener_.reset(nullptr);
     GetOrCreateLoader()->StopLoading();
     // TODO(dougarnett): Consider ability to cancel SavePage request.
   }
+  callback.Run(request_id);
 }
 
 bool PrerenderingOffliner::HandleTimeout(const SavePageRequest& request) {
   if (pending_request_) {
     DCHECK(request.request_id() == pending_request_->request_id());
     if (GetOrCreateLoader()->IsLowbarMet() &&
         (request.started_attempt_count() + 1 >= policy_->GetMaxStartedTries() ||
          request.completed_attempt_count() + 1 >=
              policy_->GetMaxCompletedTries())) {
       GetOrCreateLoader()->StartSnapshot();
@@ skipping 34 matching lines @@
   return loader_.get();
 }
 
 void PrerenderingOffliner::OnApplicationStateChange(
     base::android::ApplicationState application_state) {
   if (pending_request_ && is_low_end_device_ &&
       application_state ==
           base::android::APPLICATION_STATE_HAS_RUNNING_ACTIVITIES) {
     DVLOG(1) << "App became active, canceling current offlining request";
     SavePageRequest* request = pending_request_.get();
-    Cancel();
-    completion_callback_.Run(*request,
-                             Offliner::RequestStatus::FOREGROUND_CANCELED);
+    // This works because Bind will make a copy of request, and we
+    // should not have to worry about reset being called before cancel callback.
+    Cancel(base::Bind(&PrerenderingOffliner::HandleApplicationStateChangeCancel,
+                      weak_ptr_factory_.GetWeakPtr(), *request));
   }
 }
 
+void PrerenderingOffliner::HandleApplicationStateChangeCancel(
+    const SavePageRequest& request,
+    int64_t offline_id) {
+  // This shouldn't be immediate, but account for case where request was reset
+  // while waiting for callback.
+  if (pending_request_ && pending_request_->request_id() != offline_id)
+    return;
+  completion_callback_.Run(request, RequestStatus::FOREGROUND_CANCELED);
+}
 }  // namespace offline_pages