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..1135024c4cbcdb974c903566bcb7713ecc160d05 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)

[Ilya Sherman, 2016/04/25 19:48:40]

In what case(s) might the allocator be null?  (And why is the answer here
different than the answer in chrome_browser_field_trials.cc?)

[bcwhite, 2016/04/25 20:37:17]

I don't imagine it will be.  But this code is so far away from where it is set
that I prefer to be fail-safe.

Who knows what code paths could be across the myriad of Chrome products, not to
mention all the tests.

[Ilya Sherman, 2016/04/25 21:12:11]

You're trading predictable crashes or DCHECK failures in the case of bugs, to
mysterious behavior in the case of bugs.  I'd much rather have a clear
indication that something is awry, rather than potentially papering over a real
issue.  If tests are failing, we should fix the tests, rather than bending the
production code to their whims.

+    metrics::FileMetricsProvider::RegisterPrefs(registry, allocator->Name());
+
 #if defined(OS_WIN)
   metrics::FileMetricsProvider::RegisterPrefs(
       registry, installer::kSetupHistogramAllocatorName);
@@ -146,22 +152,48 @@ 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.
+  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"));
+      allocator->SetPersistentLocation(metrics_file);
+
+      file_metrics_provider->RegisterFile(
+          metrics_file,
+          metrics::FileMetricsProvider::FILE_HISTOGRAMS_ATOMIC,
+          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,
       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