Support saving browser metrics to disk and reading them during next run.

chrome/browser/chrome_browser_field_trials.cc

 // Copyright (c) 2012 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/chrome_browser_field_trials.h"
 
 #include <string>
 
 #include "base/command_line.h"
 #include "base/feature_list.h"
+#include "base/files/file_path.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/persistent_histogram_allocator.h"
+#include "base/path_service.h"
 #include "base/strings/string_util.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
+#include "chrome/common/chrome_paths.h"
 #include "chrome/common/chrome_switches.h"
 #include "components/metrics/metrics_pref_names.h"
 
 #if defined(OS_ANDROID)
 #include "chrome/browser/chrome_browser_field_trials_mobile.h"
 #else
 #include "chrome/browser/chrome_browser_field_trials_desktop.h"
 #endif
 
 namespace {
 
 // Check for feature enabling the use of persistent histogram storage and
 // create an appropriate allocator for such if so.
 void InstantiatePersistentHistograms() {
   if (base::FeatureList::IsEnabled(base::kPersistentHistogramsFeature)) {
     const char kAllocatorName[] = "BrowserMetrics";
     // Create persistent/shared memory and allow histograms to be stored in it.
     // Memory that is not actualy used won't be physically mapped by the system.
     // BrowserMetrics usage peaked around 95% of 2MiB as of 2016-02-20.
     base::GlobalHistogramAllocator::CreateWithLocalMemory(
         3 << 20,     // 3 MiB
         0x935DDD43,  // SHA1(BrowserMetrics)
         kAllocatorName);
     base::GlobalHistogramAllocator::Get()->CreateTrackingHistograms(
         kAllocatorName);
+
+    // Set the destination for where these will be persistent. This shouldn't

[Ilya Sherman, 2016/04/19 00:57:46]

nit: s/persistent/persisted

[bcwhite, 2016/04/19 16:33:38]

Done.

+    // be necessary because it's done in chrome_metrics_service_client.cc where
+    // the reading of this file is configured but field trials don't get done
+    // until after that point so there is no global allocator to configure
+    // when that code runs. This won't be a problem when this (eventually)
+    // becomes standard and is enabled during early startup.
+    base::FilePath metrics_file;
+    if (base::PathService::Get(chrome::DIR_USER_DATA, &metrics_file)) {
+      metrics_file = metrics_file.AppendASCII(kAllocatorName)
+                         .AddExtension(FILE_PATH_LITERAL(".pma"));
+      base::GlobalHistogramAllocator::Get()
+          ->SetPersistentLocation(metrics_file);
+    }

[Ilya Sherman, 2016/04/19 00:57:46]

I think I'd prefer to always set the location from the correct location, and to
just call a boolean Enable() method based on field trial state.  WDYT?

[bcwhite, 2016/04/19 16:33:38]

I could add Disable/Enable methods to the GlobalHistogramAllocator class.  I'll
look into it.

   }
 }
 
 }  // namespace
 
 ChromeBrowserFieldTrials::ChromeBrowserFieldTrials(
     const base::CommandLine& parsed_command_line)
     : parsed_command_line_(parsed_command_line) {
 }
 
 ChromeBrowserFieldTrials::~ChromeBrowserFieldTrials() {
 }
 
 void ChromeBrowserFieldTrials::SetupFieldTrials() {
   // Field trials that are shared by all platforms.
   InstantiateDynamicTrials();
 
 #if defined(OS_ANDROID)
   chrome::SetupMobileFieldTrials(parsed_command_line_);
 #else
   chrome::SetupDesktopFieldTrials(parsed_command_line_);
 #endif
 }
 
 void ChromeBrowserFieldTrials::InstantiateDynamicTrials() {
   // Persistent histograms must be enabled as soon as possible.
   InstantiatePersistentHistograms();
 }