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

chrome/browser/metrics/chrome_metrics_service_client.cc

Index: chrome/browser/metrics/chrome_metrics_service_client.cc
diff --git a/chrome/browser/metrics/chrome_metrics_service_client.cc b/chrome/browser/metrics/chrome_metrics_service_client.cc
index ed4fe07e3de76fd0b621c1c19d21b31f16e43770..06bcd97b00f6730052616e841a27e4afe96d48b3 100644
--- a/chrome/browser/metrics/chrome_metrics_service_client.cc
+++ b/chrome/browser/metrics/chrome_metrics_service_client.cc
@@ -15,6 +15,7 @@
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram.h"
+#include "base/metrics/persistent_histogram_allocator.h"
 #include "base/path_service.h"
 #include "base/rand_util.h"
 #include "base/strings/string16.h"
@@ -138,6 +139,11 @@ bool ShouldClearSavedMetrics() {
 }
 
 void RegisterInstallerFileMetricsPreferences(PrefRegistrySimple* registry) {
+  base::GlobalHistogramAllocator* allocator =
+      base::GlobalHistogramAllocator::GetEvenIfDisabled();
+  if (allocator)
+    metrics::FileMetricsProvider::RegisterPrefs(registry, allocator->Name());
+
 #if defined(OS_WIN)
   metrics::FileMetricsProvider::RegisterPrefs(
       registry, installer::kSetupHistogramAllocatorName);
@@ -146,22 +152,50 @@ void RegisterInstallerFileMetricsPreferences(PrefRegistrySimple* registry) {
 
 void RegisterInstallerFileMetricsProvider(
     metrics::MetricsService* metrics_service) {
-#if defined(OS_WIN)
-  std::unique_ptr<metrics::FileMetricsProvider> file_metrics(
+  // Create an object to monitor files of metrics and include them in reports.
+  std::unique_ptr<metrics::FileMetricsProvider> file_metrics_provider(
       new metrics::FileMetricsProvider(
           content::BrowserThread::GetBlockingPool()
               ->GetTaskRunnerWithShutdownBehavior(
                   base::SequencedWorkerPool::CONTINUE_ON_SHUTDOWN),
           g_browser_process->local_state()));
+
+  // Build the pathname for browser's persistent metrics file. Set it as the
+  // destination for the GlobalHistogramAllocator during process exit and add
+  // it to the file-metrics-provider so one written from a previous run will
+  // be loaded.

[Alexei Svitkine (slow), 2016/05/09 21:24:10]

In this CL, these files are not yet being created right?

If so, please mention this in the comment.

Maybe: TODO(bcwhite): These files are not yet being created. crbug.com/xyz

[bcwhite, 2016/05/10 15:42:40]

Right.  It used to be part but was removed because it's largely unnecessary
(metrics are dumped just before exit) and would cause a startup impact on the
next run.

TODO added.

+  base::FilePath metrics_file;
+  if (base::PathService::Get(chrome::DIR_USER_DATA, &metrics_file)) {
+    base::GlobalHistogramAllocator* allocator =
+        base::GlobalHistogramAllocator::GetEvenIfDisabled();
+    if (allocator) {
+      const char* allocator_name = allocator->Name();
+      metrics_file = metrics_file.AppendASCII(allocator_name)
+                         .AddExtension(FILE_PATH_LITERAL(".pma"));

[Alexei Svitkine (slow), 2016/05/09 21:24:10]

Didn't we have this extension as a constant somewhere?

[bcwhite, 2016/05/10 15:42:40]

Yes.  It's in another CL still in review.  But I'm going to move it right up to
base/ (other CL puts it in content/) so that it's more universally available.

+      allocator->SetPersistentLocation(metrics_file);
+
+      file_metrics_provider->RegisterFile(
+          metrics_file,
+          metrics::FileMetricsProvider::FILE_HISTOGRAMS_ATOMIC,
+          metrics::FileMetricsProvider::ASSOCIATE_PREVIOUS_RUN,
+          allocator_name);
+    }
+  }
+
+#if defined(OS_WIN)
+  // Read metrics file from setup.exe.
   base::FilePath program_dir;
   base::PathService::Get(base::DIR_EXE, &program_dir);
-  file_metrics->RegisterFile(
+  file_metrics_provider->RegisterFile(
       program_dir.AppendASCII(installer::kSetupHistogramAllocatorName)
-          .AddExtension(L".pma"),
+          .AddExtension(FILE_PATH_LITERAL(".pma")),
       metrics::FileMetricsProvider::FILE_HISTOGRAMS_ATOMIC,
+      metrics::FileMetricsProvider::ASSOCIATE_CURRENT_RUN,
       installer::kSetupHistogramAllocatorName);
-  metrics_service->RegisterMetricsProvider(std::move(file_metrics));
 #endif
+
+  // Give the new provider to the metrics service.
+  metrics_service->RegisterMetricsProvider(std::move(file_metrics_provider));
 }
 
 }  // namespace