Service Worker: Don't write to disk during update until proven necessary

content/browser/service_worker/service_worker_context_request_handler.cc

 // Copyright 2014 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 "content/browser/service_worker/service_worker_context_request_handler.h"
 
 #include "base/time/time.h"
 #include "content/browser/service_worker/service_worker_context_core.h"
 #include "content/browser/service_worker/service_worker_provider_host.h"
 #include "content/browser/service_worker/service_worker_read_from_cache_job.h"
@@ skipping 55 matching lines @@
     // Bypass the browser cache for initial installs and update
     // checks after 24 hours have passed.
     int extra_load_flags = 0;
     base::TimeDelta time_since_last_check =
         base::Time::Now() - registration->last_update_check();
     if (time_since_last_check > base::TimeDelta::FromHours(24) ||
         version_->force_bypass_cache_for_scripts()) {
       extra_load_flags = net::LOAD_BYPASS_CACHE;
     }
 
-    return new ServiceWorkerWriteToCacheJob(request,
-                                            network_delegate,
-                                            resource_type_,
-                                            context_,
-                                            version_.get(),
-                                            extra_load_flags,
-                                            response_id);
+    ServiceWorkerVersion* stored_version = registration->waiting_version()
+                                               ? registration->waiting_version()
+                                               : registration->active_version();
+    int64 incumbent_response_id = kInvalidServiceWorkerResourceId;
+    if (stored_version && stored_version->script_url() == request->url())
+      incumbent_response_id =
+          stored_version->script_cache_map()->LookupResourceId(request->url());

[nhiroki, 2015/06/03 05:36:14]

nit: Can you wrap this if-statement body with {}?

[falken, 2015/06/03 09:07:40]

Done.

+    return new ServiceWorkerWriteToCacheJob(
+        request, network_delegate, resource_type_, context_, version_.get(),
+        extra_load_flags, response_id, incumbent_response_id);
   }
 
   int64 response_id = kInvalidServiceWorkerResponseId;
   if (ShouldReadFromScriptCache(request->url(), &response_id)) {
     return new ServiceWorkerReadFromCacheJob(
         request, network_delegate, context_, version_, response_id);
   }
 
   // NULL means use the network.
   return NULL;
@@ skipping 27 matching lines @@
     const GURL& url, int64* response_id_out) {
   // We don't read from the script cache until the version is INSTALLED.
   if (version_->status() == ServiceWorkerVersion::NEW ||
       version_->status() == ServiceWorkerVersion::INSTALLING)
     return false;
   *response_id_out = version_->script_cache_map()->LookupResourceId(url);
   return *response_id_out != kInvalidServiceWorkerResponseId;
 }
 
 }  // namespace content