Support experiment to store persistent metrics in memory-mapped file.

chrome/browser/metrics/chrome_metrics_service_client.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 "chrome/browser/metrics/chrome_metrics_service_client.h"
 
 #include <stddef.h>
 
 #include <vector>
 
@@ skipping 195 matching lines @@
       start_time_(base::TimeTicks::Now()),
       has_uploaded_profiler_data_(false),
       weak_ptr_factory_(this) {
   DCHECK(thread_checker_.CalledOnValidThread());
   RecordCommandLineMetrics();
   RegisterForNotifications();
 }
 
 ChromeMetricsServiceClient::~ChromeMetricsServiceClient() {
   DCHECK(thread_checker_.CalledOnValidThread());
+
+  // If there is a global metrics file being updated on disk, mark it to be

[Alexei Svitkine (slow), 2016/06/16 11:36:57]

Please make a function in the anon namespace that does this and just call it
from here.

By the way, are you sure this object is guaranteed to be deleted?

[bcwhite, 2016/06/16 14:21:08]

Done.

I checked that the object is deleted on clean shutdown.  I don't know if it's
guaranteed but it's not a problem if it is not and probably desired to keep it
around it if anything abnormal happens.

+  // deleted when the process exits. A normal shutdown is almost complete
+  // so there is no benefit in keeping a file with no new data to be processed
+  // during the next startup sequence.
+  base::GlobalHistogramAllocator* allocator =
+      base::GlobalHistogramAllocator::Get();
+  if (allocator) {
+    base::FilePath path = allocator->GetPersistentLocation();
+    if (!path.empty()) {
+      base::File file(path, base::File::FLAG_OPEN | base::File::FLAG_READ |
+                            base::File::FLAG_DELETE_ON_CLOSE);
+    }
+  }
 }
 
 // static
 std::unique_ptr<ChromeMetricsServiceClient> ChromeMetricsServiceClient::Create(
     metrics::MetricsStateManager* state_manager,
     PrefService* local_state) {
   // Perform two-phase initialization so that |client->metrics_service_| only
   // receives pointers to fully constructed objects.
   std::unique_ptr<ChromeMetricsServiceClient> client(
       new ChromeMetricsServiceClient(state_manager));
@@ skipping 507 matching lines @@
   }
 }
 
 void ChromeMetricsServiceClient::OnURLOpenedFromOmnibox(OmniboxLog* log) {
   metrics_service_->OnApplicationNotIdle();
 }
 
 bool ChromeMetricsServiceClient::IsUMACellularUploadLogicEnabled() {
   return metrics::IsCellularLogicEnabled();
 }